Expression and trans-specific polymorphism of self-incompatibility RNases in coffea (Rubiaceae).

Michael D Nowak,Aaron P Davis,Anne D Yoder,François Anthony,Daniel Ortiz-Barrientos

doi:10.1371/journal.pone.0021019

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.

Highlights

Many bisexual flowering plants avoid the deleterious effects of inbreeding by employing genetically controlled self-incompatibility (SI) mechanisms to ensure outcrossing [1]
Microscopic observations of self pollen tube growth in C. canephora were found to be consistent with a gametophytic SI mechanism [38,61], but SI in all other Coffea species has been assumed by the observation of reduced seed set in self versus outcross pollinations [62,63]
These results are consistent with a strong gametophytic self-incompatibility response as previously documented both in C. canephora [38] and other GSI species [9]

Summary

Introduction

Many bisexual flowering plants avoid the deleterious effects of inbreeding by employing genetically controlled self-incompatibility (SI) mechanisms to ensure outcrossing [1]. The biochemical components of only three SI mechanisms have been well-characterized (see [8] for review): 1) the Papaveraceae calcium signaling system [9,10,11]; 2) the Brassicaceae sporophytic SRK and SP11/SCR SI system [12,13]; and 3) the Eudicot gametophytic S-RNase system [14]. The S-RNase SI mechanism is a type of gametophytic selfincompatibility (GSI) that functions through the selective deactivation of RNase cytotoxicity in non-self pollen tubes as they grow through the style. The S-locus in Eudicot species utilizing the S-RNase GSI system carries genes with reproductive tissue-specific expression patterns (i.e. pistil or pollen tube; [18,19,20,21,22]). While species in at least 23 other Eudicot families are known to employ some mechanism of GSI [3], there has been a conspicuous lack of published data from any of these families that would demonstrate utilization of the ancestral Eudicot S-RNase SI mechanism

Methods

Results

Conclusion