Hierarchical genetic structure and effective population sizes in Phalacrus substriatus

In this paper we present an analysis of the hierarchical population structure of the mycophagous beetle Phalacrus substriatus. The analysis showed that P. substriatus is substructured at both hierarchical levels studied, among islands and among local populations within islands. The level of differentiation among local populations was about three times as high as among different islands (F(PL) = 0.043 and F(LT) = 0.013, respectively). This stands in marked contrast to the patterns expected, based on the dispersal of P. substriatus, as the average dispersal distance of individual beetles is less than a metre per generation. Several explanations are discussed which can explain the observed patterns. We also estimated the effective population size for both hierarchical levels. The results show that the Ne/N ratios are well below unity, both for local populations and for islands. The average Ne/N ratio for local populations was only 0.210 (geometric mean, 0.172), whereas for islands the observed Ne/N ratios ranged from 0.75 to 0.98. Population subdivision is expected to increase the global (island) effective population size under equilibrium situations. However, random extinctions and recolonizations can lead to significant reductions in the global effective population size. Because population turnover is a commonly occurring phenomenon in P. substriatus, we argue that this is, at least partly, responsible for the low Ne/N ratios observed in this species. The low effective population sizes, both for local populations and for islands, will result in rapid erosion of a large proportion of the genetic variation present. The present study thus highlights the need to take random processes, such as extinction-recolonization dynamics, into account when studying effects of spatial subdivision.