Did greater burial depth increase the seed size of domesticated legumes?

The independent domestication of crop plants in several regions of the world formed the basis of human civilizations, and attracts considerable interest from archaeologists and biologists. Selection under cultivation led to a suite of domestication traits which distinguish crops from their wild progenitors, including larger seeds in most seed crops. This selection may be classified as 'conscious' or 'unconscious' selection according to whether humans were aware of the changes that they were driving. The hypothesis that human cultivation buried seeds deeper than natural dispersal, exerting unconscious selection favouring larger seeds with greater reserves, was tested. Using a comparative approach, accessions of eight grain legumes, originating from independent domestication centres across several continents, were sampled. Seeds were planted at different depths in a controlled environment, and seedling emergence scored for 5 weeks after sowing. Domestication in all species was associated with increased seed mass. In three species, greater mass was not correlated with increased ability to emerge from depth. In five species, emergence depth did correlate with mass, suggesting that selection during domestication may have acted on emergence depth. However, domestication only had a significant effect in two of these species (lentil and mung bean), and the increase in depth was no more than predicted by a cube-root allometric relationship with seed mass. The results do not support the hypothesis that burial under cultivation was a general selection mechanism for increased seed mass during the domestication of grain legumes, but it may have acted in particular species or regions.