Demography, selection and comparative molecular evolution in brood parasitic indigobirds (vidua spp.) and their firefinch (lagonosticta spp.) hosts
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Abstract
The interaction of natural selection and genetic drift is key to understanding patterns of genomic variation and the adaptation of organisms to their environments. I examined how demographic and selective factors shape patterns of genetic variation at the Major Histocompatibility Complex (MHC) in three species of brood parasitic indigobirds ( Vidua spp.) and their firefinch hosts (Lagonosticta spp.). These two genera experience similar ecological and physiological conditions due to their close contact and shared environment as nestlings, but they have vastly different demographic histories. I characterized variation at the second exon of Class IIB MHC loci by 454 amplicon sequencing. Results suggest a minimum of three to four putatively functional MHC Class IIB loci. Balancing selection appears to outweigh genetic drift to maintain nearly equal variation at MHC loci in the two genera, despite estimates from neutral loci indicating that indigobirds have a long-tetm effective population size about one-tenth that of firefinches.
Balancing selection at MHC is likely influenced by avian haematozoan parasites. I compared malarial infections in indigobirds and firefinches by amplifying and sequencing a portion of the haematozoan cytochrome b gene from avian blood samples. Indigo birds exhibit a lower prevalence of avian malaria than firefinches and other sympatric estrildids, but share more parasite lineages with their hosts than they do with non-hosts. A reduction in parasite load due to reduced reproductive effort may be a general life history advantage of brood parasitism, while close association in the nest may facilitate host-switching in some endoparasites. I found no significant relationships between MHC genotype and infection status in either indigobirds or firefinches.
Shared genetic variation among species can also represent retained ancestral polymorphism, which is now widely recognized as a complicating factor in phylogeny estimation. The extent of retained ancestral polymorphism and discordance among gene trees should be a function of variable demographic history among lineages. Using firefinches as an example of a genus with a typical bifurcating evolutionary history, I reconstructed demographic and speciation history using a multi-locus coalescent method (BEST), which more appropriately incorporates the stochastic effects of genetic drift than single-locus or concatenation approaches, and found that gene tree incongruence correlated with time between divergence events but not with population sizes.
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Thesis (Ph.D.)--Boston University
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.