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Hybridization and isolating mechanisms in Catostomus commersonii and Catostomus macrocheilus Nelson, Joseph Schieser

Abstract

Catostomus commersonii and Catostomus macrocheilus are sympatric in three major drainages of British Columbia. Morphological analysis employing a character index and discriminant function analysis of allopatric and sympatric populations revealed that hybrids occurred in nine lakes, representing virtually every lake examined where both species occur. Presumed F1 hybrids made up an average of 7% of the combined populations where hybrids occurred. There was no evidence of backcrossing or swamping. Spawning migrations were studied in eight confined inlets of lakes which have both species. Four inlets supported migrations of both species. Collecting was also done throughout the spawning migration in Cluculz River, an outlet with diversified habitat supporting both species. Postmating isolating mechanisms were studied from artificial crosses of the species, two back-crosses involving hybrid females, and microscopic examination of hybrid testes. An interaction of temporal, habitat, and postmating isolating mechanisms was not considered sufficient to prevent swamping. Ethological isolation is inferred to be of major importance in reducing initial hybridization, while selection against hybrid crosses is inferred to prevent swamping. Hybridization could not be attributed to environmental factors commonly noted in the literature to be associated with hybridization. Males of one species entering the spawning act with males and a female of the other species is considered the main cause of hybridization. Mis-mated individuals, however, have much opportunity for conspecific matings at other times. There was no evidence of reinforcement of isolating mechanisms despite the probable occurrence of hybridization in some areas for at least several hundred years. Natural selection against individuals producing hybrids is apparently low.

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