Evolution Of Self-incompatibility Locus In The Brassicaceae

Self-incompatibility is a widespread plant reproductive system that promotes cross-pollination bysuppressing self-pollination. In the tribe Brassiceae, which includes several agriculturally importantcrops, such as cabbage (Brassia oleracea), Chinese cabbage (Brassica rapa) and radish (Raphnussativus), SI lines was widely used to produce commercial F1hybrids that have an advantage in uniformmaturity, high yield, good flavor, as well as resistance to insect pests, diseases and adverse weatherconditions. Previous studies mainly focus on that what genes are responsible for SI and how SRK andSCR evolve. Due to absence of whole-genome sequences, few investigations of SI locus at thewhole-genome scale were reported, let alone taking the impact of recent whole-genome duplication on Slocus into account.The rapid development of the second-generation high-throughput sequencing technology makesmore and more plant genomes to be sequenced. Published genome sequences provide the opportinuty toanalyze the evolution history of SI locus at the whole-genome scale. In this study, a total of sixteengenomes were selected including species from Vitaceae, Salicaceae, Caricaceae, Cleomaceae andBrassicaceae. Firstly, SRK-, Lal2-and SCR-like genes were identified based on their respective proteindomains in these genomes and phylogenetic trees were constructed. Syntenic regions of SI locus insixteen species were then analyzed, and our findings were as follows.1. A total of428SRK-like genes, averaging nearly27per genome, were detected in these sixteengenomes, whereas192Lal2-like and138SCR-like genes were found with an average of12and nearly9per genome, respectively. Tandem duplication has an important impact on the evolution of these genes.Due to tandem duplication, SRK-like genes exist in gene clusters and some gene clusters locate close toSI locus (S locus).2. In the SRK tree, SRK-and Lal2-like genes formed two main clades. Many Lal2-like genes wereapparently scattered among SRK-like genes. In the SRK-like clade, Brassica class I and Brassica class IIbelonged to a monophyletic group with good bootstrap value support, whereas these was no confidentevidence to support that Arabidopsis and Brasscia SRK cluster in a single group, indicating theirevolutinary divergence. In the SCR tree, Brassica class I and class II SCR, Arabidopsis SCR andLeavenworthia SCRL, the SCR-like gene involved in the Leavenworthia-type SI, formed three differentsubgroups, suggesting that SCRs have different origins in Brassica, Arabidopsis and Leavenworthia.Different from SRK tree, the above three subgroups only consisted of functional SCR or SCRL geneswithout any SCR-like genes that are not involved in the SI, but clustered with SCR-like genes, implyingthat SCRs diverged from SCR-like genes and then underwent allelic diversification. Synonymousmutation rate (Ks) revealed that SCRs in Arabidopsis and Brassica diverged before the split of these twospecies.3. To date, three types of S loci, Brassica-type, Arabidopsis-type and Leavenworthia-type, werefound in Brassicaceae family. Arabidopsis-type and Leavenworthia-type S loci are ancient and Brassiceae S locus origined in the hexaploid ancestor. Whole-genome triplication events occurred inCleomaceae, Brassiceae and L.alabamica simultaneously. In these species, one or three syntenic regionsof S locus was present, however, only one SI gene pair was retained.