Allozyme variation in relation to ecotypic differentiation and population size in marginal populations of Silene nutans

Abstract

In Belgium, at the north-western margin of its geographical range, Silene nutans is a rare species, which has evolved a silicicolous (Si) and a calcicolous (Ca) ecotype, with contrasting morphometric traits. Genetic diversity and population genetic structure were examined for seven allozyme loci in 16 Si and 18 Ca populations (a total of 567 individuals). High genetic variation was found at both the ecotypic and population level, and no significant correlation was found between population size and any measure of genetic variation. The maintenance of high levels of genetic diversity in small, marginal populations might be explained by the perennial, long-lived life form and the outcrossing breeding system of the species. Additionally, low F ST -values suggested that efficient gene flow was occurring within both ecotypes. Genetic distance measures and cluster analysis using UPGMA on the distance matrix revealed that the populations were differentiated according to their ecotypic property in two distinct gene pools. It is argued that the congruence of allozymic and morphometric differentiation between edaphic races is unusual for an outcrossing species. This finding, together with previous observations of isolating mechanisms between ecotypes, strongly suggests that incipient speciation is occurring within Silene nutans at the margin of its geographical range.

Structure of allozyme variation in Nordic Silene nutans (Caryophyllaceae): population size, geographical position and immigration history

“…High F IS values may result from selfing, inbreeding, restricted pollen and/or seed dispersal, clonality and/or the spatial effects of selection in different habitats (Linhart & Premoli, 1994;Berg & Hamrick, 1997;Tyler et al, 2002;Van Rossum et al, 2002). In contrast to the high values of F IS in Nordic S. nutans, mean (over populations) F IS values that are near to Hardy-Weinberg equilibrium have been reported for the silicicolous and calcicolous ecotypes (-0.009 and 0.023, respectively) of S. nutans in Belgium (Van Rossum et al, 1997). Whereas the Belgian populations show high outcrossing rates (E. Gratia, unpubl.…”

“…Silene nutans has a markedly disjunct distribution in northern Sweden and Finland, where it reaches the north-ern limit of its European range, whereas it is more continuously distributed in southern Sweden (Hultén, 1971). Earlier studies of S. nutans in Belgium, on the western margin of its distributional range, showed that the Belgian populations maintain high levels of genetic diversity and are differentiated into two parapatric ecotypes that are associated with calcareous or siliceous substrates (Van Rossum et al, 1997, 1999, but which have also had different histories (Van Rossum et al, 2003). Edaphic adaptation (De Bilde, 1977, 1978, in combination with the partial isolation of the two ecotypes by pre-and postzygotic reproductive barriers (Van Rossum, De Bilde & Lefèbvre et al, 1996), suggests a process of incipient parapatric speciation (Van Rossum et al, 1997).…”

Population genetics of threatened wild plants in Japan

“…Van Rossum et al (1997) suggested that high levels of gene flow are responsible for maintaining high levels of variation in all populations of Silene nutans . Populations of an outcrossing species with little interpopulation isolation may tend to have greater genetic diversity even in a small population, due to the immigration of varieties of genes by pollen from other large populations.…”

Divergence and variation of quantitative traits between allozyme genotypes ofAvena barbatafrom contrasting habitats

“…Where ecotypes have been initially identified based on morphological and/or habitat differences, subsequent molecular marker surveys typically reveal significant allele frequency differences. For example, in the silicicolous‐calcicolous ecotypes of Silene nutans identified by De Bilde & Lefèbvre (1990) subsequent allozyme investigations accurately distinguished the ecotypes (Van Rossum et al . 1997).…”