College of Liberal Arts & Social Sciences Theses and Dissertations

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Biological Sciences


threespine stickleback, species pair, Mud Lake, microsatellites, mitochondrial control region


The formation of new species is responsible for all the biological diversity we see on Earth. This process of speciation is not yet fully understood and is most often studied by comparing individuals of closely related species. The threespine stickleback, Gasterosteus aculeatus, a fish species complex found in marine and coastal freshwater environments of the northern hemisphere, has become a model species used to study speciation. Many morphological and genetically divergent forms within this complex have adapted to freshwater conditions. Studying the mechanisms that allow stickleback to adapt to freshwater conditions has provided much insight into the process of speciation. Here, we present the first genetic analysis of a lake-resident/anadromous species pair of threespine stickleback. This analysis is preformed on a lake-resident/anadromous pair found in Mud Lake, a freshwater lake in the Jim Creek drainage of Alaska. Anadromous and resident freshwater stickleback in Mud Lake differ substantially in morphology, including body size, body shape, armor, and trophic morphology. Based on previous morphological research, this species pair is believed to be breeding sympatrically with little evidence of hybridization. Seven additional lake, stream and anadromous populations from the Jim Creek drainage and neighboring drainages were included for comparison. Analysis of genetic diversity and tests of differentiation from a survey of neutral genomic markers (microsatellites) and DNA sequences from the mitochondrial control region indicate significant genetic divergence between sympatric anadromous and resident freshwater fish in the Jim Creek drainage. Genetic diversity was greater in anadromous populations than resident freshwater populations, consistent with previous genetic analyses of stickleback from other areas. Neighbor-joining trees based on genetic distances among populations revealed that resident freshwater populations in the Jim Creek drainage clustered together, forming a separate cluster from the anadromous populations with which they breed sympatrically. Similarly, Bayesian cluster analysis revealed four genetic clusters: one composed of the anadromous populations, and three composed of resident freshwater groups segregated by geographic region, with the Jim Creek resident populations falling in one discrete cluster. Overall, the genetic data indicate a primary axis of genetic divergence corresponding to anadromous vs. resident freshwater populations, with the latter secondarily diverging by geographic location. Consistent with previous morphological research, we found little evidence that significant hybridization between the two forms is occurring. The similarity in the magnitude of genetic divergence between anadromous populations and resident freshwater populations breeding sympatrically in the Jim Creek drainage, compared to the divergence between anadromous populations and neighboring resident freshwater populations living in allopatry supported our hypothesis that these forms are reproductively isolated. They have likely been isolated for a period of time similar to that of other resident freshwater populations in the region. These results improve our understanding of the relationship of the species pair found in Mud Lake, and will provide a genetic baseline for research into the early mechanisms that support the formation of new species.