Sequence diversification and phylogenetic analysis of selfincompatibility specific determinant genes in Brassicaceae

Abstract

It is a long history of theoretical interest in understanding the genetic causes and consequences of shifts between outcrossing and inbreeding in plant mating systems. Selfincompatibility (SI) systems genetically promote outcrossing, and its transitions to inbreeding potentially changes genetic diversity in the evolution of species. SI in the Brassicaceae family is sporophytically controlled by a single locus (S-locus), which locates the female determinant gene SRK (S-locus receptor kinase) and the male determinant gene SCR (S-locus cysteine rich). Operation of diversifying selection maintains the diversity of SRK and SCR during the evolutionary course of S haplotypes. Here we used deduced amino acid sequences of SRK and/or SCR publicly available from seven species in three genera, Brassica, Arabidopsis and Raphanus, to investigate the sequence diversification and phylogenetic distribution of S haplotypes in the Brassicaceae family. The results reveal that SCR shows higher sequence distance than SRK within or between genera, and also in the interspecific pairs of S haplotypes. Phylogenetic analysis supports the conclusion that SI has a single origin but evolves differently within the Brassicaceae. The genetic mechanisms and consequences of the loss of SI in the Brassicaceae family are discussed.