title: Secondary evolution of a self-incompatibility locus in the Brassicaceae genus Leavenworthia.
abstract: 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.
authors.affiliation: Department of Biology, McGill University, Montreal, Quebec, Canada.
authors.forename: Sier-Ching
authors.initials: SC
authors.lastname: Chantha
authors.forename: Adam C
authors.initials: AC
authors.lastname: Herman
authors.forename: Adrian E
authors.initials: AE
authors.lastname: Platts
authors.forename: Xavier
authors.initials: X
authors.lastname: Vekemans
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Schoen
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D019607:Brassicaceae / Q000145:classification / Q000235:genetics*; D019143:Evolution, Molecular; D018506:Gene Expression Regulation, Plant; D006801:Humans; D008969:Molecular Sequence Data; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D010940:Plant Proteins; D011494:Protein Kinases; C079832:S-receptor kinase