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.H OMOMORPHIC self-incompatibility (SI) systems are widespread physiological mechanisms preventing self-fertilization in Angiosperms by controlling pollen germination or pollen tube growth (De Nettancourt 2001). Pollen and pistil are incompatible when they both express identical alleles. The recognition involves specificity molecules usually encoded by one genome region containing several multiallelic genes (De Nettancourt 2001). In gametophytic (GSI) systems, the pollen phenotype is encoded by its own haploid genome, whereas in sporophytic (SSI) systems, the pollen phenotype is determined by the sporophyte (diploid pollen parent) and can involve dominance interactions among alleles. For instance, two classes of alleles are known in Brassica oleracea (Nasrallah 1991). Class I alleles are dominant over the class II alleles in the pollen, while alleles within class I and class II are mutually codominant. In the pistil, all alleles are codominant. This scheme is also found in B. campestris, with the exception that alleles occur in three dominance levels in the pollen and a few alleles are recessive in the stigma (Hatakeyama et al. 1998). At the molecular level, the SSI system of the Brassicaceae is among the best known (for a recent review see Hiscock and McInnis 2003). Both pistil and pollen genes have been identified (Schopfer et al. 1999;Takayama et al. 2000). In this system, recognition proceeds through receptor-ligand interaction between S-locus cysteinerich protein (SCR), a small hypervariable ligand peptide, and S-locus receptor kinase (SRK), a transmembrane receptor with an intracellular kinase domain (Kachroo et al. 2001).SI evolutionary properties have also long aroused the population geneticist's interest because selection pressures are known a priori. C...

In tropical forests, selective logging removes large trees that are often the main contributors to pollination. We studied pollination patterns of the African mahogany, Entandrophragma cylindricum (Sapelli). We investigated two plots in Cameroon corresponding to three tree densities: unlogged forest (Ndama 2002), a mildly logged forest 1 year after logging (Ndama 2003) and a severely logged forest 30 years after logging (Dimako). We used four microsatellite markers to perform paternity analysis. Selfing remained below 2% in all treatments. Pollen flow was mainly long distance but with some proximity effects. Average observed within-plot pollination distances were 338, 266 and 385 m, and pollination by trees outside the plots was 70% (Ndama 2002), 74% (Ndama 2003) and 66% (Dimako). Despite sampling a limited number of seeds from a limited number of mother trees, we obtained seeds sired by 35.6-38.3% of the potential within-plot pollen donors. While trees 20 cm in diameter contributed to pollination, results in Dimako suggest that individual larger trees contribute more to pollination than small ones. This effect was not detected in the other treatments. The results suggest extensive pollen flow in Sapelli. Hence, in Sapelli, the main limiting factor for regeneration after logging may be a reduction in the number of trees capable of producing seeds rather genetic effects due to limits to pollen dispersal.

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.