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Conservation Genetics and Hybridisation of the Forbes’ Parakeet (Cyanoramphus Forbesi) in the Chatham Islands

This study describes the isolation and characterisation of microsatellite loci in Forbes’ parakeet (Cyanoramphus forbesi). These molecular markers are used to assess the status of interspecific hybridisation between Forbes’ parakeets and Chatham Island Red-crowned parakeets (C. novaezelandiae chathamensis) on Mangere and Little Mangere Islands in the Chatham Islands group. The evolution of these microsatellite loci in parrots is also investigated. Forbes’ parakeets are facing the problem of genetic introgression due to extensive hybridisation with Chatham Island Red-crowned parakeets. Hybrids show a spectrum of crown morphologies between the parent species (Nixon 1982), but identification of hybrids based on morphology alone is not foolproof. Mitochondrial DNA studies by Boon (2000) showed that Forbes’ parakeet is a distinct lineage, basal to all other New Zealand parakeets but several Forbes’ morphotypes have Chatham Island Redcrowned parakeet haplotype. However, mitochondrial DNA markers only probe the maternally inherited lineages and cannot tell the whole story. The microsatellite markers used in this study show that interspecific hybridisation between Forbes parakeet and Chatham Island Red-crowned parakeet is more extensive than previously expected. Microsatellite data combined with results from scoring mitochondrial DNA haplotypes show that crown morphology alone under-represents the proportion of hybrids in the population, and that a large number of cryptic hybrids (77.9%) show Forbes’ parakeet morphotypes. A three factor scoring system is suggested in which a parakeet must pass both genetic (microsatellites and mitochondrial) and morphological criteria to be considered a “pure” Forbes’ parakeet. Using this system, 17.8%, 1.2%, and 81.0% of the Mangere Island parakeet population are “pure” Forbes’ parakeets, “pure” Chatham Island Red-crowned parakeets, and interspecific hybrids respectively. The results of this study have implications for the future conservation of Forbes’ parakeet. Because interspecific hybridisation makes no positive contribution to the long-term survival of Forbes’ parakeets, conservation measures to limit contact between the two species should be taken to control further hybridisation of parakeets on Mangere Island. The microsatellite loci isolated in this study are found to evolve following the patterns best described by either the Stepwise Mutation Model (SMM; Ohta & Kimura 1973) or the Two-Phase Model (TPM; Di Rienzo et al. 1994) at population level. By mapping the evolutionary changes in repeat motif variations to a parrot phylogeny, it is suggested that these loci may evolve through a more complex model than sole repeat number changes.