Indirect Measures of Gene Flow

Genetic markers can be used to infer indirectly the gene flow between populations (Slatkin, 1985; Excoffier et al., 1992). By studying the distribution of allele frequencies and the degree of heterozygosity in different populations, indexes of subdivision can be established according to the theory of neutral evolution (Wright, 1951). One common measure of population subdivision (Wright's subdivision) Fst, can be translated into gene flow through the formula Fst = 1/ (1+4 Nm) for diploid or dikaryotic organisms, N is the effective population size and m is the fraction of migrants in the population per generation. Provided that the system is in equilibrium, Fst values below 0.1 indicate fairly high-gene flow, 0.1—0.2 low-gene flow and >0.2 indicate isolated populations. However, very large populations will only change gene frequencies extremely slowly through random drift and it might therefore be hard to detect recent subdivision.

Studies of genetic differentiation in wood-decomposing basidiomycetes show that very little subdivision normally occurs in common species such as Fomitopsis pinicola (Hogberg et al., 1995; Norden, 1997; Hogberg and Stenlid, 1999), H. annosum (Stenlid et al., 1994), Phlebiopsis gigantea (Vainio and Hantula 2000), Cylindrobasidium evolvens (Vasiliauskas et al., 1998), Chondrostereum purpureum (Gosselin et al., 1999) and Trichaptum abietinum (Kauserud and Schumacher, 2003a). In rare species, substantially higher Fst values have been calculated (Hogberg et al., 1999; Franzen et al., 2007), indicating that small populations are isolated from each other and the populations might undergo processes leading to inbreeding. To date, studies on inbreeding effects on fitness of wood-decay fungi are scarce, but nevertheless indicate reduced viability of basidiospores (Edman et al., 2004a). However, in the rare species Fomitopsis rosea low Fst values have also been reported. The interpretation of the conflicting results in this species can be that founder effects in small populations such as those sampled by Hogberg et al. (1999) influence the Fst values, while the rather large populations sampled by Kauserud and Schumacher (2003b) probably still reflect the genetic structure of the previously continuous distribution that was present before modern forestry fragmented the habitats.

In Phlebia centrifuga, a relatively uncommon decayer of conifer wood in managed forests, populations outside the regions of continuous distribution of suitable forests, show signs of reduced genetic variation. The genetic isolation in the fragmented habitat (Franzen et al., 2007) also matches the measured lack of spore catches on the same geographic scale (Edman et al., 2004a, 2004b).

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