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

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

Laying dates (mean ± SD in days, sample size, and range in parentheses) of conspecific and Type A mixed-species pairs (southern ♂ x northern ♀) of giant petrels at Bird Island, South Georgia, 2005/06 to 2011/12.

<p>Laying dates (mean ± SD in days, sample size, and range in parentheses) of conspecific and Type A mixed-species pairs (southern ♂ x northern ♀) of giant petrels at Bird Island, South Georgia, 2005/06 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

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

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

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