On the domestication genetics of self-fertilizing plants

Abstract

Recently, Allaby et al. (2008) suggested genetic expectations of a protracted model for the origins of domesticated crops (3,000 years) and also argued that DNA results indicating a monophyletic origin of, e.g., Einkorn wheat (Heun et al. 1997) are artifacts. In their Supporting Information, Allaby et al. (2008) stated that they simulated panmixis, i.e. out-crossing scenarios. In fact they actually simulated random-pair-mating, which excludes all selffertilization. However, Allaby et al. (2008) focused their critique on barley, Einkorn, and Emmer wheat and simply ignored the fact that these crops are self-fertilizing plants. Self-fertilizing plants have a fundamentally different population structure compared to out-crossing species (e.g., Allard 1960), and therefore, it is elementary wrong to not realize this distinction. Also, modeling a shift from the wild to the domesticated state by random genetic drift only, without considering genetic hitchhiking or natural/unconscious (Heiser 1988) selection and possibly also intentional selection, is a very unlikely scenario. Further, the occurrence and accumulation of mutations over a long time should be considered. So, what we really need is a proper genetic explanation for the domestication of self-fertilizing plants. We outline some elements of this below and briefly show the fundamental difference between out-crossing and self-fertilizing species and summarize the classical experiments of Allard and his co-workers, since both issues remained unaddressed by Olsen and Gross (2008) when discussing whether a protracted model is realistic for crop evolution. We briefly mention experimental domestication of the above-mentioned three crops and support the conclusion that the domestication of a plant like Einkorn wheat can be as short as 20–30 years. The fact that the archaeobotanical record shows that remains of wild and domesticated forms of the same plant overlap for a long time (up to 3,000 years) appears not to be consistent with rapid domestication. However, this question cannot be answered without a proper genetic model for self-fertilizing species like barley, Einkorn, and Emmer wheat and other self-fertilizing plants.