Cytogenetics of the Herbert River ringtail possum, Pseudocheirus herbertensis (Diprotodonta: Pseudocheiridae): evidence for two species

The two Herbert River ringtail possum subspecies, Pseudocheirus herbertensis ssp. herbertensis and P. h. ssp. cinereus, have diploid chromosome numbers of 12 and 16, respectively. The sex chromosomes of both subspecies are exceptionally large, with the X and Y chromosomes being approximately 16 and 12% of the haploid autosomal complement, respectively. A sex chromosome bivalent cannot be identified during male meiosis and a sex vesicle is not present during pachytene. The two karyotypes are most likely related by two centric fusion events affecting the autosomal complement. We conclude that the X and Y chromosomes have been translocated onto homologous autosomes to give t(XA) t(YA) ♀ t(XA) t(XA) ♂. Our data also strongly support the separation of P. h. cinereus as a distinct species.Key words: karyotypes, sex chromosomes, speciation.