Gene flow across ocean barriers: investigating genetic connectivity and mechanisms for dispersal in an enigmatic seabird, the red-tailed tropicbird (Phaethon rubricauda)

Romney Edwards-Francis

doi:10.25907/01003

The red-tailed tropicbird (Phaethon rubricauda, RTTR) is a pelagic seabird that occurs throughout the tropical regions of the Pacific and Indian oceans. In Australia, RTTR are listed as Vulnerable in Queensland under the ‘Nature Conservation Act 1992’ and in NSW under the ‘Biodiversity Conservation Act 2016’, and are considered a high conservation priority. Research on RTTR is largely conducted on island colonies, as their pelagic lives are cryptic and difficult to study, so information on their dispersal patterns is lacking, and colonies on the eastern coast of Australia have received little attention. RTTR exhibit nest-site fidelity, and colonies are often treated as separate populations. However, the formation of a new colony on Lady Elliot Island in the 1980’s suggests that dispersal between islands is occurring, and raises questions about the population genetics of RTTR in Australia. The aim of this project was to investigate the genetic connectivity and mechanisms for dispersal of red-tailed tropicbirds in eastern Australia. Five colonies are found in the region: Lady Elliot Island in the southern Great Barrier Reef (GBR), Raine Island in the northern GBR, the Coringa-Herald Cays in the Coral Sea, and Lord Howe Island and Phillip Island (Norfolk Island) in the Tasman Sea. The first chapter of this thesis examines sex-biased dispersal as a mechanism for genetic variability in the Lady Elliot Island colony, the smallest (~6 pairs) and youngest (founded in 1983) of the five colonies found of eastern Australia. The aim of this chapter was to examine the mechanisms of colony formation in RTTR, map genetic relatedness in the colony, and to assess the effects of inbreeding on nesting success and mate fidelity, to determine if inbreeding avoidance is occurring. The genetic sex of 38 Lady Elliot Island individuals was determined through PCR amplification of extracted DNA, and genetic markers were identified in the single nucleotide polymorphisms (SNPs) of all individuals. Combined with mark-recapture data from Lady Elliot Island, this enabled the examination of sex-biased philopatric behaviour on Lady Elliot Island, and analyses of genetic variability between males and females in the entire dataset. Relatedness was found to be high within the colony, with only eight individuals identified as not originating on Lady Elliot Island. A family tree was produced, based on pedigree reconstruction, re-trap data, and estimates of kinship (φ). This showed that four core breeding pairs have produced the majority of offspring in the colony, and relatedness of these individuals means that only two family groups are present on Lady Elliot Island. Despite this, average inbreeding was not found to be high (FIS = 0.021, SD = 0.192). A negative relationship was found between kinship and nesting success, but due to a small sample size, was not statistically significant (P >0.005). Philopatry was observed in the population, and showed a male-bias in the re-trap data, but this was not supported by genetic data. Mechanisms for inbreeding avoidance were not found statistically, so this colony may be avoiding inbreeding depression through dispersal of fledglings from other colonies, and random mating. Continued sampling of this colony will allow for larger sample sizes and more robust data. In Chapter 2, the genetic connectivity of five RTTR colonies in eastern Australia was examined with Ashmore Reef in Western Australia as an outgroup. Individuals from each colony were sampled for genetic analyses using SNPs. After filtering and data preparation, these data were used to investigate genetic structuring and diversity both within and between the six colonies. Genetic structuring was found to be present, aligning with the geographic distribution of colonies. Two genetic clusters were identified, consisting of colonies located in the Tasman Sea (Lord Howe Island and Phillip/Norfolk Island), and two colonies in the Coral Sea and GBR (Coral Sea Islands and Raine Island). Lady Elliot Island showed even admixture of both of these clusters, suggesting that dispersal is occurring from all four colonies to this island. Ashmore Reef also demonstrated significant admixture of both clusters, indicating that connectivity exists between colonies in eastern Australia and western Australia. Estimates of genetic health through observed (HO) and expected heterozygosity (HE), allelic richness (AR), and inbreeding (FIS) were not found to be significantly different between colonies, supporting the finding that dispersal is occurring between them. This thesis contributes new knowledge to our current understanding of red-tailed tropicbirds in eastern Australia, but also our broader knowledge of the species globally. Evidence from this thesis, and specifically, genetic connectivity throughout the region, suggests that management of this species should broaden to encompass the entire RTTR population, and provides baseline information about the means of dispersal and colony establishment crucial for aspects of management and conservation.

Gene flow across ocean barriers: investigating genetic connectivity and mechanisms for dispersal in an enigmatic seabird, the red-tailed tropicbird (Phaethon rubricauda)

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