N. I. Vavilov’s Fundamental Research: Centers of Origin of Cultivated Plants (a review)

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Historical Background of the Theoretical Concept

The idea to penetrate the problem of the origin of cultivated plants came to Nilolai Vavilov during his college years. It was further shaped after his internship in the United Kingdom and familiarization with Charles Darwin’s personal library. Throughout his whole life, Vavilov considered himself a disciple of the founder of the theory of the origin of species, and Darwin’s portrait was always present in his office. For the first time, Vavilov published the results of his investigations into the origin of crop plants in 1917 in his essay On the Origin of the Cultivated Rye [1]. This publication summarized Vavilov’s experience during his first scientific mission to Persia and Pamir in 1916. His notions on the subject were promoted in his article On the Eastern Centres of Origin of Cultivated Plants [2] that followed his plant explorations in Mongolia and Transcaucasia and an examination of the materials supplied from Asia and Africa. Preparing this publication in 1923, Vavilov wrote in one of his letters, “I am rereading de Candolle, Darwin and Hehn for the tenth time and in all honesty I feel that we have gone a little further than de Candolle… It became possible to clarify the question of the centres of origin of cultivated plants. It appeared that barley, for example, had two centres: Eastern Asia and Eastern Africa, while Northern Africa and South-Western Asia were for flax. Crop centres, the centres of origin for cultivated plants, turned out to be in mountainous areas, but not along the valleys of great rivers, as I thought earlier” [3, p. 136]. As the material collected by him or his colleagues arrived and new data on the distribution of plant resources over the globe were obtained, all of those findings were described in Vavilov’s publications [4].

He mentioned in his article The Doctrine of the Origin of Cultivated Plants after Darwin [5] that during his studies at Darwin’s personal library in 1913–1914 he was impressed by the scientist’s obvious persistence in scrutinizing the works of his predecessors on the history of crop cultivation and plant breeding. He also noted in the same publication that while approaching the variability and evolution of cultivated plants, Charles Darwin [6] had relied primarily on Alphonse de Candolle’s Géographie botanique raisonnée [7]. However, unlike de Candolle, Darwin had been interested in the evolution of species and the hereditary changes undergone by a species when taken into cultivation. De Candolle, on the other hand, had focused on establishing the homeland for cultivated plants. “Darwin’s great and immortal merit is that he focused attention on this dialectics in the relationships between variability, heredity, and selection, opening up opportunities for progressive breeding. That is why Darwin’s works became the foundation of the theory and practice of breeding, the basis for creative breeding work. Never before had this idea of variability and the enormous creative role of selection been advanced with such clarity, certainty, and validity” [8, p. 159].

Alphonse de Candolle’s Origine des plantes cultivées [9] was published after Darwin’s death and became the pivotal work in this field of knowledge. This classic volume, rich in factual content, seemed one-sided to Vavilov, as it addressed only the question of the original native land for crop plants and their connections to wild parent species or related ones. Favoring Darwin’s approach over de Candolle’s, Vavilov paid much more attention to both the main areas of species’ origin and the evolutionary stages passed by plant species during their dispersal under the influence of cultivation, environmental conditions, and natural and artificial selection. Inspired by the fundamental principles of Darwin’s and de Candolle’s theories, Vavilov formulated long-term research objectives for the activities of the institute he headed, “Based on Darwin’s idea of geographical evolution, a systematic study of the most important plants was planned to encompass all evolutionary stages, beginning from primary areas where the connections with wild forms are still traceable, where phylogenetic relationships between various wild species and cultivated forms can be established, tracking down the consequent historical dispersal of species to the final links of contemporary breeding. Such work, of course, could only be implemented by a team of researchers who would follow a specific plan based on the evolutionary principle.

“The research consistently covered a large portion of the globe’s agricultural lands: naturally enough, it first of all focused on the Soviet Union itself and adjacent countries. All agricultural areas in North and South Americas were explored comparatively well. Plant collecting missions traversed the entire Cordillera, covered the main arable territories in Africa, and travelled across a significant part of the farming regions in Asia. The enormous amount of collected material, now estimated at over 200,000 accessions, underwent comprehensive field studies under various conditions with all available methods.

“These studies not only expanded the knowledge of individual cultivated plants but also disclosed numerous new species and a great number of botanical varieties of both cultivated plants and their closest wild relatives, previously unknown to botanists and even more so to breeders” [8, pp. 162–163].

Vavilov concluded this article by stating, “The basic and sole evolutionary theory, unshakable for 80 years, is Darwin’s teaching. A botanist, a zoologist, a geneticist, a plant breeder, an ecologist, and a biogeographer by their direct activities confirm this comprehensive teaching; only on its basis it is possible to understand the process of evolution and control over organisms” [8, p. 176].

Studies on the Origin of Cultivated Plants − 1926

In 1926, the Bulletin of Applied Botany and Plant-Breeding presented N. I. Vavilov’s fundamental publication Studies on the Origin of Cultivated Plants (Fig. 1), with a dedication to A. de Candolle, paying tribute to this famous crop researcher, “To the Memory of Alphonse DE CANDOLLE these essays are dedicated by the Author as a tribute to his ‘L’origine des plantes cultivées’ 1882, ‘Géographie botanique raisonnée’ 1855, ‘Phytographie’ 1880 etc.” Summarizing the results of theoretical speculations in this publication, Vavilov used his law of homologous series to emphasize the parallelism and cyclicality in the shaping of the most diverse genera and families, thus simplifying to some extent the solution to the problem of their origin and making it possible to predict the presence of certain plant forms [4]. As a result, he identified five main foci for the most important field, vegetable and horticultural crops, pointing out that “Further investigations will probably enable us to establish a series of secondary centres, besides the above mentioned principal ones, as well as to locate with more geographical accuracy the principal centres themselves… The regions of the origin of the most important cultivated plants and their forms, the actual centres of varietal riches, belong chiefly to the mountainous areas of Asia, the Himalaya with its branches, the mountain ranges North-Eastern Africa, the mountainous regions of Southern Europe (Pyrenees, Apennines, Balkan range), the Cordilleras and the Southern branches of the Rocky Mountains. In the Old World the region of the origin of cultivated plants is located within the zone from 20° to 40° N. lat.” [10, p. 243].

Such diversity of conditions—from deserts to oases, from poor rocky soils to humus-rich alpine and subalpine meadows, abundance of water from melting snow and ice for irrigation and watering, isolation of sites and their protection from attacks—contributed, as Vavilov believed, to the concentration and development of exceptionally diversified vegetation in those locations.

Vavilov’s idea was that the area with the greatest variability, usually including a number of endemic forms and traits, could be considered a center of morphogenesis. He wrote, “In locating the centres where the cultivated plants have originated we come near or establishing the principal homesteads of human culture…

“The plants and their varieties are not so easily transplanted from one region into another; in spite of the millennial wanderings of the peoples and tribes, there are no difficulties at the present time to establish the principal centres where the majority of cultivated plants have originated. The presence in Northern Africa and in South-Western Asia of vast groups of endemic species and varieties of cultivated plants, out of which independent crops have arisen, settles the question of the autonomy of these crops as regards the general history of culture” [10, p. 244].

In conclusion, Vavilov opined, “The ultimate purpose of the above exposed investigations, besides their direct practical importance for utilizing the sources of varietal wealth, was to bring us nearer to the solution of the general biological problem of the origin of species.

Evolution proceeded in time and space and time, and only by coming quite near the geographical centres of the origin of forms, by establishing the links connecting species, shall we be able to master the synthesis of Linnean species (considering them as systems of forms). Only systematics and the knowledge of the geography of plants enable the geneticist to select consciously the initial forms for his crosses and to solve problems of experimental phylogenetics. The problem of the origin of species is thus considered as the problem of the origin not of separate varieties which in Darwin’s opinion differentiated into individual species, but of complex systems, such as are true Linnean species.

As it follows from the above, the solution of the problem of the origin of species lies in a synthesis of thorough investigations of separate groups of plants by the differential systematical methods, by that of botanical geography (in the sense of establishing the centres of the origin of forms) and the methods of genetics and cytology” [10, p. 245].

It was in this publication that Vavilov described the eight ancient foci and centers of the world’s agriculture:

1. Chinese
2. Indian

IIa. Indo-Malaysian

1. Middle Asian
2. West Asian
3. Mediterranean
4. Abyssinian
5. South Mexican and Central American (including the Antilles)
6. South American (Peruvian-Ecuadorian-Bolivian)

VIIIa. Chiloan

VIIIb. Brazilian-Paraguayan

Vavilov regarded this work as the initial stage for further research that would help to supplement, refine and delineate with more precision the boundaries of the centers of crop origin. He published a brief summary of his views in the proceedings of an international congress [10].

Fig. 1. Cover of the first edition of Studies on the Origin of Cultivated Plants, 1926.

Further Advanced Substantiation of the Theoretical Concept

Nikolai Vavilov worked extensively on this problem for two decades, regarding it as the most important. In 1927, he published Geographical Regularities in the Distribution of the Genes of Cultivated Plants [11, 12]; in 1928, Geographical Localization of Wheat Genes on the Globe; in 1931, The Role of Central Asia in the Origin of Cultivated Plants [13], Wild Relatives of Fruit Trees of the Asian Part of the USSR and the Caucasus and the Problem of the Origin of Fruit Trees [14], The Problem of the Origin of Cultivated Plants in Its Modern Interpretation [8], Mexico and Central America as the Principal Centre of Origin of Cultivated Plants of the New World [15], and The Problem of the Origin of World Agriculture in the Light of Modern Research [8, 16]; in 1935, Botanical and Geographical Fundamentals of Breeding [17]; in 1939, Les grandes civilisations agricoles de l’Amérique d’avant Christophe Colomb et leurs rapports mutuels (Great Agricultural Civilizations of Pre-Columbian America and Their Interrelations); and finally, in 1940, The Doctrine of the Origin of Cultivated Plants After Darwin [5, 18].

In his Botanical and Geographical Fundamentals of Breeding, included into the collective fundamental publication Theoretical Basеs of Plant Breeding, Vavilov wrote, “Our initial aspirations were targeted primarily at the study of the most difficult objects, such as wheat, rye, barley, maize, and cotton, which are now widely cultivated throughout the globe and have long since dispersed from their primary centres of domestication...

“As more new objects were involved in the research, the coincidence of the areas of the primary development of forms became increasingly clear for many species or even genera. In a number of cases, one can speak of exactly the same areas for literally dozens of species. Geographical studies has led to the identification of entire independent cultivated floras specific to individual regions” [17, p. 28].

“Summing up the outcome of the work of the Soviet team of plant scientists and the numerous expeditions conducted within Asia, Africa, Southern Europe, North and South Americas, covering up to 60 countries, as well as the entire USSR, and summarizing the results of the detailed comparative study of the colossal new varietal and specific diversity, we have come to the identification of eight independent global foci of origin for the most important cultivated plants. The work toward this goal is not yet finished; our knowledge of South-Eastern Asia is still far from being sufficient. A series of further expeditions to China, Indochina and India is required to clarify the foci where cultivated plants first originated and acquire new materials. However, with a degree of precision that would have been unthinkable ten years ago, we can now speak of eight ancient primary foci of the world’s agriculture or, more accurately, eight independent regions where various plants were first introduced into cultivation. In our earlier works, we limited ourselves to establishing the foci of agriculture on the basis of a few key indicator crops. The data were insufficient for an exhaustive approach. In the present essay, we will attempt to provide as complete a list as possible to include the crops typical for individual foci. Significant corrections and additions have had to be made to our previous concepts, for the first time formulated in 1926 in the book Studies on the Origin of Cultivated Plants. Most of the expeditions and the greatest amount of work on the study of the world’s varietal resources date back to the period of 1923–1933” [17, p. 29].

For each of the centers or foci of origin, Vavilov specified a basic list of cultivated plant species characteristic of the given geographic area. The list included: cereal grasses and other cereal crops; grain legumes; bamboos, root crops, tuber crops, bulbous and aquatic food plants; vegetables and cucurbits; fruit plants; fodder crops; sugar-producing plants; oilseeds and essential oil plants, resin-yielding and tannin-producing plants; spices, industrial and medicinal plants; fiber plants; dye plants, and multipurpose plants, including endemic species.

This publication was updated and supplemented with new facts, and in 1936, Les bases botaniques et géographiques de la sélection [19] was published in French in three issues of a French journal, followed by Vavilov’s articles in English and Spanish in 1937 [20, 21].

In 1940, in his work The Doctrine of the Origin of Cultivated Plants after Darwin, Vavilov wrote: “The continent which provided the greatest number of cultivated plants is Asia, accounting for about 700 out of the 1,000 species considered, i.e., about 70% of the entire cultivated flora. The New World accounts for approximately 17%. Before the arrival of Europeans, Australia knew no cultivated plants, and it was only in the last century that its eucalypti and acacias started to be widely utilized in the cultivation of tropical and subtropical regions of the world.

“The following seven major geographical centres of the origin of cultivated plants stand out within the continents” [8, p. 164].

“1. The Southern Asiatic tropical centre, including the territories of tropical India, Indo-China, Southern tropical China, and the islands of South-Eastern Asia... Within this large geographical centre, or region, three foci can be marked out, differing significantly in the composition of their inherent cultivated plants:

1. a) The Indian (with the richest cultivated flora).
2. b) The Indo-Chinese, including Southern China.
3. c) The Insular, including the Sunda Islands, Java, Sumatra, Borneo, the Philippines, etc.

“2. The Eastern Asiatic centre includes the temperate and subtropical parts of Central and Eastern China, the greater part of Taiwan, Korea, and Japan... Within this centre, one can distinguish the primary Chinese and the secondary, predominantly Japanese, foci.

“3. The South-Western Asiatic centre. It incorporates the territory of the inner highland Asia Minor (Anatolia), Iran, Afghanistan, Middle Asia, and North-Western India. The latter, floristically, as regards its cultivated plants, has appeared to be definitely linked with Iran. The Caucasus, the cultivated flora of which is genetically linked to Western Asia, is also part of this.

“This centre can be subdivided into the following foci:

1. a) The Caucasian, with numerous endemic species of wheat, rye, and fruit plants. For wheat and rye, as evidenced by comparative and in-depth cytogenetical and immunological studies, it is the most important global focus of the origin of species. The same applies to a number of European fruit crops.
2. b) The Near Eastern, including the inner Asia Minor (Anatolia), inner Syria and Palestine, Transjordan, Iran, northern Afghanistan, and Middle Asia (together with Chinese Turkestan).

Fig. 2. Cover of the N.I. Vavilov’s collection of articles “The Origin, Variation, Immunity and Breeding of Cultivated Plants” (transl. by K.S. Chester). Ronald Press, New York, 1951.

Fig. 3. Cover of the N.I. Vavilov’s collection of articles “Origin and Geography of Cultivated Plants” (transl. by Doris Löve). Cambridge University Press, 1992.

1. c) The North-Western Indian, including, in addition to the Punjab and adjacent provinces of Northern India, Baluchistan, southern Afghanistan, and Kashmir...” [8, p. 166]

“Wild relatives of wheat, rye, and various fruit crops are concentrated here in exceptional specific diversity. For many of the most important cultivated plants, a continuous series from cultivated to wild forms can still be traced here, and the links between wild and cultivated forms can still be established. 

“4. The Mediterranean centre includes the countries located along the shores of the Mediterranean Sea... Meanwhile, close connections can still be traced here between the origin of individual crops and definite territories—the Iberian Peninsula, the Apennines, the Balkans, Syria, and Egypt. Each of these foci is characterized by its own unique species of forage plants, such as sulla (Hedysarum coronarium), Alexandrian clover, giant creeping clover, gorse Ulex europaeus, single-flowered lentil, French lentil, Gorgonia vetch…

“Within the African continent, small Abyssinia stands out as an independent geographical centre, with a number of endemic species and even genera… Adjacent to it is the somewhat peculiar Mountain-Arabian (Yemeni) focus, mirroring on itself the influence of both Abyssinia and South-Western Asia; it is characterized by extremely early-maturing forms of cereals, grain legumes, and alfalfa. Characteristic of Abyssinia, along with generic endemics, is the presence of original cultivated endemic species and subspecies of wheat and barley… It is known to botanists that the flora on the entire mountain range running through East Africa from Cape Land to the Himalayas is notable for a certain unity of genera and even species…

“Within the New World, a striking localization of the speciation among major cultivated plants has been established.

“6. Within the vast territory of North America, the Central American geographical centre stands out first and foremost, including southern Mexico, which can be subdivided into three foci:

1. a) the mountainous Southern Mexican;
2. b) the Central American;
3. c) the West Indian insular.

“7. The Andean centre within South America, confined to a part of the Andean mountain range. We identify three foci inside it:

1. a) The Andean focus proper, encompassing the mountainous regions of Peru, Bolivia, and Ecuador. This original focus was the homeland for many tuber-bearing plants, in the first place for a large number of cultivated potato species…
2. b) The Chiloan (Araucanian) focus, located in southern Chile and on the adjacent Chiloé Island, which gave rise to the species of the common potato Solanum tuberosum  In contrast to Peruvian, Bolivian, and Ecuadorian potato species, which normally produce tubers under the shortened equatorial day, the common potato emerged in southern Chile, under conditions of a longer day, at 38–40° S. Lat...
3. c) The Bogotan focus in eastern Colombia, established by the Soviet researchers Dr S. M. Bukasov and Dr S. V. Juzepczuk. Cultivation here reaches great altitudes (up to 2,800 m above sea level)” [8, pp. 167–168].

“In fact, the seven major centres identified correspond to the localization of the most ancient agricultural civilizations. The Southern Asiatic tropical centre is linked to the great ancient Indian and Indo-Chinese culture. Recent excavations have demonstrated the profound antiquity of this culture, synchronous with that of South-Western Asia. The Eastern Asiatic centre is associated with the ancient Chinese culture; the South-Western Asiatic one, with the ancient cultures of Iran, Asia Minor, Syria, and Palestine. The Mediterranean, as early as several millennia BC, agglomerated the Etruscan, Hellenic, and Egyptian cultures that existed for about six thousand years. The relatively primitive Abyssinian culture has deep roots, possibly synchronous with, or perhaps even preceding, the ancient Egyptian culture. Within the New World, the Central American centre is linked to the great Maya culture, which achieved enormous progress in science and art prior to Columbus. The Andean centre is associated with the remarkable pre-Inca and Inca civilizations” [8, p. 169].

This article, where the work of a large group of researchers under Vavilov’s leadership was summarized, appeared the last in a series of publications dedicated to the origin of cultivated plants. Using vast volumes of factual material, Vavilov detailed in it all of his postulations concerning the centers, or foci, of crop origin and morphogenesis. The study of patterns in the geographic distribution of the Earth’s plant resources and the disclosure of immense intraspecific diversity for most crops made it possible not only to localize the origin of the most important cultivated plants but also provided an opportunity to ascertain its timing.

“The history of the origin of human civilizations and agriculture is, of course, much older than the documentation in the form of pyramids, inscriptions and bas-reliefs or tombs can tel1 us. А close acquaintance with cultivated plants and with the multitude of types and their differentiation into geographical groups as well as their frequently sharp physiological isolation from each other compel us to refer the very origin of cultivated plants to such remote epochs, where periods of 5–10000 years such as concern archeologists represent but а brief moment” [18, p. 129] (Fig. 3).

The method of differential systematics offered a possibility to trace the migration of several hundred crop species, identifying the stages of their evolution and introduction into agricultural practice. Studying the genesis of individual cultivated plants drove Vavilov toward the shaping of new notions: about the primary (more ancient) and secondary crops. It helped him to describe the centers of origin for agriculture and the routes of crop migration with more accuracy. In a relatively short time span, the number of the centers in Vavilov’s publications shifted from three in 1924 to five in 1926–1927, six in 1929–1930, seven in 1931, eight in 1934–1935, and back to seven in 1940. Each publication came as a result of his deep pondering over new facts. In his 1934–1935 articles, he proposed the division of the Southwest Asian center in two—into the Middle Asian and the Near Eastern centers. In 1937, he changed the name of the Middle Asian center to the Central Asian, classified as one of the five most important regions of origin for cultivated plants in Asia, encompassing Northwest India, Afghanistan, Uzbekistan, Tajikistan, and part of eastern Turkmenistan. Vavilov explained his subsequent decision to abandon the division of the Southwest Asian focus by the uniform species composition of the cultivated flora across this territory. The concepts of the Mediterranean and Abyssinian centers have not significantly changed since their identification in 1926. Botanical surveys of the countries over the American continent, conducted by N. I. Vavilov, S. M. Bukasov, S. V. Yuzepchuk, and other scientists of the Institute of Plant Industry (VIR), helped to establish autonomous centers of origin for local agricultural practice there, based on the crop species and even genera alien to the Old World. The relative youth of the New World’s civilizations facilitate observation of the processes that set cultivated plant species apart from their wild relatives. Within the New World, Vavilov identified two geographic centers, the Central American and the South American (the latter renamed the Andean center in 1940), where a striking localization of crop plant speciation was established [22].

In all his publications, starting with Studies on the Origin of Cultivated Plants [10], Nikolai Vavilov used the terms “center” and “focus”, as well as “region”, of crop origin as equivalent designations to localize geographically ancient agricultural civilizations. These definitions are of significant importance: a geographic center is the main independent focus where an agricultural crop has emerged, while a geographic region is the area where a group of crop species has concentrated. Moving from one of Vavilov’s publications on the origin of cultivated plants to another, it is possible to track down that the terms “center” and “focus” become increasingly associated with vast territories. Meanwhile, he already writes about the conditional nature of the concept of the “center” of origin—a term originally proposed by Charles Darwin. In his final work on this topic, The Doctrine of the Origin of Cultivated Plants After Darwin, published in 1940, he clearly differentiated between the notions of “center” and “focus”" [5]. According to Vavilov, “centers” referred to the seven centers of crop origin, further divided, in their turn, into several foci of morphogenesis.

It should be emphasized that Vavilov’s ideas were groundbreaking for his time. According to his theory of the centers of crop origin, cultivated vegetation emerged and took shape in a few specific areas—usually situated in mountainous regions—which served as primary centers of polymorphism. The river valleys where the great civilizations of antiquity flourished were merely homelands for the development of agriculture and served as secondary centers of diversity for cultivated plants. However, Vavilov believed that one the same region could be the center of origin for some crops and the secondary center of polymorphism for others. Furthermore, he also distinguished independent secondary centers of crop morphogenesis [4].

After his travels through the Mediterranean countries and Ethiopia, Vavilov discovered regularities in the geographic distribution of genes—the patterns that were subsequently confirmed. In his report at the V International Congress of Genetics in Berlin in September 1927, and later in his publication Geographical Regularities in the Distribution of the Genes of Cultivated Plants [11, 23, 24], he summarized the results of his analysis of the collected material. In these work, he examined the geography of plant gene distribution and formulated the core notion that the dominant genes of any cultivated plant species were concentrated within its center of origin, whereas recessive genes manifested themselves on its periphery. At the same time, populations with high concentrations of some recessive traits could be found in certain isolated areas even within the centers of origin. According to Vavilov’s observations, while dominant genes prevailed in plants within primary centers of their origin, many recessive traits emerged naturally in plants at the margins of a species’ area of distribution. Thus, secondary centers of morphogenesis were also able to supply crops with valuable recessive genes to plant breeders. The idea of the geographic distribution of gene alleles, which was considered as significant as the law of homologous series in variation, held great practical importance for plant breeding [22].

In his publication (Fig. 4) on the law of homologous series in hereditary variation, Nikolai Vavilov explained: “1. Species and genera that are genetically closely related are characterized by similar series of heritable variations with such regularity that knowing the series of forms within the limits of one species, we can predict the occurrence of parallel forms in other species and genera. The more closely related the species and Linneons in the general system, the more resemblance will there be in the series of variations.

2. Whole families of plants in general are characterized by definite cycles of variability occurring through all genera and species making up the family” [25, p. 75] (Fig. 2).

Fig. 4. Cover of the first edition of The Law of Homologous Series, 1920.

Development of an Integrated Concept Based on N. I. Vavilov’s Key Theoretical Principles

 “The multitudinous chaos of innumerable forms obliges investigators to look for some way of simplification. The process of differentiation will go on inevitably, adding to the records of existing forms, and giving a true conception of Linneons. But parallel to differentiation it is natural to search for ways of integration of our knowledge of Jordanons and Linneons themselves” [26, p. 52] (Fig. 5).

The conclusion from all of the above was as follows: “Thus the Linnean species, in our conception, appears to be a distinct, complex, mobile, morpho-physiological system related in its origin to a definite environment and area, and in its intraspecific hereditary variability, subject to the Law of Homologous Series...

“The Law of Homologous Series shows investigators and breeders the directions of their search. It aids in the discovery of systematic links and extends the horizon of the worker, disclosing the great amplitude of species variations” [25, p. 91].

“The problem of the origin of species cannot be separated from the problem of variation. A great many forms are undoubtedly only different combinations of the same genes, some primary types. The study of variation will give us the possibility of establishing these primary types, the fundamental series of variation of organisms” [25, p. 89].

 “We have laid the law of homologous series at the basis of the differential systematics of cultivated plants. Here it has made it possible to encompass into rigorous systems the exceptional diversity of forms that represent individual species, dissected by practice into a multitude of varieties” [8, pp. 219–220].

Summarizing the work of the entire Institute and the initial results of the agroecological classification of cultivated plants, Vavilov wrote in his work The New Systematics of Cultivated Plants [27], published in English in 1940 and translated into Russian [28], “In our study of cultivated plants we have advanced step by step. Some twenty years ago, coming to the study of cereals, we soon found that the previously existing classification into botanical varieties based on a few easily determined spike and kernel characters—which was elaborated by the German taxonomist, Friedrich Körnicke, and has been accepted by most investigators, including Professor John Percival in his monograph on the wheat plant (1921)—was not adequate. We found it necessary to elaborate a new, more detailed morphological and physiological system based on a study of the evolution of plants from their primary regions, which are usually characterized by the presence of a great diversity of botanical varieties. As a result of the establishment of the law of homologous series in variation [26], according to which closely allied species and genera to a great extent repeat one another in their differentiation, we came to the discovery of a huge number of varieties unknown before. Many expeditions to the various primary regions of the origin of cultivated plants and thorough and many-sided studies (in sowings) of the collected material were conducted by the Institute of Plant Industry of the U.S.S.R. The evolutionary and geographical principle was taken as the chief basis of our studies of the species systems. We have tried as far as possible to follow in detail the steps of evolution from the primary regions where the differentiation into Linnean species took place. Fortunately, the location of these regions may be established on the basis of historical, archaeological, and, particularly, botanical data” [27, p. 550].

Vavilov’s report The Problem of the Origin of the World’s Agriculture in the Light of Recent Investigations [16] was presented before the Second International Congress of the History of Science and Technology, held in 1931 in London. In this report, Vavilov summarized some of the results from the crop collecting expeditions and studies conducted by the Institute of Plant Industry over the previous decade. He stated in it, “In the course of our work on the practical questions connected with plant-breeding, we have approached some of the problems of the world history of agriculture…

“…It became evident that so far neither the botanist, nor the agronomist, nor the breeder has yet, with any degree of completeness, approached the study of the world's resources even of the most important cultivated plants, whose centres of evolution, as investigations have shown, are located chiefly in ancient agricultural countries. Contemporary European and American horticulture and agriculture know only fragmentary details, derived from ancient centres of diversity of cultivated plants” [16, p. 2].

“As a result of the investigation of several hundreds of cultivated plants we succeeded in establishing the fundamental world centres of the chief cultivated plants. Some of these results are probably of general interest.

“In general our investigations have led to the establishment on the earth of seven fundamental, independent centres of origin of cultivated plants, which at the same time were the probable foci of the independent development of world agriculture…” [16, p. 3].

“The geographical location of the primary agricultural centres is rather peculiar. All seven centres are chiefly confined to the tropical and sub-tropical mountain regions. The centres of the New World are confined to the tropical Andes, those of the Old World to the Himalaya, the Hindu-Kush, mountainous Africa, the mountain regions of the Mediterranean countries, and to mountainous China.

“After all, only a narrow strip of the dry land of the earth has played an important role in the history of world agriculture.

“From the point of view of dialectics, considered in the light of the latest investigations, the geographical concentration of the great primeval agricultures in this limited zone, becomes comprehensible. The tropics and the sub-tropics provide optimum conditions for the unfolding of the process of species origination. The maximum species diversity shown by the wild vegetation obviously gravitates towards the tropics…” [16, p. 7].

“The processes of mountain formation indubitably have played an important role in the differentiation of the vegetation into species, promoting the process of the divarigation of species. Isolators, mountain barriers checking the spread of species and genera have been of great importance to the differentiation of separate forms and whole species. The various climates and soils found in the mountain zones to which gravitate the principal centres of the origin of cultivated plants, promote the development of diversity among the species, as well as within the species composition of these plants….

Fig. 5. Cover of the first English edition of The Law of Homologous Series in Variation, 1922.

 “If the moist subtropics chiefly favour the development of trees, the tropical and subtropical mountain regions, where the primeval agricultures settled, are characterized by the development of herbaceous species, to which belongs the majority of the most important plants of the earth.

“The tropical and subtropical mountain regions afford optimum conditions for human settlement. Primeval man was afraid, and is still afraid, of the moist tropics, with their luxuriant vegetation and their tropical diseases, though the vast subtropics with their highly fertile soils occupy one-third of the dry land on the whole earth (Sapper). For his domicile man turned, and continues to turn, to the borders of the tropical forests. The tropical and subtropical mountain regions offered the most favourable conditions of warmth and abundance of food to the first settlers. In Central America and Mexico man still utilizes a multitude of wild plants. It is not always easy to distinguish the cultivated from their corresponding wild plants…

“This knowledge of the initial centres of agriculture, throws light on the whole history of mankind, and the history of general culture” [16, p. 8-9].

The data on crops and animals, as well as on the customs and living conditions of various peoples, collected by Vavilov during his expeditions—and corroborated by information from history, geography, archaeology, and other fundamental sciences—helped to build up a coherent system of views on the origin of agriculture and, ultimately, civilization on the Earth.

Continuation of Research by Associates and Followers

Theoretical and methodological studies conducted by Nikolai Vavilov and his associates stimulated subsequent large-scale and detailed research into the world’s plant diversity, contributing to the disclosure of utterly new facts that changed the understanding of the origin of the most important crops. New information and materials obtained during collecting missions underpinned the further development of the doctrine of the centers of crop origin.

A botanical study of extensive plant material and the quantitative distribution of species from their centers of origin motivated Dr. E. V. Wulff to identify 16 floristic regions on the globe [29]. This system became the basis for studying and systematizing the vast herbarium material accumulated by the Institute of Plant Industry. Furthermore, this research played an important role in understanding the problem of the interdependence of countries on plant genetic diversity. It was established that the areas with the greatest concentrated diversity of plant species, especially the tropical floras, still hid inexhaustible riches of valuable plant products [30].

The Institute’s collecting activities and the studies of the collected material helped VIR’s scientists to supplement and develop Vavilov’s ideas concerning the centers of crop origin. For example, Dr. N. A. Bazilevskaya (Vavilov’s student) identified five additional centers of origin for ornamental plants: North American, South African, Australian, the temperate zone of Europe, and the Canary Islands [31].

Professor P. M. Zhukovsky, Vavilov’s associate and follower, offered to add four more centers to the seven primary ones ascertained by Vavilov: the Euro-Siberian, African, Australian, and North American centers [32, 33]. Vavilov regarded the abovementioned as secondary gene centers, or areas of adoption, since the prevailing part of crop plants within them—with the exception of a relatively small number—was adopted from the seven primary centers established by him. Prof. Zhukovsky introduced new terms: “megacenters”, which corresponded to Vavilov’s centers of origin, and “microcenters”, the foci where narrowly endemic species (forms) had entered into cultivation, identifying more than 100 such microcenters.

According to P. M. Zhukovsky, the Euro-Siberian center primarily includes the central and northern territories of Europe and Asia, where many species of fruit crops (apple, pear, sweet cherry, and sour cherry), nut crops (Russian almond, almond, and Persian walnut), and berry crops (grapevine, rowan, raspberry, currant, gooseberry, strawberry, wild strawberry, sea buckthorn, etc.) originated. It is also the region of origin for fodder crops (red and alsike clovers, and alfalfa), vegetables (cabbage, purslane, onion, and horseradish), industrial crops (fiber flax, dogbane, hemp, and hops), as well as peas, buckwheat, and others.

The African center covers the entire territory of Africa, including the Abyssinian focus identified by N. I. Vavilov and the South African (Cape) region identified by E. V. Wulff. Originating from the African center are several endemic species and varieties of wheat (Triticum durum Desf., T. turgidum L., T. dicoccum Schrank., and T. aethiopicum Jakubz.), rye (Secale africanum Stapf.), oat (Avena abyssinica Hochst., and A. vaviloviana Mord.), and barley (Hordeum aethiopicum Vavilov et Bacht.). It is also the center of origin for sorghum, pearl millet, rice, coffee, rattlepods, Bambara groundnut, cowpea, castor bean, watermelon, cotton, cultivated flax, Abyssinian cabbage (Ethiopian mustard), date palm, banana, and others [34].

The Australian center is an important source of new gene pools for breeding valuable cultivated plants: cotton (about 10 species), tobacco (more than 20 species), five endemic Microcitrus Swing species, easily crossable with Citrus L. spp., three species of rice, about 400 species of acacias, many of which have excellent wood. It is the homeland of the valuable nut-bearing plant macadamia (Macadamia ternifolia F. v. Muell.), and the secondary center for subterranean clover, representatives of the genera Panicum, Brachiaria, Eragrostis, Danthoma, Atriplex, Dioscorea, Rubus, and others.

The North American center covers the territories of the United States and Canada. Its flora is the birthplace of numerous endemic species representing such genera as Vitis, Helianthus, Prunus, Ribes, Rubus, Fragaria, Grossularia, Lupinus, Hordeum, Zizania, Gossypium, Nicotiana, Solanum, etc. [34].

1. N. Sinskaya, pursuing Vavilov’s initiative in studying crop geography and using her previous ideas [35], further developed the concept of the centers of origin of cultivated plants [36]. Summarizing new monographic works by plant breeders, archaeological data, and collecting mission results, she shed new light on the course of development of cultivated plants and the ways of their dissemination from the centers of origin. She introduced a new notion into historical phytogeography − “zones of influence”, and suggested that five principle areas of the development of cultivated flora should be distinguished, for the first time highlighting the African, as well as the Ancient Mediterranean, East Asian, South Asian, and New World areas. It served as the basis for the conclusion that agriculture in North America had evolved from the crops grown in Mexico and Central America, and later from those of the Old World. Agricultural practice in Central and Northern Europe, the Russian steppes, and Siberia was based, first of all, on varietal assortment from Asia Minor and the Mediterranean countries. The history of agriculture in the “zones of influence” was not ancient, although the period of its development was not too brief: sometimes this can be witnessed by the large number of plants introduced into cultivation from the poor wild flora of these territories [36].

Analyzing Nikolai Vavilov’s heritage and employing anthropological and ethnographic data, A. I. Kuptsov [37] identified 10 foci of the world’s agriculture as centers of origin for cultivated plants: Western Asian, Mediterranean, Middle Asian, Ethiopian, Indian, Indonesian, Mexican, Peruvian, North Chinese, and West Sudanese [38].

The research conducted by Acad. V. F. Dorofeev established the Transcaucasia as the epicenter of speciation and evolution for the genus Triticum L. [39], providing extensive new data for the development of Vavilov’s theory of the centers of crop origin [40]. Prof. R. A. Udachin confirmed Vavilov’s viewpoint that the centers of origin, morphogenesis, and species diversity for Triticum aestivum L. and T. compactum Host. coincided within the territory of Middle Asia [41].

Later, as a result of intensive plant explorations in the western, central and southern parts of the African continent, two independent gene centers of the origin and diversity of cultivated plants and their wild relatives were identified—West African and Central African [42].

1. I. Vavilov’s phytogeographic method and the collecting missions to South America made it possible to demarcate more accurately the centers of origin and genetic diversity of potato forms—the areas where species with traits valuable for breeding were concentrated—and outline a strategy for introducing cultivated and wild species [43].

Phenotyping Plant Genetic Resources Collections: Vavilov’s Geographic Principle and Modern Approaches

The fall of 1923 was the starting point for the Institute’s geographic plantings devised to study the ecogeographic patterns of ontogenesis in major cultivated plants, utilizing permanent and temporary experimental stations and trial plots in contrasting environments. The foundation for such research had been laid by the Bureau of Applied Botany during R. E. Regel’s tenure—at the Bureau’s three stations with contrasting climate conditions. In his 1923 letter to G. S. Zaitsev, Nikolai Vavilov wrote, “The Department of Applied Botany and Plant Breeding of the Agricultural Scientific Committee is organizing, as of this year, systematic research into the variability of cultivated plant varieties across various regions of European and Asiatic Russia. For the vast territory of European and Asiatic Russia, the question of morphological and physiological changes undergone by the same varieties in different regions and conditions is of immense significance” [44].

Initially, these plantings were limited to only 25 sites, but by 1927, their number had grown to 115. The northernmost site was located near Murmansk, the southernmost ones were in Turkmenistan, the westernmost in Kaunas, and the easternmost in Vladivostok. Over 40 crop species were covered by these geographic plantings. Plant monitoring and maintenance were carried out strictly according to instructions [23, 24, 45].

Vavilov organized these plantings, above all, to find out the potential geographic limits of crop distribution and to use the findings as a platform for practical measures to regulate crop cultivation across the country. The task was to identify the patterns by which individual plant variability depended on geographic factors. How morphological and physiological traits, as well as biochemical properties in plants, changed; which traits were conservative and thus suitable for taxonomic purposes; what the relationship was between environment and heredity—these were the problems that the geographic experiments were expected to resolve [44].

VIR currently conducts annual studies of about 20,000 accessions across 15 experiment stations (see Table) with environmental conditions contrasting in temperature and total precipitation. Furthermore, over 100 scientific institutions—recipients of plant germplasm from VIR (ca. 5,000 accessions per year)—also perform phenotyping in various regions of the Russian Federation. Phenotyping is carried out according to VIR’s guidelines. The results are published in the VIR catalogues; by now, 945 issues of them have been released. Phenotyping data obtained by comparative analyses are published in peer-reviewed journals. Both the raw data published in catalogues and the comparative analytical data serve as a valuable tool for domestic breeders. At the same time, they are a source of vital fundamental knowledge.

A comprehensive study of oat species and a geographic analysis of the areas of distribution for populations and species have clarified that the primary center of origin for the genus Avena L. is situated in the western Mediterranean, while the secondary center of the species’ morphogenesis is located within the Southwest Asian center of crop origin. The centers of morphogenesis for cultivated oat species are: for the diploid A. strigosa Schreb., the territory of Spain and Portugal; for diploid naked forms, Great Britain (Scotland); for the tetraploid species A. abyssinica Hochst., the territory of Ethiopia; for the hexaploid species A. byzantina K. Koch, Algeria and Morocco, with a secondary center in Turkey; for hulled forms of A. sativa L., the territory of Iran, Georgia, and Russia (Tatarstan), and for its naked forms, Mongolia and China [46].

Table. Experimental stations of VIR and their geographic and climatic characteristics

The phylogenetic analysis of the representative intraspecific diversity of cultivated and wild Avena spp. performed through next-generation sequencing (NGS) showed that diploid species with A-genome variants were not in fact primary diploids, but a peculiar Mediterranean introgressive hybridization complex of species that sporadically entered into interspecific hybridization. It was established that the tetraploid cultivated species A. abyssinica most likely originated from the wild A. vaviloviana. An analysis of the ways of A. sativa and A. byzantina domestication showed that the most widespread ribotype of the A. sativa hexaploid was inherited from A. ludoviciana, and the second most widespread one, from A. magna, while A. byzantina had two unique ribotype families, most likely inherited from an extinct oat species or a still undiscovered cryptospecies [47].

Thus, long-term studies conducted at the same geographic site make it possible to identify the effects of climate change on the adaptability of crop accessions [48]. Analyzing the same sets of accessions across diverse geographic planting sites helps to select excellent models for comparative omics research. For example, a comparison of Vigna unguiculata (L.) Walp. landraces from the crop’s secondary center of origin with cowpea accessions introduced to other countries led to the selection of models for establishing the mechanism, through further comparative transcriptomic analysis, that contributes to the maintenance of compact V. unguiculata plant architecture under humid conditions [49, 50].

In recent years, phenotyping chickpea, soybean and barley from the VIR collection has been combined with genotyping, thus providing data for GWAS [51-54]. Such studies enable researchers not only to find genomic markers for further breeding purposes, but also to trace events that occurred after their domestication. A panel of 147 Cicer arietinum L. accessions, representing the genetic diversity of the center of origin for chickpea and a major location of its secondary diversity, was characterized. As a result, over 70% of polymorphisms affecting plant characters appeared to be associated with multiple characteristics localized on the same chromosome. The authors hypothesized that selection and introgression via inadvertent hybridization between more and less advanced morphotypes might have resulted in agricultural improvement genes aggregated to the so-called genomic ‘agro-islands’ [51].

GWAS was performed using data from a three-year field study conducted at the Adler experimental station (branch of VIR), and GBS data for a set of 169 soybean accessions from the VIR collection. Forty-eight significant SNPs distributed across 14 chromosomes were identified. These include known QTLs involved in flowering regulation and potentially novel chromosome loci Gm1 (115 cM), Gm6 (117 cM), Gm7 (34-36 cM), and Gm8 (116 cM), which have not previously been described in the literature. So, a study of soybean varieties with different geographical origins in the coastal zone of the Black Sea allowed us to identify new markers associated with flowering time [54].

Recently, genetic resources of soybean and potato from the VIR collection have been increasingly utilized to develop high-throughput phenotyping methods [55-57]. These methods are applied, inter alia, to wild relatives of cultivated plants, revealing the hereditary diversity of plant materials mobilized from the centers of crop origin. Key morphological parameters for epidermal morphology in wild potato leaves were quantified using computer analysis for image recognition. Principal component analysis of the data obtained highlighted two main axes that aligned well with the historical and geographical origins of the species, separating South American species from Central American ones [57].

Importance of Vavilov’s Fundamental Concepts for the Systematic Collecting of Plant Genetic Resources and Worldwide Conservation of Agricultural Biodiversity

The theory of the centers of origin of cultivated plants and the law of homologous series in hereditary variation, formulated by Nikolai Vavilov, remain the theoretical foundation for all collecting missions launched by VIR. These theoretical developments provide for the prediction of plant genetic resources collecting sites where valuable traits and properties are concentrated. The following criteria are applied to enrich the existing collections: collecting botanical forms and cultivars absent in them, prioritizing sources of valuable breeding traits and donors of key genes, and accumulating the maximum possible phenotypic and genotypic diversity of all collected cultivated species and their wild relatives [44].

To date, the VIR global genetic resources collection of cultivated plants and their wild relatives is one of the largest in the world in terms of the botanical, genetic, geographic, and environmental diversity of its accessions. The collection represents 64 botanical families, 376 genera, and 2,169 species, totaling more than 320,000 unique accessions. The breakdown includes cereal crops (137,500 accessions); grain legumes (46,500); forage crops (32,000); vegetables and cucurbits (52,000); industrial crops (28,500); root and tuber crops (8,300); fruit crops (23,000) [58].

It is worth mentioning that VIR’s collecting missions were not the only ones to spend decades visiting the centers of origin of cultivated plants discovered by Vavilov. While some foreign experts do not share Vavilov’s views on the “centers of origin” − preferring to call them “centers of diversity” [59] − countries such as the United States, Australia, India, Italy, Japan, Germany, and many others have organized collecting missions to these centers. In the period following the publication of Vavilov’s theory of the centers of crop origin until 1970, U.S. scientists launched over 100 plant explorations, about 75 of which were to the regions identified by Vavilov. Between 1967 and 1976, Australian scientists organized more than 50 expeditions, with roughly 45 of them targeting Vavilov’s centers of crop origin. Meanwhile, the international community utilized Vavilov’s classification of the world’s centers of origin to select locations for the international plant genetic resources organizations (Future Harvest Centers) of the Consultative Group on International Agricultural Research (CGIAR) and to implement their programs for the most important food crops [22, 60].

By the end of 2020, the international CGIAR genebanks held a total of 736,210 accessions of mandate crops, including 26,224 accessions maintained in vitro and 32,930 accessions kept in field genebanks. Approximately 78% of this stock is safely duplicated and conserved in the Svalbard Global Seed Vault (SGSV), which opened in 2008 [61].

There are now some 1,750 gene banks in the world, with about 130 of them each holding more than 10,000 accessions. Of the total 7.4 million accessions conserved worldwide, national genebanks hold about 6.6 million. Seven of the world’s largest genebanks—including the N.I. Vavilov Institute of Plant Genetic Resources (VIR)—preserve 45% of the global plant genetic diversity. The access to these resources is regulated by Standard Material Transfer Agreements adopted under the International Treaty on Plant Genetic Resources for Food and Agriculture supported by the Food and Agriculture Organization of the United Nations [62]. The genebank in Russia has been recently transformed to the National Center for Plant Genetic Resources, and the access to the genetic resources collections in Russia is regulated now by the Federal Law No. 428-FZ (November 30, 2024) “On Bioresource Centers and Biological (Bioresource) Collections and on Amendments to Article 29 of the Federal Law On the Animal World…” [44].

Conclusion

1. I. Vavilov’s theory of the centers of origin of cultivated plants, along with his other major fundamental concept—the law of homologous series in variation—is not only fundamentally important for the development of contemporary biological research into the mechanisms of crop evolution and adaptation, but also practically significant for the conservation, systematic study, and sustainable utilization of global agricultural biodiversity.

About the authors

IGOR G. Loskutov

Email: i.loskutov@vir.nw.ru
ORCID iD: 0000-0002-9250-7225
SPIN-code: 2715-2082
Russian Federation, N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR), St. Petersburg, Russia

Elena K. Khlestkina

Author for correspondence.
Email: director@vir.nw.ru
ORCID iD: 0000-0002-8470-8254
SPIN-code: 3061-1429
Russian Federation, N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR), St. Petersburg, Russia

References

Supplementary files