Hybridization and back-crossing in giant petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a summary of hybridization in seabirds.

Ruth M Brown,Richard A Phillips,N M S Mareile Techow,Andrew G Wood,Hans-Ulrich Peter

doi:10.1371/journal.pone.0121688

Abstract

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.

Highlights

Speciation, the process by which taxa evolve mechanisms conferring reproductive isolation, is generally considered to occur when geographically isolated populations gradually acquire genetic differences, either through selection or drift [1,2]
Three different types of mixed species pairs have been identified at Bird Island, referred to as Type A [17], Type B [17] and Type C
Average breeding success was 57.3% for northern giant petrels, 44.9% for southern giant petrels and 15.6% for Type A mixed-species pairs, sample sizes for this last group each year are small

Summary

Introduction

Speciation, the process by which taxa evolve mechanisms conferring reproductive isolation, is generally considered to occur when geographically isolated populations gradually acquire genetic differences, either through selection or drift [1,2]. Hybridization in Giant Petrels when genetic incompatibilities between sibling taxa become so great that offspring are infertile or unviable. If allopatric populations come into contact before this point is reached there is the potential for interbreeding and genetic homogenization, unless behavioural isolating mechanisms, such as differences in the timing of breeding or incompatibility in mating displays, are sufficient to maintain reproductive isolation [3]. The study of hybridization in natural populations provides important insights into the processes of evolutionary diversification, as well as contributing to accurate identification of species boundaries

Methods

Results

Conclusion