Genetic substructure in isolated and non-isolated populations of an endangered arboreal marsupial, the Leadbeaters possum (Gymnobelideus leadbeateri)

A project undertaken at the School of Biological Sciences, Monash University, and supervised by Andrea Taylor

Leadbeaters possum is a small cryptic non-gliding petaurid, which is patchily distributed and at low density within the central highlands region of Victoria. Since a significant proportion of the possum's distribution occurs on land set aside for timber production, its viability is heavily influenced by current timber management prescriptions (Lindenmayer and Lacy 1995). In 1986 a population was discovered in lowland swamp habitat at Yellingbo Flora and Fauna Reserve, on the fringe of but separate from other highlands populations. Subsequent to this discovery, another population was found in sub-alpine woodland at Lake Mountain. At both sites, nest boxes have been erected and inhabited by Leadbeaters possum colonies, providing relatively easy access for the purposes of ecological and sociobiological study.

Our previous genetic analyses have identified that the Yellingbo population is most likely an endemic remnant colony (rather than a recent introduction as rumoured) and is demographically isolated from its conspecifics. We have also identified strong genetic structuring within the Yellingbo population, indicating minimal exchange of individuals between the north and south sections. Analysis of genetic parentage at Yellingbo has indicated substantial discrepancies between parentage inferences based on the ecological data collected by Harley (2005), and those based on genetic data. In a large proportion of colonies, the presumed parents (breeding adults present in boxes during multiple visits) were genetically incompatible as the true parents of the offspring present in the box at the time of inspection. The extent to which this pattern is typical of Leadbeaters possum colony structure is entirely unknown, and we therefore aimed in this project to characterise colony kinship and social structuring in the non-isolated population at Lake Mountain.

Lake Mountain was visited three times over the course of the study, and samples taken from 26 nestbox colonies. A Bayesian clustering analysis of these genotypes has suggested strong population substructure similar to that at Yellingbo. In both cases this structure appears to be driven by colony structure and territory philopatry. Inspection revealed a single (apparently) breeding pair in each nest box at Lake Mountain. In most boxes, the breeding pair was accompanied by one or more juvenile and/or sub-adult animals. Each female in a pair was lactating, as revealed by the presence of enlarged teats. Parentage analysis has produced similar results to those obtained at Yellingbo, whereby the genetic data regarding family relationships are at odds with prior assumptions about colony kin structure.

Our analyses strongly indicate that the mating system and social organisation in this species is more complex than the previously assumed strict monogamy, regardless of the degree of population isolation.

References cited

Harley, D.K.P., (2005) The life history and conservation of Leadbeater's Possum (Gymnobelideus leadbeateri) in lowland swamp forest. PhD thesis, Monash University, Clayton, Victoria.

Lindenmayer, D.B., Lacy, R.C., 1995. Metapopulation viability of Leadbeater’s Possum, Gymnobelideus leadbeateri, in fragmented old-growth forests. Ecological Applications 5, 164-182.

Fig. 1. Juvenile Leadbeaters Possum at Yellingbo Nature Conservation Reserve

Fig. 2. Nest box in riparian thicket at Lake Mountain

Fig.3. Nest box in sub-alpine woodland at Lake Mountain