Reconstructing the Demographic History of Ad´elie Penguins (Pygoscelis adeliae) Using mtDNA and Coalescent Methods

Abstract

Stratigraphy indicates that Ad´elie penguin population size has varied in time. The aim of this project will be to use phylogenetic inference and coalescent methods to test this idea. Ad´elie penguins inhabit the Antarctic coast with stringent habitat requirements. These penguins show high breeding site fidelity and breed in colonies of <100 to >440,000 individuals. The overall population is structured into regional populations composed of individual colonies. It is possible to infer changes in population size over time from the characteristics of phylogenetic trees. Such trees will be estimated from mitochondrial DNA using neutrally evolving gene sequences. DNA sequences were retrieved from GenBank, for both living birds and subfossil birds buried by new nests and preserved by the cold and dry environment. We can infer past population levels using the Bayesian skyline plot applied to these trees. This method uses information from the coalescent events, the timing of which reflects the effective population size. The analysis will be applied at differing levels: within individual colonies, over the Ross Sea region and over the entire species. Skyline plots are implemented in the software package BEAST, which will be used to undertake this analysis. Results show that the Ad´elie population size is strongly correlated with temperature, yet restricted by ice sheets. Population size was low during periods of low temperature and increased subsequent to temperature increases. Population growth was delayed due to the timing of retreat of the east and west Antarctic ice sheets. Migration appeared to impact the variation between colonies, rendering colony level analysis inappropriate. A strong genetic signal for population structure was discovered, a likely relict from retreat into refugia during the Last Glacial Maximum. This structure has no relationship with the overlaying physical structure of the Ad´elie penguins. Two putative locations for the refugia were discovered, the Antarctic haplotype originating in Enderby land and the Ross Sea haplotype originating in Oates land.