Phylogeography and population structure of an ecotonal marsupial, Bettongia tropica, determined using mtDNA and microsatellites

The northern bettong, Bettongia tropica, is an endangered species of Potoro idae with a restricted distribution in the wet tropics of north Queensland, Australia. The species is only found within a thin strip of sclerophyll fo rest along the western margin of rainforest. This tight association with ra inforest boundaries is predicted to have resulted in population isolation a s rainforest contracted during the Pleistocene, though some have proposed t hat the northern bettong was not present in the wet tropics until the late Pleistocene, The dispersal ability of the species, and of the family, is no t known. This study examined gene flow among populations within areas of co ntinuous habitat complemented by a broader analysis of phylogeography. Indi viduals trapped at each of the four known regions (one region was subsample d at three different sites), were sequenced for 547 base pairs of the mitoc hondrial DNA (mtDNA) control region and typed for seven microsatellite loci . The mtDNA phylogeny showed congruence with a biogeographical hypothesis, a relatively deep split suggesting historical isolation in separate norther n and southern refugia. The two divergent clades were both present within t he Lamb Range, indicating an expansion from these refuges and subsequent ad mixture at one site. mtDNA allele frequencies indicated relatively limited gene flow within the Lamb Range over distances as short as nine km. Tests o f population divergence using microsatellites (F-ST and assignment tests) s trongly supported this result. A molecular signal indicative of a recent bo ttleneck was unexpectedly detected in one of the Lamb Range subpopulations. This lead us to examine the behaviour of the statistics used in this bottl eneck test under a linear stepping-stone model with varying migration rates . We found that it may be more difficult to detect molecular signatures for recent bottlenecks under conditions of very low migration rates than for i solated populations and, conversely, that 'false' bottleneck signatures may be observed at higher migration rates. The Lamb Range F-ST estimate clearl y fell within the category of potentially 'false' bottleneck signals. Despi te relatively limited gene flow, evidence for asymmetric dispersal suggests more complicated population dynamics than a simple linear stepping-stone m odel.