Population genetic structure and gene flow across arid versus mesic environments: a comparative study of two parapatric Senecio species from the Near East

To shed light on the potential effects of xeric/arid versus mesic environments on plant population genetic structure and patterns of gene flow, we have compared allozyme and cpDNA haplotype variation in populations of two closely related, highly outcrossed, and largely wind-dispersed winter annuals of Senecio (Asteraceae). The species form a distinctive zone of parapatric distribution in the Near East by differing in their ecogeographical regimes. Senecio vernalis mainly thrives in the mesic Mediterranean life zone of Israel, whereas S. glaucus inhabits either xeric maritime or arid (semi-) desert sites. Significant differences in allozymic population subdivision among S. vernalis (theta(n) = 0.04; Nm(n) = 5.85) and S. glaucus (theta(n) = 0.12; Nm(n) = 1.85) largely resulted from topogeographical substructuring present within the latter species. Because of the similarity of within-region estimates of population structure for S. glaucus with those measured among populations of S. vernalis, it appears unlikely that ecological "aridity" factors per se are important in influencing levels of population differentiation in these species. Based on hierarchical F-statistics and tests of isolation by distance, we further conclude that geographical topologies influence the level and mode of nuclear gene flow (via pollen and/or seed) among and within subsets of S, glaucus populations, although without providing a complete barrier to interregional dispersal (dNm(reg) = 2.16) and without promoting allopatric differentiation via drift. The allozymic data further suggested that S. vernalis and S. glaucus form a zone of secondary contact in the Near East, accompanied by an almost complete interspecific barrier to nuclear gene how (dnNm(sp) = 0.253). However, to account for the considerable sharing of cpDNA haplotypes, both at the intra- and interspecific level, it is necessary to invoke either (1) selection acting against alien nuclear but not cytoplasmic DNA; or (2) the sporadic immigration of cpDNA via seed with large homogenizing effects on cytoplasmic population structure over time.