Ian J. Mason

Relationships of the New Guinean subspecies, Gymnorhina tibicen papuana, of the Australian Magpie: an assessment from DNA sequence data

ABSTRACT Although it is well understood that New Guinea and Australia are essentially one landmass that connects periodically during sea-level fluctuations, the biogeographic history and effects of environmental change for species that span the two islands has not been studied in depth. The New Guinean subspecies of the Australian Magpie, Gymnorhina tibicen papuana, is known to share morphological traits with subspecies from north-western and south-western Australia (G. t. longirostris and G. t. dorsalis, respectively). Here, we use molecular data to explore the relationships of the New Guinean subspecies. Mitochondrial DNA supports a close relationship with a previously identified Australian phylogroup, which includes the two western Australian subspecies G. t. longirostris and G. t. dorsalis and western populations of what has been recognised as a northern subspecies G. t. eylandtensis. We infer that the formation of land bridges, most likely during the most recent glacial period, allowed movement of magpies between New Guinea and Australia but not, evidently, involving Cape York Peninsula, which is that part of Australia closest today to the range of G. t. papuana. Dispersal would have ceased following the most recent sea-level rise 16 500 years ago. Overall, our study is consistent with a hypothesis that savannah-associated species dispersed between Australia and New Guinea via a western route across land bridges associated with the Sahul Shelf.

Dissection by genomic and plumage variation of a geographically complex hybrid zone between two Australian non-sister parrot species, Platycercus adscitus and Platycercus eximius

The study of hybrid zones advances understanding of the speciation process, and approaches incorporating genomic data are increasingly used to draw significant conclusions about the impact of hybridisation. Despite the progress made, the complex interplay of factors that can lead to substantially variable hybridisation outcomes are still not well understood, and many systems and/or groups remain comparatively poorly studied. Our study aims to broaden the literature on avian hybrid zones, investigating a potentially geographically and temporally complex putative hybrid zone between two native Australian non-sister parrot species, the pale-headed and eastern rosellas (Platycercus adscitus and Platycercus eximius, respectively). We analysed six plumage traits and >1400 RADseq loci and detected hybrid individuals and an unexpectedly complex geographic structure. The hybrid zone is larger than previously described due to either observer bias or its movement over recent decades. It comprises different subregions where genetic and plumage signals of admixture vary markedly in their concordance. Evidence of contemporary hybridisation (later generation and backcrossed individuals) both within and beyond the previously defined zone, when coupled with a lack of F1 hybrids and differential patterns of introgression among potentially diagnostic loci, indicates a lack of post-zygotic barriers to gene flow between species. Despite ongoing gene flow, species boundaries are likely maintained largely by strong pre-mating barriers. These findings are discussed in detail and future avenues for research into this system are proposed, which would be of benefit to the speciation and hybrid zone literature.