The history of giants - population genetics of the world's largest raptor

A project undertaken at the Alan Wilson Centre for Molecular Ecology and Evolution, Department of Anatomy, University of Otago, and supervised by Michael Knapp

New Zealand's extinct Haast's Eagle is an iconic example of island gigantism. The largest raptor in the world descended from a lineage of small eagles and went through the most extreme size increase known of any vertebrate species in the world. To date only one study has yielded insights into the genetics of Haast's Eagle. Based on genetic divergence reconstructed from a 1 kilobase (kb) fragment of the mitochondrial genome, Bunce et al. (2005) estimated the age of the most recent common ancestor of Haast's Eagle and its two closest relatives, the Little Eagle and the Booted Eagle, to be between 0.7 and 1.8 million years. As Haast’s Eagle was up to 15 times larger than these two relatives, this suggests that, in adapting to the New Zealand environment, Haast's Eagle went through one of the largest size increases (relative to time) known of any vertebrate species in the world.

Haast’s Eagle is therefore an ideal model for identifying the molecular adaptations underlying the evolution of island gigantism. However, due to previous technical limitations associated with sequencing ancient DNA, no functional genomic study or population study on Haast’s Eagle has so far been conducted. Thus, there is no information on effective population size or phylogeography of this formidable predator. This basic knowledge of Haast's Eagle biology is fundamental to any study investigating the evolution of a species as it helps to determine the size of the gene pool and the amount of gene flow between populations, both key components determining how selective forces influence a species.

With the support of the Australia and Pacific Science Foundation we have started to fill these gaps in knowledge. As part of a larger research program we have sequenced complete mitochondrial genomes from a significant proportion of known Haast's Eagle remains. We have also obtained the first nuclear genome data from this enigmatic species. Analyses of these datasets are ongoing and will provide insights in population dynamics of Haast's Eagle throughout the late Pleistocene and Holocene. The analyses will help identifying potential causes for Haast's Eagle extinction as well as the functional genomic basis for island gigantism in this iconic New Zealand bird species. Based on preliminary results from this work, further funding could be obtained and will be used to extend the research into the evolution and extinction of Haast's Eagle.

Fig. 1: (A) An artist's impression of Haast’s Eagle attacking the extinct New Zealand moa. (Artwork: John Megahan.) (B) Comparison of the huge claws of H. moorei with those of its close relative Little Eagle. (Figure modified from Bunce et al. 2005).

Fig. 2: Haast’s Eagle tarsometatarsus after sampling by drilling (drill hole on the right).

Fig. 3: DNA extraction in the University of Otago ancient DNA facility.