Time-calibrated phylogeny and ecological niche models indicate Pliocene
aridification drove intraspecific diversification of brushtail possums
in Australia.
Abstract
Major aridification events in Australia during the Pliocene may have had
significant impact on the distribution and structure of widespread
species. To explore the potential impact of Pliocene and Pleistocene
climate oscillations we estimated the timing of population fragmentation
and past connectivity of the currently isolated but morphologically
similar subspecies of the widespread brushtail possum (Trichosurus
vulpecula). We use ecological niche modelling (ENM) with the current
fragmented distribution of brushtail possum to estimate the
environmental envelope of this marsupial. We projected the ENM on models
of past climatic conditions in Australia to infer the potential
distribution of brushtail possums over six million years. D-loop
haplotypes were used to describe population structure. From shotgun
sequencing we assembled whole mitochondrial DNA genomes and estimated
timing of intraspecific divergence. Our projections of ENMs suggest
current possum populations were unlikely to have been in contact during
the Pleistocene. Although lowered sea level during glacial periods
enabled colonisation of Tasmania, climate fluctuation during this time
would not have facilitated gene flow. The most recent common ancestor of
sampled intraspecific diversity dates to the early Pliocene when
continental aridification caused significant changes to Australian
ecology and Trichosurus vulpecula distribution was likely fragmented.
Phylogenetic analysis revealed that the subspecies T. v. hypoleucus
(koomal; southwest), T. v. arnhemensis (langkurr; north) and T. v.
vulpecula (bilda; southeast) correspond to distinct mitochondrial
lineages. Despite little phenotypic differentiation, Trichosurus
vulpecula populations probably experienced little gene flow with one
another since the Pliocene, supporting the recognition of several
subspecies and explaining their adaptations to the regional plant
assemblages on which they feed.