Abstract
BackgroundThe S-domain serine/threonine receptor-like kinases (SRLKs) comprise one of the largest and most rapidly expanding subfamilies in the plant receptor-like/Pelle kinase (RLKs) family. The founding member of this subfamily, the S-locus receptor kinase (SRK), functions as the female determinant of specificity in the self-incompatibility (SI) responses of crucifers. Two classes of proteins resembling the extracellular S domain (designated S-domain receptor-like proteins, SRLPs) or the intracellular kinase domain (designated S-domain receptor-like cytoplasmic kinases, SRLCKs) of SRK are also ubiquitous in land plants, indicating that the SRLKs are composite molecules that originated by domain fusion of the two component proteins. Here, we explored the origin and diversification of SRLKs by phylogenomic methods.ResultsBased on the distribution patterns of SRLKs and SRLCKs in a reconciled species-domain tree, a maximum parsimony model was then established for simultaneously inferring and dating gene duplication/loss and fusion /fission events in SRLK evolution. Various SRK alleles from crucifer species were then included in our phylogenetic analyses to infer the origination of SRKs by identifying the proper outgroups.ConclusionsTwo gene fusion events were inferred and the major gene fusion event occurred in the common ancestor of land plants generated almost all of extant SRLKs. The functional diversification of duplicated SRLKs was illustrated by molecular evolution analyses of SRKs. Our findings support that SRKs originated as two ancient haplotypes derived from a pair of tandem duplicate genes through random regulatory neo-/sub- functionalization in the common ancestor of the Brassicaceae.
Highlights
The S-domain serine/threonine receptor-like kinases (SRLKs) comprise one of the largest and most rapidly expanding subfamilies in the plant receptor-like/Pelle kinase (RLKs) family
SRLKs emerged in early land plants and expanded greatly in Angiosperms SRLKs are composed principally of three modular domains in a configuration of S domain (SD)-transmembrane domain (TM)-kinase domain (KD)
Our results suggest that almost all SRLKs of land plants are derived from a single ancient domain fusion event
Summary
The S-domain serine/threonine receptor-like kinases (SRLKs) comprise one of the largest and most rapidly expanding subfamilies in the plant receptor-like/Pelle kinase (RLKs) family. Two classes of proteins resembling the extracellular S domain (designated S-domain receptor-like proteins, SRLPs) or the intracellular kinase domain (designated S-domain receptor-like cytoplasmic kinases, SRLCKs) of SRK are ubiquitous in land plants, indicating that the SRLKs are composite molecules that originated by domain fusion of the two component proteins. (RLCKs) resembling the intracellular kinase domains of SRLKs but lacking the extracellular S-domain (designated S-domain receptor-like cytoplasmic kinase, SRLCKs) were defined by their close phylogenetic relationship to the kinase domains of SRLKs [8,12] Another class of proteins resembling SLGs, designated S-domain receptor-like proteins, SRLPs, is ubiquitous in plants [13,14,15], suggesting that the composite SRLKs likely originated by fusion of the two split component proteins. Despite the fact that multiple-domain proteins in super-protein families are normally composed of abundant and versatile domains and tend to undergo more independent gene fusion/fission events [27], analysis of gene fusion/fission events in a large gene subfamily such as SRLK subfamily is still lacking
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