Karyotypic evolution and phylogeny of Mexican Passalidae (Coleoptera: Polyphaga: Scarabaeoidea)

The chromosomes of 26 taxa from Mexico of the tribes Passalini (three species) and Proculini (23 species) have been studied, increasing the karyotypically known species of the family Passalidae to 56. Karyotypic dynamism is high since the diploid number varies from 18 to 44 in the tribe Proculini. and from 25 to 31 in the tribe Passalini. In addition, supernumerary chromosomes, chromosome heteromorphism, translocations and possible sex multivalents have been found. These results contrast with the numerical conservatism found in related families of the superfamily Scarabaeoidea. However, both tribes are conservative with regard to sex determination, as all species of Proculini have male XY chromosomes whereas species of the Passalini have male XO chromosomes. It is postulated that differences in patterns shown by these two tribes are mainly due to population structure, because many species of Proculini are endemic to restricted areas of Meso and South America, favouring the settlement of karyotypic changes, whereas species of Passalini are distributed over large areas in the lowlands. It is also postulated that the ancestral karyotypic formula of the family is close to 12–14 pairs of autosomes although the ancestral male sex determination may be either XY or XO. At present only a weak relationship between morphological and karyotypic evolution has been found, which together with the marked numerical variability found within and between genera make it difficult to obtain phylogenetic conclusions from karyotypic results.