Evolution of diploid chromosome number, sex-determining systems, and heterochromatin in Western Mediterranean and Canarian species of the genus Pimelia (Coleoptera: Tenebrionidae)

A compilation of the diploid chromosome numbers and karyotype formulae of 30 species of the genus Pimelia from Morocco, Iberian Peninsula, Balearic and Canary Islands is presented. All species show a conservation of diploid numbers and karyotype formulae 2n = 18 (8 + Xy(p)) except for Pimelia cribra, Pimelia elevata, and Pimelia interjecta 2n = 20 (9 + Xyp) and Pimelia sparsa sparsa 2n = 18 (8 + neoXY). The ancestral state for the genus Pimelia is suggested to be 2n = 18 (8 + Xyp) in accordance with a previously described phylogeny of these species based on mitochondrial and nuclear DNA. The derived state 2n = 20 (9 + Xyp) is present in a monophyletic clade, which originated about 2.5-5 Mya. The male meiotic formula 8 + neoXY found in P. sparsa sparsa seems to have originated by the reorganization of the Xyp pair resulting in two homomorphic sexual chromosomes and the lost of most of the heterochromatin from the former X chromosome. In all chromosomes C-banding revealed conspicuous pericentromeric heterochromatic blocks, except in the Y chromosome in most of the species, and in situ hybridization of satellite DNA probes revealed the correspondence between heterochromatin and satellite DNA. Finally, the possible role of heterochromatin and satellite DNA is discussed in relation to the uniformity of the Tenebrionidae alpha-karyology.