1.1  N.I. Vavilov — Background

• Nikolai Ivanovich Vavilov (1887–1943) was a Soviet botanist, plant breeder, and geneticist who conducted massive plant exploration expeditions across the world.
• 'N.I. Vavilov developed the concept of centres of origin and that of homologous series in variation from the study of a vast collection of plant types.' He built what was at the time the world's largest seed collection at the Plant Industry Institute, Leningrad (now the Vavilov Institute of Plant Industry, N.I. Vavilov Research Institute of Plant Industry or VIR, St. Petersburg).
• This collection had approximately 1,60,000 entries.
• Vavilov published his concept of centres of origin in 1926 in 'Studies on the Origin of Cultivated Plants'.
• He proposed that regions where a crop species shows the greatest genetic diversity (highest number of distinct forms, greatest range of variation) represent the area where the crop originated — because diversity accumulates at the centre of origin over the longest period of time.
• Vavilov's life ended tragically. He was arrested in 1940 on orders of Stalin and Trofim Lysenko (who rejected Mendelian genetics). He died in a Soviet prison camp in 1943 of starvation — a deeply ironic fate for a man who spent his life fighting hunger. 

2.2  The Concept of Centre of Origin

A centre of origin is defined as a region of maximum genetic diversity of a crop species, where the crop originated and was first domesticated. Vavilov recognised that:

• The area of origin of a cultivated plant is the area where the maximum number of forms and varieties of the plant are found.
• Wild relatives and progenitor species are found in or near the centre of origin.
• The centre of origin is the richest source of germplasm for crop improvement.
• Exploration teams should target centres of origin first when collecting germplasm for gene banks.

There is an important distinction between a centre of origin and a centre of diversity. The two often coincide but not always:

• Centre of origin: The geographic area where a species first originated and was domesticated.
• Centre of diversity (Vavilov's secondary centres): A region where high genetic diversity exists, which may be different from the actual place of origin. Crops can develop secondary centres of diversity as they spread and adapt — the Hindustan centre is considered both a primary and secondary centre for several crops.

2.3  Vavilov's 8 Centres of Origin 

Vavilov originally proposed 8 centres (with some having sub-centres). 

2.4  Harlan's Microcentre Concept

• Jack R. Harlan, an American botanist and plant explorer, modified Vavilov's concept by proposing that within the larger centres of origin, there exist smaller areas of exceptionally high diversity. He called these microcentres.
• Microcentre definition: A small geographic area within a larger centre of origin that contains exceptionally high genetic diversity — often a valley, an isolated mountain range, or a specific ecological zone that served as a refugium during climate change events.
• Example: Within the Hindustan centre, the northeastern India region (Assam, Meghalaya, Nagaland) is a microcentre for rice diversity — harbouring thousands of rice landraces, wild Oryza species, and primitive cultivars.
• Significance: Microcentres should be the primary targets for germplasm collection expeditions. They hold the greatest diversity per unit area. NBPGR's proposal to establish gene sanctuaries in Meghalaya for Citrus and in Northeast India for Musa, Citrus, Oryza, Saccharum, and Mangifera is directly motivated by the microcentre concept.

2.5  Vavilov's Law of Homologous Series in Variation (1922)

• This is a critically important and frequently tested concept. The Law of Homologous Series was proposed by Vavilov in 1922
• Law: Species and genera which are genetically closely related are characterized by similar series of heritable variations.
• Alternatively stated: 'Related species and genera show parallel series of heritable variation.
• What this means in practice: If a particular type of heritable variation (e.g., waxy endosperm, or rust resistance, or dwarfism) is found in one species, a similar variation can be predicted to exist in related species. The variation series run 'parallel' across related species — like homologous chromosomes are parallel in structure.

Examples

• Awned, awnless, and hooded grain forms are found in wheat, barley, and rye — all three members of the grass tribe Triticeae show the same range of awn variation.
• Glabrous (hairless), hairy, and glaucous (waxy-blue) leaf forms exist in all Triticum species.
• Red, yellow, white, and purple grain colour is found in wheat, rice, barley, and maize — all major cereal grains show the same colour variation series.
• Waxy endosperm (amylose-free starch) is found in maize (waxy corn), sorghum (kafir corn waxy type), rice (glutinous/sticky rice), barley, and millet — a parallel series across cereals.

Significance of the Law of Homologous Series:

• Predicts the existence of useful variation: If resistance to a rust race is found in wheat, a similar type of resistance may exist in barley or rye — this guides the breeder where to look.
• Guides germplasm exploration: If a character is found in one species, the breeder knows to look for it in related genera.
• Suggests new crosses: If a desirable trait variant exists in one species, the law suggests where to find a source for transfer through wide hybridization.
• Foundation for systematic taxonomy: The parallel series of variation is itself evidence of evolutionary relationship between taxa.
• Vavilov compared this law to Mendeleev's periodic table in chemistry — just as the periodic table predicted the existence of undiscovered elements, the law of homologous series predicted the existence of undiscovered useful variants in related plant species.

GENE POOL CONCEPT — HARLAN AND DE WET (1971)

• Jack R. Harlan and J.M.J. de Wet (1971) developed the gene pool concept to classify the genetic resources available to a crop breeder based on the ease or difficulty of crossing them with the cultivated species.
• This is one of the most practically useful frameworks in plant breeding.
• The gene pool concept divides all species that could potentially contribute genes to a given crop into three gene pools:

3.1  Primary Gene Pool (GP-1)

The primary gene pool consists of the biological species — the cultivated variety/ies plus wild and weedy races that cross easily with the cultivated species to produce fertile F1 hybrids. Gene transfer from GP-1 to the cultivated species is straightforward and can be accomplished using conventional hybridization methods without any special technique.

Characteristics:

• Crossing produces fertile F1 hybrids with little or no sterility
• Chromosomes pair normally at meiosis in the F1
• Genes segregate and recombine as expected by Mendel's laws in F2
• Gene transfer from GP-1 requires only conventional backcrossing

Examples:

• Wheat: Triticum aestivum (cultivated bread wheat) + T. spelta, T. macha, T. compactum (all hexaploid wheats, AABBDD genome). Crossing is easy; hybrids are fertile.
• Rice: Oryza sativa (cultivated) + O. rufipogon (wild progenitor). These are the same species complex; hybridization is straightforward.
• Maize: Zea mays + Z. mexicana (teosinte, now classified as Z. mays subsp. mexicana by some authorities). Teosinte is the progenitor of cultivated maize; the two cross readily.

3.2  Secondary Gene Pool (GP-2)

• The secondary gene pool consists of species that can cross with the cultivated species but the resulting F1 hybrids show partial sterility — chromosome pairing is incomplete or meiosis is abnormal.
• Gene transfer requires special techniques such as embryo rescue, chromosome doubling, or repeated backcrossing to overcome sterility.

Characteristics:

• Hybridization occurs but with difficulty (low crossability), or the F1 is partially fertile
• F1 hybrids show chromosome pairing failures or meiotic abnormalities
• Gene transfer is possible but requires embryo rescue or chromosome manipulation
• Useful genes can ultimately be introgressed but with more effort than GP-1

Examples:

• Wheat: Triticum monococcum (einkorn, AA genome), T. timopheevii (AAGG genome), and Aegilops species related to the wheat genomes. Crosses are possible but F1 is often partially sterile.
• Rice: Oryza glaberrima (African rice, same AA genome but different from O. sativa), O. nivara (source of grassy stunt resistance). Crossable but some sterility barriers; embryo rescue sometimes needed.
• Maize: More distant Zea species; also sometimes Tripsacum (a genus related to maize) for certain traits, though Tripsacum is arguably GP-3.

3.3  Tertiary Gene Pool (GP-3)

• The tertiary gene pool consists of species so distantly related to the cultivated species that direct hybridization is extremely difficult, and even when hybridization is achieved, the F1 hybrids are completely sterile or the embryo aborts.
• Gene transfer requires special techniques including embryo rescue, chromosome doubling to produce amphidiploids, somatic hybridization (protoplast fusion), or genetic transformation (transgenics).

Characteristics:

• Hybridization usually fails; when achieved, F1 is highly sterile or embryo aborts
• Gene transfer requires bridge crosses, embryo rescue, somatic hybridization, or transgenic methods
• The effort required is substantial but the genetic resources available here may include traits completely absent from GP-1 and GP-2

Examples:

• Wheat: Agropyron (wheatgrass — source of alien addition lines for Fusarium resistance), Hordeum (barley), rye (Secale) — though note that rye x wheat crosses produce Triticale, which is an important allopolyploid. The rye x wheat cross produces completely sterile F1 unless chromosome doubling is used.
• Rice: Porteresia coarctata (mangrove rice — a completely different species with extreme salt tolerance; source of salt tolerance genes for rice improvement). Hybridization is very difficult and embryo rescue is essential.
• Cotton: Distant Gossypium species in other sections of the genus — F1 may be lethal or completely sterile.

3.4  Practical Application of the Gene Pool Concept