Color and parasitization of the threespine stickleback, Gasterosteus aculeatus

The threespine stickleback, Gasterosteus aculeatus , is a fish utilized for various evolutionary and behavioral studies by scientists all over the world. As such a model, the significance of color expression and its role in understanding color signal communication stands at the forefront of understanding reproductive behaviors in the marine and freshwater threespine stickleback that inhabit the holarctic region of the northern hemisphere. The nuptial coloration seen in males is important in intrasexual selection as it relates to aggression or intimidation, and in intersexual selection as it is a quality indicator for males in their quest to lure females to their nests for reproductive purposes. In order to best understand color expression and the pigment cell types responsible, I needed to begin my study of these fish from the embryonic stage through development to maturity and then again after infection with a parasite that seems to affect color expression;First, I utilized light microscopy to determine pigment genesis by the chromatophores responsible for coloration in these fish. Specifically, I quantified the ontogeny of pigment expression in various marine crosses by these pigment cells during development. Also, I quantified chromatophores expressed during the nuptial coloration of the marine male and female, both of which serve as an extant ancestor to the more recent adaptive radiation of the freshwater lake derived threespine stickleback.;Next, I studied the chromatophore changes apparent as a result of infection by the tapeworm Schistocephalus solidus. The parasite manipulates its intermediate host, the threepine stickleback, in order to ensure trophic transmission to its definitive host, a piscivorous bird. As a result of this manipulation, both phenotypic color change and a behavioral change occur in these fish. I utilized light microscopy to detail chromatophore changes expressed as a result of infection by this parasite.;Next, I obtained blood samples via exsanguinations techniques and employed hematological tests to provide support that a physiological consequence is induced by the parasite and is responsible for the behavioral changes seen. As a result of parasitic infection, it is the combination of the phenotypic color that is concurrent with the behavior changes induced as a result of a physiological manipulation that further ensures trophic transmission and supports the parasite manipulation hypothesis in this model system.