Using molecular allelic variation to understand domestication processes and conserve diversity in Brassica crops

The harvested curd of cauliflower (Brassica oleracea var. botrytis L.) consists of proliferating, arrested inflorescence and floral meristems. The origins and events leading to the domestication of this important crop trait remain unclear. A similar phenotype observed in the ap1-1/cal-1 mutant of Arabidopsis thaliana resulted in speculation that the orthologous genes from B. oleracea may be responsible for this characteristic trait. A recent molecular and genetic study has led to development of a genetic model based on segregation of recessive alleles at specific, mapped loci of the candidate genes BoCAL and BoAP1. This model, although incomplete, accounts for differences in stage of arrest between cauliflower and Calabrese broccoli (B. oleracea var. italica Plenck), and predicts the intermediate stages of arrest similar to those observed in Sicilian Purple types. There appears to be a reduction in allele diversity at two key loci associated with this crop type. Surveys of ex situ genetic resource collections have demonstrated the association of alleles of BoCAL-a with curding phenotypes of B. oleracea. Strong correlations exist between specific alleles of BoCAL-a and discrete inflorescence morphologies. These complementary lines of evidence suggest that the cauliflower curd arose in southern Italy from a heading Calabrese broccoli via an intermediate Sicilian crop type. The close association of BoAP1-a and BoAP1-c with the self-incompatibility locus S may have contributed to the observed reduced number of S-alleles within the modern cauliflower gene-pool. The results indicate that it is important to consider the effect of strong selection for crop type on genetic erosion of loci linked in coupling to genes underlying the crop phenotype.