The number, age, sharing and relatedness ofS-locus specificities inPrunus

Abstract

SummaryIn gametophytic self-incompatibility systems, many specificities (different ‘lock-and-key’ combinations) are maintained by frequency-dependent selection for very long evolutionary times. In Solanaceae, trans-specific evolution (the observation that an allele from one species may be more closely related to an allele from another species than to others from the same species) has been taken as an argument for the very old age of specificities. In this work, by determining, for the first time, the age of extantPrunusspecies, we show that this reasoning cannot be applied to Prunoideae. Furthermore, since our sample size is large (allS-RNaseencoding the female component andSFBencoding the male component GenBank sequences), we were able to estimate the age of the oldestPrunusspecificities. By doing so, we show that the lower variability levels at thePrunus S-locus, in comparison with Solanaceae, is due to the younger age ofPrunusalleles, and not to a difference in silent mutation rates. We show that the ancestor to extantPrunusspecies harboured at least 102 specificities, in contrast to the maximum of 33 observed in extantPrunusspecies. Since the number of specificities that can be maintained in a population depends on the effective population size, this observation suggests a bottleneck inPrunusevolutionary history. Loss of specificities may have occurred during this event. Using only information on amino acid sites that determine specificity differences, and a simulation approach, we show that a model that assumes closely related specificities are not preferentially lost during evolution, fails to predict the observed degree of specificity relatedness.