Doctor of Philosophy
Integrative Life Sciences
Brian C. Verrelli
In urban population genetic studies, the "urban fragmentation model" predicts that urbanization acts as a barrier that isolates native populations, and can lead to reduced gene flow and increased genetic drift between populations. The “urban facilitation model” predicts urban areas act as corridors to increase dispersal among urban areas, and can lead to higher genetic diversity within and lower differentiation between urban areas.
In a review of the current literature, we found that there is no consistent signature of reduced within-population genetic diversity or increased between-population genetic differentiation. Analyses that investigate the urban barriers to gene flow also found no consistent results. Thus, the response to urbanization may be species and city specific.
We used social network genetic analyses, which can identify connections that both fragment and facilitate gene flow, to investigate the impact of anthropogenic disturbance on connectivity in a model urban pest of significant medical-relevance, the Western black widow spider, Latrodectus hesperus. In comparison to non-urban locales, urban locales have higher within-population genetic diversity, lower between-population genetic differentiation, and higher overall estimates of genetic connectivity. We found that not all cities are highly connected, with specific urban hubs driving gene flow among historically isolated non-urban locales.
We compared and contrasted our previous broad-scale patterns of urban gene flow with a new fine-scale locale sampling from within three Southwestern U.S. cities. Urban areas have significantly different patterns of connectivity to the overall network that generate contrasting patterns of within- and between-city genetic diversity. There is significant heterogeneity among the fine-scale city samples, such that certain urban hubs are impacting the network of urban and non-urban locales on the whole.
We examined differences in gene expression between three paired urban and non-urban populations from the cephalothorax (metabolism), ovary (fertility), and silk glands (web architecture). There is significant differential expression in each tissue type observed between urban and non-urban locales, among both urban and among non-urban locales, and specific to geographic locations independent of urban or non-urban habitat. These results imply that not all cities are created equal with respect to demographic and gene flow patterns, but also with phenotypic patterns.
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