Molecular Characterization of Lal2, an SRK-Like Gene Linked to the S-Locus in the Wild Mustard Leavenworthia Alabamica
Busch, Jeremiah W., Sharma, Julia, Schoen, Daniel J., Genetics
Single-locus sporophytic self-incompatibility inhibits inbreeding in many members of the mustard family (Brassicaceae). To investigate the genetics of self-incompatibility in the wild mustard Leavenworthia alabamica, diallel crosses were conducted between full siblings. Patterns of incompatibility were consistent with the action of single-locus sporophytic self-incompatibility. DNA sequences related to S-locus receptor kinase (SRK), the gene involved in self-pollen recognition in mustards, were cloned and sequenced. A single sequence with high identity to SRK and several other groups of sequences (Lal1, Lal2, Lal3, Lal8, and Lal14) were isolated from L. alabamica. We propose that either Lal2 sequences are divergent alleles of SRK or Lal2 is in tight linkage with SRK because (1) Lal2 alleles cosegregate with S-alleles inferred from dialleles in all 97 cases tested in five families; (2) Lal2 sequences are highly diverse at both synonymous and nonsynonymous sites and exhibit patterns of selective constraint similar to those observed at SRK in Brassica and Arabidopsis; and (3) transcripts of one Lal2 allele were detected in leaves and the styles of open flowers, but were most abundant in the stigmas of maturing buds. We discuss the utility of the S-linked polymorphism at Lal2 for studying the evolutionary forces acting on self-incompatibility in Leavenworthia.
INBREEDING avoidance is thought to have contributed to the evolution and maintenance of genetically controlled self-incompatibility (SI) systems in flowering plants (Richards 1986). In SI systems, plants that share alleles at the self-incompatibility locus (i.e., S-locus) are incapable of producing seed. SI evolved early during the diversification of angiosperms, and it has been proposed that this system may have contributed to the success of flowering plants by contributing to the maintenance of population-genetic variation (Darlington and Mather 1949; Igic and Kohn 2001, 2006). SI systems are diverse in terms of the molecular mechanisms underlying self-recognition, as well as in associated floral traits that promote pollen transfer between compatible mates (Hiscock and Tabah 2003). A feature of most SI systems is the presence of a single linked complex of genes (or more rarely, several unlinked loci) that code for proteins involved in the recognition and rejection of self-pollen. Since individuals that possess rare S-alleles have a strong fertility advantage, negative frequency-dependent selection is expected to maintain a large amount of genetic diversity at S-loci in natural populations. Indeed, these loci are among the most polymorphic in eukaryotic organisms (Wright 1939; Lawrence 2000).
Single-locus sporophytic SI has been studied in a number of genera in the mustard family (Brassicaceae) (Bateman 1954, 1955; Thompson 1957; Lloyd 1967; Sampson 1967; Schierup et al. 2001). Molecular genetic characterization of SI in Brassica and Arabidopsis species has shown that the S-locus consists of a tightly linked cluster of genes encoding proteins that function together as a receptor-ligand system (Schopfer et al. 1999; Takasaki et al. 2000; for a review see Fobis-Loisy et al. 2004). One of these genes, the S-locus receptor kinase (SRK), codes for a membrane-bound protein expressed in the stigma that binds specifically to the S-locus cysteine-rich ligand (SCR/SP11), which is expressed in the tapetum of anthers (Kachroo et al. 2001; Takayamaet al. 2001). The successful binding of SCR by SRK initiates a signaling cascade that prevents pollen tubes from penetrating the stigmatic surface, although the roles of other genes in this pathway are still under investigation (Murase et al. 2004; Liu et al. 2007). It is thought that the linked system of co-evolved SRK and SCR genes has been maintained by natural selection for at least 25-40 million years (Uyenoyama 1995). As would be expected in a system in which selection is strongly negatively frequency dependent,nucleotide polymorphism is pronounced among S-alleles in Brassica campestris and B. …