Evolutionary significance of fixed chromosomal variants in South American populations of the leopard beetle Calligrapha polyspila (Coleoptera: Chrysomelinae)

Chrysomelinae species have a striking ecological specialization due to their trophic selection on plants usually belonging to the same botanical family. Calligrapha (Chevrolat) (Coleoptera, Chrysomelidae) is a diverse genus that comprises about 80 species distributed in most of the American continent. Cytogenetic studies of Calligrapha refer to 18 species with a modal diploid chromosome number 2n = 23 = 22 + XO (male) and a meioformula n = 11 + XO (male). A population of C. polyspila (Germar) from República Oriental del Uruguay shows 2n = 24 = 22 + Xyp (n = 12 = 11 + X/yp, male), which is coincident with the karyotype currently considered as the ancestral one. We performed for the first time a detailed karyotype analysis in specimens of C. polyspila from Argentinean natural populations. We described the male karyotype and analyzed the meiotic behavior and frequency and distribution of chiasmata. We also examined the chromatin organization and constitution by determining the amount, composition, and distribution of constitutive heterochromatin and the number and location of nucleolus organizer regions. The presence of a novel 2n in C. polyspila from República Argentina allows proposing different trends of karyotype differentiation that may have occurred in the Argentinean and Uruguayan allopatric populations. Our results provide more information on the karyology of Chrysomelinae and contribute to discussing the evolutionary significance of the different cytological features and the fixed chromosomal variants observed in these South American populations of C. polyspila.