Rock-wallabies teach us about evolution, as well as looking cute.

A recent study of north Queensland rock-wallabies has contradicted a long-held theory on how species originate. Although each of these wallaby species has unique chromosomes, these chromosome rearrangements alone are not responsible for them becoming different species. Further studies of these wallabies are underway as they are likely to teach us much about how species on planet Earth have diversified.

Understanding the detailed mechanisms by which new species form (i.e., the process of speciation) remains one of the great mysteries of biology. The iconic Australian rock-wallabies (Petrogale) show extreme chromosome diversity compared to other marsupials, particularly the six species in northeast Queensland found from Rockhampton to Cape York Peninsula. These closely related species have been thought a clear example of chromosomal speciation as they are otherwise indistinguishable but differ markedly in the shape and number of their chromosomes. It has long been assumed that these chromosome changes were driving speciation in this group since genetic theory predicts they should result in significantly reduced fertility in hybrid individuals and so prevent gene flow between the chromosomally different rock-wallaby species.

However, in a recent analysis using high resolution genetic markers, we surprisingly found no association between the degree of chromosome differences and the extent of gene flow amongst these species. In fact, high levels of gene flow were detected between some species, even though they differ by multiple chromosome changes. In contrast, other species with similar chromosomes showed little evidence of gene flow between them. Clearly these wallabies have not read the text books! These species also continue to provide a head-ache for researchers and wildlife managers, since they can’t be readily identified by standard morphological or genetic tests.

Our seemingly contradictory results suggest that the causes of speciation in this fascinating group of marsupials are far more complex than had been thought and cannot be attributed to changes in chromosome shape and number alone. Additional studies of the genomes of these rock-wallaby species are now underway and it is hoped that they will reveal the secrets of speciation in this group and so lift the veil on the mechanisms by which life has proliferated on Earth.

Dr Mark Eldridge
Principal Research Scientist, Australian Museum Research Institute

Dr Sally Potter
Australian National University and Australian Museum Research Institute

More information:
Potter, S., Moritz, C. and Eldridge, M.D.B. (2015). Gene flow despite complex Robertsonian fusions amongst rock-wallaby (Petrogale) species. Biology Letters, 11 (10): DOI: 10.1098/rsbl.2015.0731

This research formed part of Sally Potter’s Chadwick Biodiversity Fellowship at the Australian Museum Research Institute.