Causes and Consequences of Contrasting Genetic Structure in Sympatrically Growing and Closely Related Species

Gene flow, natural selection and genetic drift are processes that play a major role in shaping the genetic structure of natural populations. In addition, genetic structures of individual populations are strongly correlated with their geographical position within the species distribution area. The highest levels of genetic variation are usually found in the centre of a species' distribution and tend to decrease beyond that point. Additionally, narrowly endemic taxa are expected to be characterized by lower levels of genetic variation than their widespread congeners. To understand the historical circumstances that shape populations of sympatric and closely related taxa, microsatellite markers were used, while populations of the three closely related and sympatric Mediterranean Salvia species (S. officinalis L., S. fruticosa Mill. and S. brachyodon Vandas) served as a study model. In the populations of widespread S. officinalis, located in the central parts of this species' distribution area, no population genetic disturbances were detected. The narrow endemic S. brachyodon showed heterozygote excess, clonal reproduction and a genetic bottleneck. Because the genetic bottleneck was likely caused by the disappearance of suitable open-type habitats, the recent wildfire that cleared the terrain probably saved the S. brachyodon population from gradual deterioration and extinction. At the same time, clonal reproduction could serve as a valuable mechanism in the preservation of genetic variability. The results of the disjunct S. fruticosa population indicated heterozygote deficiency, inbreeding, hybridization with S. officinalis and population expansion. The hybridization with S. officinalis along with the abandonment of the agro-pastoral system are likely the main drivers of the strong expansion of S. fruticosa in the studied location. As many relevant findings and conclusions regarding historical and contemporary demography of individual populations or species can be reached only through their comparison with closely related taxa, this study demonstrates the importance and advantages of such a multi-species approach.