title: Expression and trans-specific polymorphism of self-incompatibility RNases in coffea (Rubiaceae).
abstract: Self-incompatibility (SI) is widespread in the angiosperms, but identifying the biochemical components of SI mechanisms has proven to be difficult in most lineages. Coffea (coffee; Rubiaceae) is a genus of old-world tropical understory trees in which the vast majority of diploid species utilize a mechanism of gametophytic self-incompatibility (GSI). The S-RNase GSI system was one of the first SI mechanisms to be biochemically characterized, and likely represents the ancestral Eudicot condition as evidenced by its functional characterization in both asterid (Solanaceae, Plantaginaceae) and rosid (Rosaceae) lineages. The S-RNase GSI mechanism employs the activity of class III RNase T2 proteins to terminate the growth of "self" pollen tubes. Here, we investigate the mechanism of Coffea GSI and specifically examine the potential for homology to S-RNase GSI by sequencing class III RNase T2 genes in populations of 14 African and Madagascan Coffea species and the closely related self-compatible species Psilanthus ebracteolatus. Phylogenetic analyses of these sequences aligned to a diverse sample of plant RNase T2 genes show that the Coffea genome contains at least three class III RNase T2 genes. Patterns of tissue-specific gene expression identify one of these RNase T2 genes as the putative Coffea S-RNase gene. We show that populations of SI Coffea are remarkably polymorphic for putative S-RNase alleles, and exhibit a persistent pattern of trans-specific polymorphism characteristic of all S-RNase genes previously isolated from GSI Eudicot lineages. We thus conclude that Coffea GSI is most likely homologous to the classic Eudicot S-RNase system, which was retained since the divergence of the Rubiaceae lineage from an ancient SI Eudicot ancestor, nearly 90 million years ago.
authors: Department of Biology, Duke University, Durham, North Carolina, United States of America. michael.nowak@duke.edu
authors: Michael D
authors: MD
authors: Nowak
authors: Aaron P
authors: AP
authors: Davis
authors: François
authors: F
authors: Anthony
authors: Anne D
authors: AD
authors: Yoder
mesh_terms: D000349:Africa; D000483:Alleles; D040503:Coffea / Q000201:enzymology* / Q000235:genetics*; D017124:Conserved Sequence; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant / Q000235:genetics; D055993:Germ Cells, Plant / Q000378:metabolism; D006579:Heterozygote; D008270:Madagascar; D010802:Phylogeny; D054817:Pollination / Q000235:genetics*; D011110:Polymorphism, Genetic; D017434:Protein Structure, Tertiary; D012260:Ribonucleases / Q000737:chemistry / Q000145:classification / Q000235:genetics*; D013045:Species Specificity
chemical_list: D012260:Ribonucleases