Natural Selection on S-Linked Genes in Turnera (Passifloracease)

Investigations of the evolutionary dynamics exhibited by S-linked loci have the potential to provide evidence concerning the particular genes that determine the expression of distyly in flowering plants. Several approaches were adopted to explore the signatures of selection on S-linked genes in distylous Turnera. While dN/dS-based results revealed pervasive purifying selection at the S-locus in Turnera, average nucleotide diversity and sequence polymorphism measures were found to be elevated in two S-linked genes (AP2D and RNABP), suggesting the possible occurrence of balancing selection at these or closely-linked loci. Limited trans-species polymorphisms were identified in APETALA2, as well. Conversely, the negatively selected S-haplotype specific gene, Tssta1, also appears to be a very promising distyly gene candidate and shows significant sequence homology to known self-incompatibility proteins in Papaver. This study represents the first investigation of the molecular signatures of natural selection on S-linked genes in any heterostylous species. The implications of the results obtained for the elucidation of the genetic mechanisms that determine distyly in Turnera are discussed. Ultimately, it is hoped that understanding these mechanisms will, in turn, help to evaluate existing models regarding how distyly has evolved.