Multivariate heritability of shape in June sucker (Chasmistes liorus) and Utah sucker (Catostomus ardens): shape as a functional trait for discriminating closely related species.

Molecular genetic data suggest that June sucker (Chasmistes liorus) is only shallowly diverged from the co-occurring but phenotypically distinct Utah sucker (Catostomus ardens) in Utah Lake. Phenotypes representing both extreme morphologies (June sucker and Utah sucker) are observed in the small wild population, but relatively large numbers of intermediate phenotypes are also present. This relatively continuous variation between the two putative species could result from extensive hybridization (including reticulate evolutionary patterns) of genetically based phenotypes or incomplete divergence among lineages and extensive phenotypic plasticity with little genetic basis. To help inform the evolutionary history of June sucker and to provide critical information for management and restoration of June sucker populations, we evaluated the distribution of shape phenotypes among purebreds of each species and their hybrids and determined the heritability of shape and ecological performance between June sucker (C. liorus) and its sister species Utah sucker (C. ardens). Mouth shape of purebred June sucker and Utah sucker are located at the extremes, and hybrids are located midway between the purebreds. Multivariate heritability was relatively high for mouth shape at 0.27. Heritability for growth rate was high at 0.32-0.42, but variation was not associated with cross type. Genetically based variation in mouth shape has evolved fairly rapidly or has been maintained in the face of ongoing hybridization between the two species. Currently, there seems to be little evidence for differential selection between species that would maintain shape variation.