Genetic differentiation and natural hybridization between two morphological forms of the common woodlouse, Oniscus asellus Linnaeus 1758

The common woodlouse Oniscus asellus can be divided into two forms on the basis of morphology, particularly male accessory genitalia. Where these taxa meet, morphological intermediates are found, and the forms were therefore described as subspecies; O. a. asellus and O. a. occidentalis. In this study allozyme loci are used to test the hypothesis that intermediate forms result from hybridization, and to study the nature of hybridization. Thirteen enzyme loci were scored across five English sites representative of each subspecies and intermediates. Ten loci showed strong frequency differences between asellus and occidentalis populations, although no loci showed completely fixed differences. These data confirm that asellus and occidentalis represent genetically distinct taxa, and that intermediate populations are of hybrid origin. There is apparently substantial population substructuring in the contact zone, as indicated by deficits of heterozygotes (FIS) and sporadic gametic (i.e. linkage) disequilibria. Population structure in the Oniscus hybrid zone appears to be analogous to that seen in plant hybrid swarms rather than the narrow hybrid zones observed in many animal taxa. Values of Nei’s genetic distance between the subspecies range from 0.65 to 0.70; these are much higher than between typical conspecific taxa and are indicative of ancient genetic divergence. However, because asellus and occidentalis do not remain distinct in areas of overlap, it is simplest to regard these taxa as members of the same species.