Chromosome variation in British populations of Oncopsis (Hemiptera:Cicadellidae)

The British species of Oncopsis fall into two groups with respect to male karyotype. Four species—O. avallanae, O. carpini and O. subangulata, together with an undescribed new species close to carpini—consistently show ten autosome pairs and a single X-chromosome (2n=10AII+XO). In O. tristis too the XO state predominates but single neo-sex chromosome variants with nine and eight autosome pairs respectively have also been found. The two remaining species-O. alni and O. flavicollis—both regularly include derived neo-XY states involving the incorporation of autosomal material onto the X(2n=9AII+XY). The single population of O. alni studied was entirely neo-XY but the situation in O. flavicollis proved more complex. Montane populations which occur on Betula pubescens are XO-monomorphic whereas populations in lowland woodlands are polymorphic, including both XO and neo-XY types. In such situations the XO forms are found predominantly on B. pubescens whereas the XY morphs predominate on a second species of birch, B. pendula, which is absent from montane woodlands. In all three species where the neo-XY state is present, different autosomes have been involved in the fusion process.