Reproductive and genetic consequences of extreme isolation in Salix herbacea L. at the rear edge of its distribution

Background and Aims At the rear edge of species distribution, extreme isolation and small population size influence the genetic diversity and differentiation of plant populations. This may be particularly true for Arctic-alpine species in mid-latitude mountains, but exactly how peripherality has shaped their genetic and reproductive characteristics is poorly investigated. The present study, focused on Salix herbacea, aims at providing new insights into the causes behind ongoing demographic dynamics and their consequences for peripheral populations of Arctic-alpine species. Methods We performed a whole-population, highly-detailed sampling of the only two S. herbacea populations in the northern Apennines, comparing their clonal and genetic diversity, sex ratio and spatial genetic structure with a reference population from the Alps. After inspecting ~1800 grid intersections in the three populations, 563 ramets were genotyped at 11 nSSRs. Past demography and mating patterns of Apennine populations were investigated to elucidate the possible causes of altered reproductive dynamics. Key Results Apennine populations, which experienced a Holocene bottleneck and are highly differentiated (FST=0.15), had a lower clonal and genetic diversity compared to the alpine population (RMLG=1 and HE=0.71), with the smaller population exhibiting the lowest diversity (RMLG=0.03 and HE=0.24). An unbalanced sex ratio was found in the larger (63F:37M) and the smaller (99F:1M) Apennine population. Both were characterized by the presence of extremely large clones (up to 2500 m2) which, however, did not play a dominant role in local reproductive dynamics. Conclusions Under conditions of extreme isolation and progressive size reduction, S. herbacea has experienced an alteration of genetic characteristics produced by the prevalence of clonal growth over sexual reproduction. However, our results showed that the larger Apennine population has maintained levels of sexual reproduction enough to counteract a dramatic loss of genetic and clonal diversity.