Date of Award

Fall 2015

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Adrienne I Kovach

Second Advisor

Brian J Olsen

Third Advisor

Gregory Shriver

Abstract

Hybrid zones in nature have long been equated to “windows on the evolutionary process” providing unique environments to understand patterns of gene flow and introgression and the role of these mechanisms in maintaining biodiversity. Ongoing hybridization and introgression can lead to a number of conservation and evolutionary outcomes; as such, identifying the role of introgression in natural populations can provide new insights into species interactions while contributing to our understanding of evolutionary theory.

The research presented below characterizes hybrid zone dynamics between two tidal marsh endemics – the Saltmarsh (Ammodramus caudacutus) and Nelson’s (Ammodramus nelsoni) sparrow. Both species co-inhabit salt marshes from southern Maine to northern Massachusetts and hybridization has been documented in the overlap zone for over a century. While there is previous evidence for asymmetrical introgression and southward expansion, information regarding the rates and consequences of hybridization in this system is lacking. To address this objective, I evaluated five components that define a hybrid zone: 1) phenotypic and morphological variation; 2) strength of selection and patterns of introgression; 3) influence of habitat; 4) hybrid fitness; and 5) temporal stability. I combined genetic and ecological methods to evaluate the above components and to assess hybrid zone dynamics.

I collected morphometric and plumage data from individuals sampled extensively across the hybrid zone and evaluated both phenotypic and genetic variation in pure and admixed populations. I found that morphological and genetic variation increased in sympatric populations and that a majority of the individuals sampled were backcrossed with a lack of both intermediate phenotypes and genotypes. I also found that plumage traits related to the darkness and definition of streaking on the breast, flanks, and back of the birds correlated more strongly with genotype than traits related to the amount of streaking on an individual or the color of the plumage.

I also investigated patterns of genetic structure, selection, and differential introgression across the hybrid zone. I found that a high proportion (50%) of the sampled individuals were admixed to some degree, but that only 3% of individuals were recent generation hybrids. Genetic analyses revealed that pure and hybrid individuals were patchily distributed across the hybrid zone. I also found that introgression was highly variable among marker types and that, for a majority of the markers examined, selection was weak and introgression was asymmetrical toward Saltmarsh Sparrows. I did document abrupt patterns of selection for sex-linked markers, mitochondrial markers, and two gene-associated markers. Selection for sex-linked and mitochondrial markers supports findings of Haldane’s rule in this system. These results suggest a role for selection in maintaining pure species boundaries in the face of ongoing geneflow and introgression.

I also investigated patterns of genetic structure, selection, and differential introgression across the hybrid zone. I found that a high proportion (50%) of the sampled individuals were admixed to some degree, but that only 3% of individuals were recent generation hybrids. Genetic analyses revealed that pure and hybrid individuals were patchily distributed across the hybrid zone. I also found that introgression was highly variable among marker types and that, for a majority of the markers examined, selection was weak and introgression was asymmetrical toward Saltmarsh Sparrows. I did document abrupt patterns of selection for sex-linked markers, mitochondrial markers, and two gene-associated markers. Selection for sex-linked and mitochondrial markers supports findings of Haldane’s rule in this system. These results suggest a role for selection in maintaining pure species boundaries in the face of ongoing geneflow and introgression.

To identify patterns of differential fitness across the hybrid zone, I compared numerous reproductive parameters among pure, backcrossed, and recent generation hybrid females. I also compared the proportion of first generation hybrids among age and gender classes to test for differences in survival in hybrids. I found that pure and backcrossed Nelson’s Sparrows have reduced hatching and fledgling success compared to hybrids and pure and backcrossed Saltmarsh Sparrows. The reduced fitness in Nelson’s Sparrows is likely due to differential adaptation to coastal marshes and may be influential in shaping hybrid zone dynamics and asymmetries in this system. Also, hybrid females had lower survival than hybrid males.

Lastly, I evaluated temporal stability in the hybrid zone by comparing genetic and morphological data between two temporal replicates. I compared plumage data, morphometric measurements, and data from neutral and mitochondrial markers between five marshes sampled in 1998 and then again 15 years later in 2012/2013. I detected a southward expansion of Nelson’s Sparrow alleles, supported by a shift in the center of the hybrid zone and an increase in introgression over the 15-year time period. Despite signs of hybrid zone expansion, some markers are under stronger selection now than they were in 1998 indicating that some genetic regions may be maintained despite introgression. These regions may be critical in maintaining species boundaries.

Following the assumptions of existing theoretical frameworks, the above components were systematically evaluated to assess hybrid zone stability. The data have been used to evaluate impacts of hybridization and introgression in a naturally occurring avian hybrid zone. The work presented here identifies the mechanisms responsible for maintaining pure species boundaries in the face of ongoing introgression and gene flow. Further, this work has elucidated how hybridization shapes population dynamics while providing insight into the conservation management of hybridizing species. Although hybridization is ongoing between the two species, selective mechanisms likely play a role in reproductive isolation. Based on these findings, the management of both species, even within the hybrid zone, is warranted. This insight is particularly relevant to the management of Saltmarsh Sparrows, as this species is imminently threatened by habitat loss through sea level rise and the management and conservation of genetically “pure” populations is a high priority in the northeast.

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