The brushtail possum is a protected and treasured species in its native range of Australia, but also a devastating folivore and predator introduced into New Zealand. Here we report the first chromosome-level assembly of the possum genome, and using nuclear and mitochondrial analysis, trace invasion origins of one New Zealand location to distinct Tasmanian (Trichosurus vulpecula fuliginosus) and mainland Australian sub-species (Trichosurus vulpecula vulpecula), which have subsequently hybridised.
High nucleotide diversity in T. v. fuliginosus x T. v. vulpecula hybrid possums allowed us to distinguish expressed parental alleles throughout the genome. In doing so, we discovered candidate possum genes with imprinted, parent-specific expression not yet seen in other species, a result challenging the expectation that marsupial imprinting is restricted to orthologues of imprinted genes in humans and mice. We also confirmed imprinted expression of several genes already known to be imprinted, finding that the iconic IGF2 gene loses imprinted expression postnatally, in concert with silencing of the linked H19 non-coding RNA.
In order to understand how epigenetic marks like those controlling imprinting are reset at each generation, we examined genome-wide methylation in primordial germ cells. We find reprogramming of germline imprints in possum is reminiscent of that occurring in eutherian mammals, except that onset occurs after birth, and methylation erasure is less dramatic. Together, our data and analysis will underpin genetic-based control and conservation efforts for possums, but also contribute to our understanding of the evolution of novel mammalian epigenetic traits such as germline methylation erasure and genomic imprinting.