Polymorphism for plate morphs in Gasterosteus aculeatus on the east coast of Canada and an hypothesis for their global distribution

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Abstract
In the threespine stickleback (Gasterosteus aculeatus) three plate morphs, controlled by major genes, are easily distinguished: the low-plated, partially plated, and completely plated morphs. We give the results of a geographic survey for these morphs, with 177 samples from New Brunswick, Prince Edward Island, and adjacent regions. The low-plated morph is virtually absent. Most sites are either monomorphic for the completely plated morph or polymorphic with high frequencies of that morph. Polymorphic sites are most frequent in fresh water where the frequency of partials is much higher. Polymorphic sites are also more than twice as frequent on Prince Edward Island than on the mainland. But sites with high frequencies of the partial morph, with one exception, occur only on the mainland. Populations with high frequencies of partials predominate in lakes and their tributaries near the sea. Hybridization between the low-plated leiurus and completely plated trachurus cannot account for most of the polymorphism in North America. The global distribution of plate morphs shows an association with climate, and the virtual absence of the low-plated morph in this survey can be accounted for by the action of climatic selection or an associated variable.
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