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Test of postzygotic isolation and parallel inheritance of morphological traits in threespine stickleback (Gasterosteus aculeatus complex) Clifford, Elisabeth Anne

Abstract

This thesis investigates post-zygotic reproductive isolation, parallel evolution, and parallel inheritance of morphological characters using two pairs of ecologically differentiated populations of threespine stickleback. These populations represent two evolutionarily distinct lineages, one from Japan and one from Canada, that are each represented by one marine and one stream-dwelling population. I used hybridisation experiments to examine degree of postmating isolation among the four populations. I found no significant reduction in F₁ or F₂ hybrid fitness relative to pure-bred controls. For my fitness measures I used fertilisation and hatching success of F₁ and F₂ hybrids. The lack of postzygotic isolation between populations when genetic relationships and times of divergence are considered is consistent with other postzygotic isolation studies in frogs, salamanders, and Drosophila. I used the hybrid fish created in the first experiment to answer three questions regarding the genetic causes of parallel evolution. First, do the two ecologically similar stream populations show parallel evolution of morphology? Second, are morphological characters evolved in parallel inherited in a similar way? And third, is parallel inheritance the result of changes at exactly the same loci or merely at loci that behave similarly? I used two morphological characters for these analyses: lateral plate number and body shape as quantified by a thin-plate spline analysis. These traits represent the extremes of genetic complexity with lateral plate number being under simple genetic control and body shape representing a polygenic composite character.

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