Molecular ecology of the endangered Gouldian Finch Erythrura gouldiae

Esparza Salas, Rodrigo (2008) Molecular ecology of the endangered Gouldian Finch Erythrura gouldiae. PhD thesis, James Cook University.

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Abstract

The present distribution of extant species has been influenced by historical events of habitat change that can have an effect on cause population sizes and distribution. These types of fluctuations often have had an effect on the genetic makeup of populations. Although the effects of past climatic changes are well documented for temperate environments, these are relatively less clear for the tropics. In this study I analyse mitochondrial DNA control region variation in the Gouldian Finch (Erythrura gouldiae), a species endemic to the Australian tropical savannas that has experienced drastic population declines in recent times. Results show a lack of lineage differentiation between individuals across the present distribution of this species, and a total lack of genetic diversity for a population that is part of a captive breeding and reintroduction program. Analyses of mismatch distributions fit predictions of population growth consistent with range expansions, dating during the Holocene period from approximately 7000 to 3600 years before present. Differences in mismatch distributions between separate geographical regions suggest population expansion events happened at different times in different regions. This gradual colonisation of northern Australia can be explained in terms of important climatic and socio-cultural changes during the Holocene that were favourable for the growth of E. gouldiae populations. The apparent lack of genetic differentiation between populations from different geographical regions, and the relatively low sequence divergence suggest a recent colonisation of the tropical Savannas by E. gouldiae. Implications of these results for current management practices aimed to recover E. gouldiae populations are discussed.

Species that have experienced population reductions are expected to be at higher extinction risk because of loss of genetic diversity. This can have deleterious consequences for population survival and reduce the potential of such populations to respond to environmental change. Such deleterious effects can be reduced by high levels of migration that contribute to the maintenance of genetic diversity. The Gouldian Finch (Erythrura gouldiae) has experienced recent population reductions that have resulted in its endangerment. In order to provide effective management aimed to achieve the recovery of an endangered species, it is necessary to investigate the genetic variation in this species.

In this study I investigate genetic variation, population structure and gene flow in Gouldian Finch populations, and contrast such patterns with those of the sympatric and abundant Long-tailed finch (Poephila acuticauda). Heterozygosity is slightly higher in P. acuticauda than in E. gouldiae. There was no evidence of recent severe genetic bottlenecks and the indicators of intra-population inbreeding (f) were relatively low in both species. Inter-population differentiation (θII) was moderate for both species.

Significantly lower heterozygosity and evidence of a recent population bottleneck was found for a captive population of E. gouldiae that is part of a reintroduction program in north-east Australia.

There was evidence of population structuring between P. acuticauda populations, while E. gouldiae shows no sign of population structure. The lack of genetic differentiation in E. gouldiae can be partly explained by relatively higher migration rates compared to P. acuticauda.

Genes of the Major Histocompatibility Complex (MHC) have an important role in pathogen recognition and the initiation of the immune response. MHC genes are among the most variable genes in vertebrates. Their high diversity is thought to be maintained by gene duplication, balancing selection and gene conversion. I investigated the variability of MHC class IIβ genes of the endangered Gouldian Finch (Erythrura gouldiae) and the sympatric and relatively abundant Long-tailed Finch (Poephila acuticauda). I looked for evidence for the presence of gene conversion events between haplotypes, in order to determine its relative importance on the maintenance of genetic variability in these two species, and calculated dN/dS (the ratio of non-synonymous to synonymous substitution rates) as an indication of the relative strength of selection at this gene.

Erythrura gouldiae had an apparently lower variability at MHC class II genes than P. acuticauda. Gene conversion events between haplotypes were detected in both species, being more prevalent in E. gouldiae than in P. acuticauda. This indicates that gene conversion has been relatively more important in the maintenance of genetic variability of MHC genes in E. gouldiae. Evidence of positive selection at MHC class II genes was found for both species. The patterns of positive selection at the amino-acid level appear roughly similar between both species, although in some cases I found positions that are under purging selection in E. gouldiae, while in P. acuticauda are under strong positive selection.

Differences found between both species in variability of MHC genes, the prevalence of gene conversion events and the rates of positive selection may reflect possible differences in evolutionary pressures caused by pathogens in each species.

Item ID: 9429
Item Type: Thesis (PhD)
Keywords: genetic diversity, allele variation, gouldian finches, Erythrura, population distribution, mitochondrial DNA, tropical savannahs, genetic selection, evolutionary genetics, phylogeography, phylogenetics
Date Deposited: 24 Mar 2010 22:35
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060302 Biogeography and Phylogeography @ 50%
06 BIOLOGICAL SCIENCES > 0604 Genetics > 060411 Population, Ecological and Evolutionary Genetics @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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