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The evolution through natural hybridizations of the Umatilla dace (Pisces : Rhinichthys umatilla) , and their associated ecology and systematics Haas, Gordon Robert

Abstract

Umatilla dace (Rhinichthys umatilla Gilbert and Evermann 1894) are determined to be a distinct species that has sympatrically speciated and evolved through ancient hybridizations between leopard and speckled dace. Principal components, univariate, and allometric analyses reveal three distinct morphotypes, with Umatilla dace intermediate and convergent. A field identification key is developed. Their body morphologies remain largely consistent across broad mutual ranges, and Umatilla dace are found in abundance and predominance in allopatry, parapatry, and strict sympatry with either or both leopard and speckled dace. All show reduction in their overall morphometric variability when in strict sympatric combinations. Mitochondrial and two nuclear DNA (ITS and D3B) sequences provide consistent evidence of the overall distinct identity, increased variability and past hybridization, and multiple hybrid origins of Umatilla dace. Umatilla dace are composed of three main types each more closely related to the leopard or speckled dace through their comparative nearby numerical dominance. Laboratory crosses demonstrate morphologies remain distinctive when reared under identical conditions. The two combinations of artificial hybrids of leopard and speckled dace strongly resemble Umatilla dace, with Umatilla dace still remaining more derived. Second generation artificial crosses demonstrate that Umatilla dace and both laboratory hybrids are viable with no differential mortality. Each species is specialized to particular water flow regimes as determined in the field and at spawning times in a laboratory flow tube using mature artificial crosses. Umatilla dace are specialized to intermediate water flows, as are both laboratory hybrids. The laboratory hybrids show a maternal relationship in their water flow tolerances that matches the molecular genetic data. Umatilla dace are only found in the interior Columbia River drainage in range overlap between leopard and speckled dace, and have an intermediate postglacial northern recolonization pattern and distance. Their main distribution strongly coincides with large Wisconsinan glacial lakes that existed in this area. The associated breakdown of water flow regimes is hypothesized to have caused the past interbreeding of leopard and speckled dace, with the difficult glacial / postglacial environment favouring their hybrids' increased genetic and ecological variability. The dace genus has disproportionately higher hybridization, as does its minnow Family Cyprinidae, particularly in western North America.

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