Heterozygosity-fitness correlations and inbreeding in a reintroduced population of Stewart Island robins (Petroica australis rakiura)

Abstract

Inbreeding occurs when related individuals mate. Inbred individuals are more likely to inherit identical alleles at a given locus, decreasing heterozygosity and increasing the expression of deleterious recessive alleles, resulting in fitness losses or inbreeding depression. Since small, isolated populations will generally become increasingly inbred overtime, inbreeding depression is a potentially important factor influencing the persistence of threatened species.

Inbreeding within populations can be detected with pedigrees. Due to the difficulty of obtaining data, however, pedigrees are often unavailable for wild populations. Consanguineous mating and other demographic processes such as genetic drift or population admixture can result in increased non-random associations between diploid genotypes across loci or identity disequilibria (ID). When ID occurs, correlations between heterozygosity at neutral molecular markers (e.g. microsatellites) and traits that reflect fit- ness, or heterozygosity-fitness correlations (HFCs), may arise. Because of the association between inbreeding and ID, HFCs may facilitate the study of inbreeding depression when wild pedigrees are unavailable.

The primary aim of this thesis was to determine the effects of pedigree in- breeding (f) on HFCs in a wild, recently bottlenecked population of Stewart Island robins (Petroica australis rakiura). Stewart Island robins are small forest passerines endemic to New Zealand and a subspecies of the South Island robin (P. australis). On Stewart Island, population declines prompted the translocation of 25 individuals in 2000 to an open sanctuary on Ulva Island. This isolated and closely monitored population represents a rare opportunity to study a pedigreed wild avian population with frequent inbreeding.

In this thesis I present the development of 35 polymorphic microsatellite markers for use in Stewart Island robins. I also review the suitability of the Ulva Island robin pedigree for studying inbreeding depression and HFCs. The Ulva Island robin population represents a relatively large dataset (957 banded individuals studied over 10 years), with low levels of extra-pair paternity (EPP), a small proportion of unknown individuals, relatively high variance in f and high incidences of close and moderately close inbreeding. In addition, I found no evidence that unequal founder representation within the population has introduced a potentially confounding correlation between parental and offspring f.

I examined inbreeding depression using the pedigree, and HFCs in three life-history traits (hatching, fledging and juvenile survival). I report significant HFCs for hatching and fledging success that appear to reflect weak inbreeding depression. I also report, for the first time in an empirical study of a wild population, within-inbreeding class HFCs for juvenile survival that are influenced by pedigree f. More heterozygous same-nest siblings experience increased juvenile survival but only when inbred.

My results provide optimism for the continued use of molecular markers in the study of HFCs in evolution and conservation, particularly when populations are small and may accumulate inbreeding. My findings reinforce the need for further research specifically designed to consider factors that influence the detectability and strength of HFCs in wild populations. They also highlight the importance of considering not only pedigree f, but variation in realised inbreeding among individuals that share inbreeding coefficients.