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

Ashlee Shipham,Leo Joseph,Jane M Hughes,Ian Mason,Daniel J Schmidt,Alex Drew

doi:10.1038/s41437-018-0127-5

Abstract

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.

Highlights

Hybridisation between recognised species is common and well-known
When plotting individuals by latitude (Fig. 3, Figures S1, S2), a clear zone emerged between latitudes of −27.836° and −30.167° within which individuals exhibit a range of plumages including predominantly pure P. adscitus and P. eximius
When generating hybrid indices based on all six characteristics (Fig. 3b), there was some variation in score throughout the sampled range of P. adscitus

Summary

Introduction

Hybridisation between recognised species is common and well-known (occurring in ~10% of species: Mallet 2005). The evolutionary impact of this depends on the strength and nature of pre- and post-zygotic barriers to gene flow (Jiggins and Mallet 2000). Even between multiple contact zones within a given species pair, there can be considerable variation in the outcome of hybridisation (Gompert et al 2017; Mandeville et al 2015, 2017). Outcomes of renewed contact include erosion of genetic boundaries (e.g. Taylor et al 2006), strengthening of premating barriers (i.e. reinforcement: Jiggins and Mallet 2000), maintenance of ‘tension zones’ through dispersal into the zone and selection against hybrids (Barton and Hewitt 1985), evolution of new species of hybrid origin (Lavretsky et al 2015; Mallet 2007), expansion or contraction of hybrid zones, and variable rates of introgression

Methods

Results

Discussion

Conclusion