Dispersal constraints and fine-scale spatial genetic structure in two earthworm species

Abstract

The limited dispersal ability of earthworms is expected to result in marked genetic isolation by distance and remarkable spatial patterns of genetic variation. To test this hypothesis, we investigated, using microsatellite loci, the spatial genetic structure of two earthworm species, Allolobophora chlorotica and Aporrectodea icterica, in two plots of less than 1ha where a total of 282 individuals were collected. We used spatial autocorrelation statistics, partial Mantel tests of isolation-by-distance (IBD) and isolation-by-resistance (IBR), and Bayesian test of clustering to explore recent patterns involved in the observed genetic structure. For A.icterica, a low signal of genetic structure was detected, which may be explained by an important dispersal capacity and/or by the low polymorphism of the microsatellite loci. For A.chlorotica, a weak, but significant, pattern of IBD associated with positive autocorrelation was observed in one of the plots. In the other plot, which had been recently ploughed, two genetically differentiated clusters were identified. These results suggest a spatial neighbourhood structure in A.chlorotica, with neighbour individuals that tend to be more genetically similar to one another, and also highlight that habitat perturbation as a result of human activities may deeply alter the genetic structure of earthworm species, even at a very small scale.