Hybridization and Isolating Mechanisms Between Catostomus commersonii and C. macrocheilus (Pisces: Catostomidae)

Abstract

Catostomus commersonii and C. 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 were in 9 of the 11 lakes examined in which both species occurred. Presumed F1 hybrids made up an average of 7% of the individuals of the combined populations where hybrids occurred. There was no evidence of backcrossing or swamping.Segregation in spawning time and habitat was not apparent in four confined inlets which have migrations of both species. Some lakes supporting both species were found to have inlets in which only one species spawned. Temporal isolation was absent but habitat isolation was present in some areas of Cluculz River, an outlet with diversified habitat. Where segregation was apparent, C. commersonii spawned in shallow gravelly areas and C. macrocheilus in deep sandy areas. Artificial crosses of the species, two backcrosses involving hybrid females, and microscopic examination of hybrid testes failed to show any effective postmating isolating mechanism. 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. No explanation was found for the apparent failure of hybrids to successfully reproduce.Hybridization could not be attributed to environmental factors commonly associated with this phenomenon. 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. Characters of each species were as distinctive in the areas of sympatry as in the areas of allopatry. Natural selection against individuals producing hybrids is apparently slow.