The evolutionary origin of a novel karyotype in Timarcha (Coleoptera, Chrysomelidae) and general trends of chromosome evolution in the genus

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Abstract
In this work, we have analysed the karyotypes of six species of Timarcha for the first time and updated the cytological information for two additional taxa, for one of them confirming previous results (Timarcha erosa vermiculata), but not for the other (T. scabripennis). We describe the remarkable karyotype of T. aurichalcea, the lowest chromosome number in the genus (2n = 18), distinctive as well for the presence of an unusual chiasmatic sexual bivalent hitherto unreported for Timarcha. This study increases the number of species studied cytologically in this genus to forty. Additional cytogenetic analyses are performed on several species, including Ag-NOR staining and fluorescent in situ hybridization (FISH) studies with ribosomal DNA probes. Karyotype evolution is analysed by tracing different karyotype coding strategies on a published independent phylogenetic hypothesis for Timarcha based on the study of three genetic markers. The implementation of a likelihood model of character change optimized onto the phylogeny is tentatively used to detect possible drifts in chromosome changes. These analyses show that karyotype is conservative in the evolution of the genus and that there is an apparent trend to reducing chromosome number. Cytological and phylogenetic data are used to explain the evolutionary origin of the karyotype of T. aurichalcea by two centric fusions involving one pair of acrocentric autosomes and the sexual chromosomes.