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A study of hybridization between two species of cyprinid fishes, Acrocheilus alutaceus and Ptychocheilus oregonensis Stewart, Kenneth

Abstract

Fish morphologically intermediate between Acrocheilus alutaceus and Ptychocheilus oregonensis have been collected infrequently from the Columbia River system. Morphological comparisons of wild Acrocheilus, Ptychocheilus, and intermediates, with artificial inter- and intra- species crosses indicated that wild intermediates were of hybrid origin. Observations of spawning habitat and behavior of the parent species suggest that hybridization in most localities is accidental. In Missezula Lake an unusually high incidence of hybrids, pronounced spatial separation of spawning groups of the parental species and presence of stray Ptychocheilus in the school of Acrocheilus all suggest that interspecific mating occurs. Hybrids are largely sterile, but hybrid males produce fertile sperm infrequently. The parent species show no evidence of gene flow, but a few intermediates are apparently backcrosses. Gene flow is probably blocked by the rarity and partial sterility of hybrids and by selection against backcrosses. The presence of wild backcrosses and partial fertility of hybrids suggest a large amount of genetic similarity between Acrocheilus and Ptychocheilus. This contention is strengthened by similarity in chromosome morphology and the presence of dominance effects in the inheritance of some parental characters in hybrids.

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