Secondary Evolution of a Self-Incompatibility Locus in the Brassicaceae Genus Leavenworthia

Self-incompatibility (si) is the flowering plant reproductive system in which self pollen tube growth is inhibited,thereby preventing self-fertilization. si has evolved independently in several different flowering plant lineages. in all brassicaceae species in which the molecular basis of si has been investigated in detail,the product of the s-locus receptor kinase (srk) gene functions as receptor in the initial step of the self pollen-rejection pathway,while that of the s-locus cysteine-rich (scr) gene functions as ligand. here we examine the hypothesis that the s locus in the brassicaceae genus leavenworthia is paralogous with the s locus previously characterized in other members of the family. we also test the hypothesis that self-compatibility in this group is based on disruption of the pollen ligand-producing gene. sequence analysis of the s-locus genes in leavenworthia,phylogeny of s alleles,gene expression patterns,and comparative genomics analyses provide support for both hypotheses. of special interest are two genes located in a non-s locus genomic region of arabidopsis lyrata that exhibit domain structures,sequences,and phylogenetic histories similar to those of the s-locus genes in leavenworthia,and that also share synteny with these genes. these a. lyrata genes resemble those comprising the a. lyrata s locus,but they do not function in self-recognition. moreover,they appear to belong to a lineage that diverged from the ancestral brassicaceae s-locus genes before allelic diversification at the s locus. we hypothesize that there has been neo-functionalization of these s-locus-like genes in the leavenworthia lineage,resulting in evolution of a separate ligand-receptor system of si. our results also provide support for theoretical models that predict that the least constrained pathway to the evolution of self-compatibility is one involving loss of pollen gene function. © 2013 chantha et al.