A genomic perspective on an Australian hybrid zone.
Ever heard of Julian Ford? During the 1970s, this Australian ornithologist studied numerous hybrid zones across Australia. His 1987 Emu paper “Hybrid Zones in Australian Birds” lists about 100 contact zones. At the time, molecular techniques were in their infancy, so Ford relied on morphological descriptions. With the advent of genetic – and later genomic – data, some researchers are revisiting these hybrid zones. A recent study in Heredity focuses on two rosella species.
A Moving Hybrid Zone?
Hybrid parrots are pretty common in captivity (see this recent series of photos), but these colorful birds also interbreed in the wild. Ashlee Shipham and her colleagues revisited a hybrid zone between pale-headed rosella (Platycercus adscitus) and eastern rosella (P. eximius), which has previously been described using phenotypic data. They sequenced the DNA of 139 birds, collected across the contact zone in eastern Australia.
A phylogenetic study indicated that past hybridization has resulted in the exchange of mtDNA between these parrot species (see this recent blog post on mitochondrial capture in another system). The present study revealed that hybridization is still ongoing, and that ‘contemporary hybridization extends beyond the recently defined limits of the hybrid zone.’ Indeed, the position of the hybrid zone uncovered in this study does not correspond to that reported in previous studies. This discrepancy can be due to observer bias in past studies or the hybrid zone has actually moved over time. More research is needed to figure this out.
Respecting Species Boundaries
Interestingly, there were no first generation hybrids in the dataset. All admixed individuals were multi-generational or back-crossed birds. This suggests that there is some degree of reproductive isolation between pale-headed and eastern rosella. The exact nature of this isolation mechanism remains to be uncovered, but it could include assortative mating based on plumage, vocalizations or odor (yes, parrots can use smell to recognize each other).
Finally, there was a strong correlation between genomic data and morphology. It thus seems feasible to identify hybrids and back-crosses based on plumage patterns (something that is not always straightforward, see for example hybrids between Saltmarsh Sparrow and Nelson’s Sparrow). This provides great opportunities for further research into this interesting hybrid zone.
Shipham, A., Joseph, L., Schmidt, D.J., Drew, A., Mason, I. & Hughes, J.M. (2018) Dissection by genomic and plumage variation of a geographically complex hybrid zone between two Australian parrot species, Platycercus adscitus and Platycercus eximius. Heredity.
The paper has been added to the Psittaciformes page.