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

The chromosomes of 26 taxa from Mexico of the tribes Passalini (three speci es) and Proculini (23 species) have been studied, increasing the karyotypic ally known species of the family Passalidae to 56. Karyotypic dynamism is h igh since the diploid number varies from 18 to 44 in the tribe Proculini, a nd from 25 to 31 in the tribe Passalini. In addition, supernumerary chromos omes, chromosome heteromorphism, translocations and possible sex multivalen ts have been found. These results contrast with the numerical conservatism found in related families of the superfamily Scarabaeoidea. However, both t ribes are conservative with regard to sex determination, as all species of Proculini have male XY chromosomes whereas species of the Passalini have ma le XO chromosomes. It is postulated that differences in patterns shown by t hese two tribes are mainly due to population structure, because many specie s of Proculini are endemic to restricted areas of Meso and South America, f avouring the settlement of karyotypic changes, whereas species of Passalini are distributed over large areas in the lowlands. It is also postulated th at the ancestral karyotypic formula of the family is close to 12-14 pairs o f autosomes although the ancestral male sex determination may be either XY or XO. At present only a weak relationship between morphological and karyot ypic evolution has been found, which together with the marked numerical var iability found within and between genera make it difficult to obtain phylog enetic conclusions from karyotypic results.