RESUMEN
Self-incompatibility (SI) systems are widespread mechanisms that prevent self-fertilization in angiosperms. They are generally encoded by one genome region containing several multiallelic genes, usually called the S-locus. They involve a recognition step between the pollen and the pistil component and pollen is rejected when it shares alleles with the pistil. The direct consequence is that rare alleles are favored, such that the S-alleles are subject to negative frequency-dependent selection. Several theoretical articles have predicted the specific patterns of polymorphism, compared to neutral loci, expected for such genes under balancing selection. For instance, many more alleles should be maintained and populations should be less differentiated than for neutral loci. However, empirical tests of these predictions in natural populations have remained scarce. Here, we compare the genetic structure at the S-locus and microsatellite markers for five natural populations of the rare species Brassica insularis. As in other Brassica species, B. insularis has a sporophytic SI system for which molecular markers are available. Our results match well the theoretical predictions and constitute the first general comparison of S-allele and neutral polymorphism.
Asunto(s)
Brassica/genética , Selección Genética , Alelos , Western Blotting , Brassica/metabolismo , Fertilidad/genética , Francia , Frecuencia de los Genes , Variación Genética , Genética de Población , Genotipo , Geografía , Glicoproteínas/genética , Glicoproteínas/metabolismo , Modelos Genéticos , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismoRESUMEN
Similar patterns of dispersal and gene flow between closely associated organisms may promote local adaptation and coevolutionary processes. We compare the genetic structures of the two species of a plant genus (Roridula gorgonias and R. dentata) and their respective obligately associated hemipteran mutualists (Pameridea roridulae and P. marlothi) using allozymes. In addition, we determine whether genetic structure is related to differences in host choice by Pameridea. Allozyme variation was found to be very structured among plant populations but less so among hemipteran populations. Strong genetic structuring among hemipteran populations was only evident when large distances isolated the plant populations on which they live. Although genetic distances among plant populations were correlated with genetic distances among hemipteran populations, genetic distances of both plants and hemipterans were better correlated with geographic distance. Because Roridula and Pameridea have different scales of gene flow, adaptation at the local population level is unlikely. However, the restricted gene flow of both plants and hemipterans could enable adaptation to occur at a regional level. In choice experiments, the hemipteran (Pameridea) has a strong preference for its carnivorous host plant (Roridula) above unrelated host plants. Pameridea also prefers its host species to its closely related sister species. Specialization at the specific level is likely to reinforce cospeciation processes in this mutualism. However, Pameridea does not exhibit intraspecific preferences toward plants from their natal populations above plants from isolated, non-natal populations.