Recent speciation and elevated Z-chromosome differentiation between sexually monochromatic and dichromatic species of Australian teals

Sex chromosomes potentially have an important role in speciation and often have elevated differentiation between closely related species. In birds, traits associated with male plumage, female mate preference, and hybrid fitness have been linked to the Z-chromosome (females are heterogametic, ZW). We tested for elevated Z-differentiation between two recently diverged species of Australian ducks, the sexually monochromatic grey teal Anas gracilis and the dichromatic chestnut teal A. castanea. Despite prominent morphological differences, these two species are genetically indistinguishable at both mitochondrial DNA (mean ST 0.0001) and 17 autosomal loci (mean ST 0.0056). However, we detected elevated Z-differentiation (mean ST 0.281) and tentative evidence of an island of differentiation on the Z-chromosome. is elevated differentiation was explained by a high frequency of derived alleles in chestnut teal that were absent in grey teal, which parallels independent evidence for a gain in dichromatism from a monochromatic ancestor. Coalescent estimates of demographic history and simulations indicated that the elevated Z-differentiation was unlikely to be explained by neutral processes, but instead supported a role of divergent selection. We discuss evidence for models of speciation with gene flow versus adaptive divergence in the absence of gene flow and find that both hypotheses are plausible explanations of the data. Overall, these teal have the weakest background differentiation documented to date for a species showing a large Z-effect, and they are an excellent model species for studying speciation genomics and the evolution of sexual dichromatism.Kirandeep K. Dhami, Leo Joseph, David A. Roshier, Jeffrey L. Peter

Population genomics and phylogeography of four Australasian waterfowl

OnlinePublBiogeographic barriers can restrict gene flow, but variation in ecological drivers of dispersal influences the effectiveness of these barriers among different species. Detailed information about the genetic connectivity and movement of waterfowl across biogeographic barriers in northern Australia and Papua New Guinea is limited. We compared genetic connectivity for four species of Australasian waterfowl that vary in their capacity and predisposition for dispersal: Radjah Shelduck (Radjah radjah), Wandering Whistling Duck (Dendrocygna arcuata), Green Pygmy Goose (Nettapus pulchellus), and Pacific Black Duck (Anas superciliosa). We obtained >3,700 loci from double-digest restriction-associated DNA sequencing for 15 to 40 individuals per species and found idiosyncratic patterns of population structure among the four species. The mostly sedentary Radjah Shelduck exhibited clear genetic differences between New Guinea and Australia as well as among locations within Australia. Although the population structure was consistent with isolation by distance, the Torres Strait and Carpentaria Barrier contributed more to genetic differences than geographic distance alone. In contrast, the presumed sedentary Green Pygmy Goose did not show obvious structure. Likewise, populations of the more dispersive Wandering Whistling Duck and Pacific Black Duck were unstructured and genetically indistinguishable between southern New Guinea and northern Australia. Our data suggest that some Australo-Papuan biogeographical barriers are insufficient to impede gene flow in waterfowl species capable of dispersing great distances. In sedentary species like the Radjah Shelduck, these barriers, perhaps coupled with its ecology and natural history, restrict gene flow. Our findings bring new insight into the population ecology of Australo-Papuan waterfowl.Sara R. Seibert, Leo Joseph, Jonathan Bowers, Philip Lavretsky, Alex Drew, Ian Mason, David A. Roshier, Bulisa Iova and Jeffrey L. Peter