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

Abstract

Background and aims: At the rear edge of the distribution of species, 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 nuclear microsatellite markers (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 lower clonal and genetic diversity compared with 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 (63 F:37 M) and the smaller (99 F:1 M) 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.

Keywords: Salix herbacea; Fragmentation; clonal growth; gene flow; genetic diversity; isolation; marginality; microsatellite markers; peripherality; sex ratio; snowbed; spatial genetic structure.

Fig. 1.

Location of the two remnant S. herbacea populations in the northern Apennines at Mt Prado and Mt Cimone. The insert at bottom left shows the Alpine distribution of the species (hatched area) according to Jalas and Suominen (1976) and the location of the Alpine population at Gavia pass (black circle). The box in the insert represents the area of the northern Apennines included in the main figure and × symbols show the location of S. herbacea remnants in the central Apennines.

Fig. 2.

Location of sampled ramets in the Prado (A) and Cimone (B) populations. Open circles represent ramets that, after genetic analyses, were classified as unique MLGs. Coloured dots represent >1 ramets MLGs. The key shows the number of ramets associated with each MLG identity code.

Fig. 3.

Correlograms from spatial autocorrelation analysis based on the correlation coefficient F_ij by Loiselle et al. (1995) and even sample size classes for the three S. herbacea populations. Filled dots represents distance classes for which significant spatial structuring of individuals was observed. Black and grey lines indicated autocorrelograms calculated at the ramet and MLG level, respectively.