Hybridization and Back-Crossing in Giant Petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a Summary of Hybridization in Seabirds

Hybridization in natural populations provides an opportunity to study the evolutionary processes that shape divergence and genetic isolation of species. The emergence of pre-mating barriers is often the precursor to complete reproductive isolation. However, in recently diverged species, pre-mating barriers may be incomplete, leading to hybridization between seemingly distinct taxa. Here we report results of a long-term study at Bird Island, South Georgia, of the extent of hybridization, mate fidelity, timing of breeding and breeding success in mixed and conspecific pairs of the sibling species, Macronectes halli (northern giant petrel) and M . giganteus (southern giant petrel). The proportion of mixed-species pairs varied annually from 0.4-2.4% (mean of 1.5%), and showed no linear trend with time. Mean laying date in mixed-species pairs tended to be later than in northern giant petrel, and always earlier than in southern giant petrel pairs, and their breeding success (15.6%) was lower than that of conspecific pairs. By comparison, mixed-species pairs at both Marion and Macquarie islands always failed before hatching. Histories of birds in mixed-species pairs at Bird Island were variable; some bred previously or subsequently with a conspecific partner, others subsequently with a different allospecific partner, and some mixed-species pairs remained together for multiple seasons. We also report the first verified back-crossing of a hybrid giant petrel with a female northern giant petrel. We discuss the potential causes and evolutionary consequences of hybridization and back-crossing in giant petrels and summarize the incidence of back-crossing in other seabird species

The origins of white-chinned petrels killed by long-line fisheries off South Africa and New Zealand

The white-chinned petrel (Procellaria aequinoctialis) is the seabird species most frequently killed by fisheries in the Southern Ocean and is listed by the International Union for the Conservation of Nature and Natural Resources as globally vulnerable. It breeds around the sub-Antarctic, but genetic data identified two subspecies: P. a. aequinoctialis from islands in the Atlantic and Indian Oceans and P. a. steadi from the New Zealand sub-Antarctic islands. We identify the region of origin of birds killed by two long-line fisheries based on differences in the mitochondrial gene cytochrome b. All 113 birds killed off South Africa had the haplotype of P. a. aequinoctialis, whereas all the 60 birds from New Zealand had P. a. steadi haplotypes. The two subspecies of white-chinned petrels thus appear to disperse to different regions irrespective of their age, which accords with the tracking data of adult birds. Our finding has significant implications for managing the bycatch of this species by regional fisheries.Keywords: Fishery bycatch; Procellaria aequinoctialis; genetic structure; genetic diversity; demographic impact.(Published: 21 June 2016)To access the supplementary material for this article, please see the supplementary file in the column to the right (under Article Tools).Citation: Polar Research 2016, 35, 21150, http://dx.doi.org/10.3402/polar.v35.2115

Breeding locations of northern (circles) and southern (triangles) giant petrels.

<p>Distribution data sourced from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121688#pone.0121688.ref009" target="_blank">9</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121688#pone.0121688.ref011" target="_blank">11</a>].</p

Numbers of conspecific and mixed-species pairs of giant petrels recorded at Bird Island, South Georgia, 1978/79 to 2011/12.

<p>Type B mixed-species pairs—females were assumed to be hybrids based on bill colour [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121688#pone.0121688.ref017" target="_blank">17</a>].</p><p>Study area was different in 1978/79 to 1980/81.</p><p>Numbers of conspecific and mixed-species pairs of giant petrels recorded at Bird Island, South Georgia, 1978/79 to 2011/12.</p

Results of t-tests (assuming unequal variance) comparing mean laying date of northern giant petrels and Type A mixed-species pairs (southern ♂ x northern ♀) in different years at Bird Island, South Georgia.

<p>Results of t-tests (assuming unequal variance) comparing mean laying date of northern giant petrels and Type A mixed-species pairs (southern ♂ x northern ♀) in different years at Bird Island, South Georgia.</p

Breeding success (%) of giant petrels at Bird Island, South Georgia; 2002/03 to 2011/12.

<p>Breeding success (%) of giant petrels at Bird Island, South Georgia; 2002/03 to 2011/12.</p

Distribution of laying dates for giant petrels at Bird Island, South Georgia; 2005/06 to 2011/12.

<p>Type A = southern ♂ x northern ♀; Type C = hybrid ♂ x northern ♀.</p