The Outcrossing-Selfing Transition and Asian Rice Domestication

The protracted domestication of Asian rice is usually defined by the fixation of the domestication syndrome consisting of six main phenotypic traits having all an agronomic importance: seed retention, reduction of seed dispersal aids, increase in seed size, reduction of seed dormancy, a more compact growth habit, and synchronised flowering and ripening. However, during this initial domestication process the mating system of rice has also changed. Indeed, domesticated rice, Oryza sativa L., is predominantly a self-fertilising species while its wild progenitor – Oryza rufipogon Griff. – is partly an outcrossing species. We explain this cross- to self-fertilisation transition and study its consequences. We demonstrate that this transition cannot be explained by mate rarity, and recall it could be a side effect of conscious selection for closed panicles. Then we suggest an alternative explanation based on the ‘time limitation hypothesis’. For this purpose, we hypothesise agronomic practices of early cultivators – fostering the rapid maturation of plants and their reproductive isolation – that are likely the selective pressures having led to this transition. Given its consequences and its origins, we contend that the outcrossing-selfing transition should be systematically included in the Asian rice domestication syndrome and even considered as its main trait.