Nuclear and chloroplast microsatellite markers were used to study population structure and gene flow among seven Cretan populations of the Aegean endemic plant species Brassica cretica (Brassicaceae). Both nuclear and chloroplast markers revealed exceptionally high levels of population differentiation (overall F(ST)=0.628 and 1.000, respectively) and relatively little within-population diversity (overall H(S)=0.211 and 0.000, respectively). Maximum-likelihood estimates of directional migration rates were low among all pairs of populations (average Nm=0.286). There was no evidence that differences in flower colour between populations had any influence on historical levels of gene flow. In addition, a haplotype network showed that all five chloroplast haplotypes found in the sample were closely related. Together, these results suggest that current patterns of diversification in B. cretica are mainly a result of genetic drift during the last half million years. The main conclusions from the present study are consistent with the prevailing hypothesis that plant diversification in the Aegean region is driven by random rather than adaptive differentiation among isolated populations.

Self-incompatibility (SI) in the Brassicaceae plant family is controlled by the SRK and SCR genes situated at the S locus. A large number of S haplotypes have been identified, mainly in cultivated species of the Brassica and Raphanus genera, but recently also in wild Arabidopsis species. Here, we used DNA sequences from the SRK and SCR genes of the wild Brassica species Brassica cretica, together with publicly available sequence data from other Brassicaceae species, to investigate the evolutionary relationships among S haplotypes in the Brassicaceae family. The results reveal that wild and cultivated Brassica species have similar levels of SRK diversity, indicating that domestication has had but a minor effect on S-locus diversity in Brassica. Our results also show that a common set of S haplotypes was present in the ancestor of the Brassica and Arabidopsis genera, that only a small number of haplotypes survived in the Brassica lineage after its separation from Arabidopsis, and that diversification within the two Brassica dominance classes occurred after the split between the two lineages. We also found indications that recombination may have occurred between the kinase domain of SRK and the SCR gene in Brassica.T O avoid self-fertilization, different kinds of selfincompatibility (SI) mechanisms have evolved in many plant families (Richards 1997). In most cases, SI is controlled by a single locus, the S locus, which, even though it often contains several genes, is inherited as a single Mendelian locus (Silva and Goring 2001); for this reason, S alleles are often referred to as S haplotypes (Boyes and Nasrallah 1993). In the Brassicaceae, which has the sporophytic type of SI where the pollen SI phenotype is determined by the S-locus genotype of the diploid parent, two S-locus genes, SRK and SCR, encode the maternal (pistil) and paternal (pollen) specificities, respectively (Stein et al. 1991;Schopfer et al. 1999;Suzuki et al. 1999). The SRK receptor consists of three domains with different functions: an extracellular S domain (encoded by exon 1 in SRK), which is the center for recognition of SCR; a transmembrane domain (exon 2) that passes through the plasma membrane; and an intracellular kinase domain (exons 4-7), which initiates the signaling cascade in the stigma cells (see Takayama and Isogai 2005). The SCR gene product is a small soluble protein molecule present at the pollen surface where it interacts with the S domain of its cognate SRK protein (Takayama et al. 2001). Because of the strong frequency-dependent selection acting on the SI system, the S locus typically maintains a large number of haplotypes (Lawrence 2000), and levels of diversity at both synonymous and nonsynonymous sites are high in the S domain of SRK and in SCR (e.g., Awadalla and Charlesworth 1999;Charlesworth et al. 2003;Takuno et al. 2007). Furthermore, in both Brassica and Arabidopsis, synonymous diversity is elevated in a region encompassing several tens of kilobases beyond the actual targets of selection due to the low level...

Self-incompatibility (SI) in plants is a classic example of a trait evolving under strong frequency-dependent selection. As a consequence, population genetic theory predicts that the S locus, which controls SI, should maintain numerous alleles, display a high level of nucleotide diversity, and, in structured populations, show a lower level of among-population differentiation compared to neutral loci. Population-level investigations of DNA sequence variation at the S locus have recently been carried out in the genus Arabidopsis, largely confirming results from theoretical models of S-locus evolutionary dynamics, but no comparable studies have been done in wild Brassica species. In this study, we sequenced parts of the S-locus genes SRK and SCR, two tightly linked genes that are directly involved in the determination of SI specificity in samples from four natural populations of the wild species Brassica cretica. The amount and distribution of nucleotide diversity, as well as the frequency spectrum of putative functional haplotypes, observed at the S locus in B. cretica fit very well with expectations from theoretical models, providing strong evidence for frequency-dependent selection acting on the S locus in a wild Brassica species.

In the Brassicaceae plant family, which includes the Arabidopsis and Brassica genera, self-incompatibility (SI) is controlled by genes at the S locus. Using experimental crosses, we studied the pattern of inheritance of S-locus alleles in the wild species Brassica cretica. Four full-sib families were established and unequal segregation of alleles at the SRK SI gene was found in one family. The segregation distortion acted in favour of a recessive (class II) allele and was best explained by some form of gametic-level selection. Our findings are discussed in the light of theoretical predictions of differential accumulation of deleterious mutations among S-locus alleles.