Gene Flow and Hybridization between Numerically Imbalanced Populations of Two Duck Species on the Subantarctic Island of South Georgia

Abstract

Hybridization is common between species of animals, particularly in waterfowl (Anatidae). One factor shown to promote hybridization is restricted mate choice, which can occur when 2 species occur in sympatry but one is rare. According to the Hubbs principle, or "desperation hypothesis," the rarer species is more likely to mate with heterospecifics. We report the second of 2 independent examples of hybridization between 2 species of ducks inhabiting island ecosystems in the Subantarctic and South Atlantic Ocean. Yellow-billed pintails (Anas georgica) and speckled teal (Anas flavirostris) are abundant in continental South America, where they are sympatric and coexist in mixed flocks. But on South Georgia, an isolated island in the Subantarctic, the pintail population of approximately 6000 pairs outnumbers a small breeding population of speckled teal 300∶1. Using 6 genetic loci (mtDNA and 5 nuclear introns) and Bayesian assignment tests coupled with coalescent analyses, we identified hybrid-origin speckled teal alleles in 2 pintails on South Georgia. While it is unclear whether introgression has also occurred into the speckled teal population, our data suggest that this hybridization was not a recent event, but occurred some time ago. We also failed to identify unequivocal evidence of introgression in a much larger sample of pintails and speckled teal from Argentina using a 3-population "Isolation-with-Migration" coalescent analysis. Combined with parallel findings of hybridization between these same 2 duck species in the Falkland Islands, where population ratios are reversed and pintails are outnumbered by speckled teal 1:10, our results provide further support for the desperation hypothesis, which predicts that scarcity in one population and abundance of another will often lead to hybridization. While the South Georgia pintail population appears to be thriving, it's possible that low density of conspecific mates and inverse density dependence (Allee effect) may be one factor limiting the reproductive output of the speckled teal population, and this situation may persist unless speckled teal increase in abundance on South Georgia.

Convergent Evolution of Hemoglobin Function in High-Altitude Andean Waterfowl Involves Limited Parallelism at the Molecular Sequence Level

“…The derived pair of β A -globin amino acid variants ‘116Ser-133Met’ that are shared between sympatric high-altitude populations of yellow-billed pintails and speckled teal are clearly identical-by-descent ( S6 Fig ). Independent evidence for hybridization between the two species [ 40 , 41 ] suggests that the ‘116Ser-133Met’ β A -globin allele in high-altitude yellow-billed pintails was derived via introgression from high-altitude speckled teals. The same is true for a shared β13(Gly/Ser) polymorphism, although the derived Ser variant is present at low-frequency in yellow-billed pintails.…”

“…One notable case of homoplasy in the β A -globin gene involved sites 116 and 133 in high-altitude yellow-billed pintails and speckled teal ( S4 Fig ), two species that are known to hybridize in nature [ 40 , 41 ]. To assess whether identical two-site ‘β116Ser-β133Met’ haplotypes from the two species were identical-by-descent, we reconstructed haplotype networks of β A -globin coding sequence using the median-joining algorithm [ 76 ], as implemented in the program Network 4.6 (Fluxus Technology, Suffolk, UK).…”

Introgression between Sphyrapicus nuchalis and S. varius sapsuckers in a hybrid zone in west‐central Alberta

“…Such density troughs create conditions that can slow the movement of, or even trap, hybrid zones (Barton and Hewitt 1985, Hewitt 1988). Immigration from larger populations to the north and/or to the south may contribute individuals to these zones, but limited opportunities to mate with conspecifics in ‘density traps' with many hybrids may further promote hybridization (Hewitt 1988, McCracken et al 2013).…”

Persistence of an endangered native duck, feral mallards, and multiple hybrid swarms across the main Hawaiian Islands

“…Hubbs's principal states that females should prefer to mate with a male of their own species, but in their absence mating with a male from a different species is better than not mating at all (Randler, ). Thus, interspecific hybridization may be more likely when conspecific mates are rare (van Dongen, Lazzoni, Winkler, Vásquez, & Estades, ; McCracken & Wilson, ; McCracken, Wilson, & Martin, ; Steeves, Maloney, Hale, Tylianakis, & Gemmell, ; White & Clausen, ). Additionally, banding data ( n = 1,093 birds; B. Dugger and C. Malachowski, unpublished data), diurnal behavioural studies ( n = 984; Malachowksi, Dugger, & Uyehara, ) and botulism records ( n = 78; K. Uyehara, unpublished data) all indicate a male‐biased sex ratio of over 3:1 at Hanalei, suggesting that breeding females are likely able to find conspecific mates (Steeves et al, ).…”