N.I. Vavilov's theory of centres of diversity in the light of current understanding of wheat diversity, domestication and evolution

N.I. Vavilov hypothesized that the geographical centres of diversity of crops indicate their geographical centres of origin. Vavilov's conclusions about the geographical origins of einkorn, durum and common wheat agree well with current population and molecular genetic studies when macro-geography is used but agree poorly when they are examined at higher resolution. We examined the causes of such disagreements for tetraploid emmer wheat and hexaploid common and club wheat. Molecular studies suggest that emmer was domesticated in the Diyarbakir region in south-eastern Turkey. Nucleotide diversity of wild emmer in the Diyarbakir region estimated earlier was compared with nucleotide diversity of wild and domesticated emmer across their distribution estimated here. Although domesticated emmer is only half as diverse as wild emmer, it is more diverse than the ancestral wild emmer population in the Diyarbakir region. Its centre of diversity is in the Mediterranean and does not coincide with the geographical centre of emmer origin. A similar disagreement exists in hexaploid wheat. Its centre of molecular diversity is in Turkey, which is west of the putative site of its origin in Transcaucasia and north-western Iran. It is shown that the primary cause of the disagreements between geographical centres of crop diversity and geographical centres of crop origin is gene flow from an ancestor subsequently to crop origin, which modifies the geographical pattern of crop diversity. When such gene flow takes place and when crop is domesticated in a peripheral population of the ancestor, the centre of crop diversity and the centre of crop origin are unlikely to coincide.