title: RNase-based self-incompatibility: puzzled by pollen S.
abstract: Many plants have a genetically determined self-incompatibility system in which the rejection of self pollen grains is controlled by alleles of an S locus. A common feature of these S loci is that separate pollen- and style-expressed genes (pollen S and style S, respectively) determine S allele identity. The long-held view has been that pollen S and style S must be a coevolving gene pair in order for allelic recognition to be maintained as new S alleles arise. In at least three plant families, the Solanaceae, Rosaceae, and Plantaginaceae, the style S gene has long been known to encode an extracellular ribonuclease called the S-RNase. Pollen S in these families has more recently been identified and encodes an F-box protein known as either SLF or SFB. In this perspective, we describe the puzzling evolutionary relationship that exists between the SLF/SFB and S-RNase genes and show that in most cases cognate pairs of genes are not coevolving in the expected manner. Because some pollen S genes appear to have arisen much more recently than their style S cognates, we conclude that either some pollen S genes have been falsely identified or that there is a major problem with our understanding of how the S locus evolves.
authors.affiliation: School of Botany, University of Melbourne, VIC 3010, Australia. edwardjn@unimelb.edu.au
authors.forename: Ed
authors.initials: E
authors.lastname: Newbigin
authors.forename: Timothy
authors.initials: T
authors.lastname: Paape
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Kohn
mesh_terms: D000483:Alleles; D044783:F-Box Proteins / Q000235:genetics / Q000378:metabolism; D005298:Fertility / Q000235:genetics / Q000502:physiology; D018506:Gene Expression Regulation, Plant; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D011058:Pollen / Q000235:genetics / Q000502:physiology*; D054817:Pollination / Q000235:genetics / Q000502:physiology*; D012260:Ribonucleases / Q000235:genetics / Q000378:metabolism*
chemical_list: D044783:F-Box Proteins; D010940:Plant Proteins; D012260:Ribonucleases; C092227:S1-RNase; C022542:ribonuclease S
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