Laboratory of Todd Vision
Department of Biology
University of North Carolina at Chapel Hill

Genetics of Domestication in the Solanaceae

Fruit size in interspecific crosses of tomato is a classic domestication trait. By crossing the domesticated tomato, with its grossly enlarged fruit, to related small-fruited species, upwards of a dozen quantitative trait loci (QTL) have been mapped. Tomato is not the only member of the Solanaceae family to be domesticated. The family includes, in addition to crops such as tobacco and potato, additional species that have been selected for highly enlarged fruits, including species of bell pepper, eggplant, and tomatillo (or husk tomato).

Interestingly, sequence homologs of the markers linked to the major tomato fruit size QTL cosegregate with major fruit size loci in cultivated-by-wild crosses of eggplant and pepper, both of which were independently domesticated. This pattern of "homologous QTL" is unexpected given our current understanding of how genes underlying complex traits respond to selection.

However, the phenomenon is not unique to tomato; similar cases of homologous QTL have been observed in other systems, most notably for domestication traits in the cereal grains. Homologous QTL may underly variation in adaptively significant traits in natural populations, as well; and they may have been seen primarily in crop domestication traits simply because of the genetic attention that has been devoted to these economically important systems.

We are currently studying potentially homologous QTL for fruit size in domesticated tomatillo (Physalis philadelphica). Unlike tomato, tomatillo is an outcrossing species with high genetic variability. We have shown that the population structure of the species is essentially panmictic, making it well suited for studies for population genetic studies of molecular variation. Thus, we are using association mapping to characterize, at a very fine level, the genetic changes within candidate genes that contribute to fruit size variation. This will allow us to test outstanding hypotheses for the nature and origin of homologous QTL.