Growth, Morphology, and Developmental Instability of Rainbow Trout, Yellowstone Cutthroat Trout, and Four Hybrid Generations

Abstract

Hybridization of cutthroat trout Oncorhynchus clarkii with nonindigenous rainbow trout O. mykiss contributes to the decline of cutthroat trout subspecies throughout their native range. Introgression by rainbow trout can swamp the gene pools of cutthroat trout populations, especially if there is little selection against hybrids. We used rainbow trout, Yellowstone cutthroat trout O. clarkii bouvieri, and rainbow trout × Yellowstone cutthroat trout F₁ hybrids as parents to construct seven different line crosses: F₁ hybrids (both reciprocal crosses), F₂ hybrids, first-generation backcrosses (both rainbow trout and Yellowstone cutthroat trout), and both parental taxa. We compared growth, morphology, and developmental instability among these seven crosses reared at two different temperatures. Growth was related to the proportion of rainbow trout genome present within the crosses. Meristic traits were influenced by maternal, additive, dominant, overdominant, and (probably) epistatic genetic effects. Developmental stability, however, was not disturbed in F₁ hybrids, F₂ hybrids, or backcrosses. Backcrosses were morphologically similar to their recurrent parent. The lack of developmental instability in hybrids suggests that there are few genetic incompatibilities preventing introgression. Our findings suggest that hybrids are not equal: that is, growth, development, character traits, and morphology differ depending on the genomic contribution from each parental species as well as the hybrid generation.

Received March 22, 2010; accepted December 21, 2010

ACKNOWLEDGMENTS

We thank Damon Keen and the staff at Henry's Lake Hatchery, Matt Campbell of Idaho Fish and Game, and Brad Dredge and the staff at Hayspur State Fish Hatchery for assistance and logistical support. S. Rubin, M. Hayes, R. Reisenbichler, R. Rodriguez, J. Harvey, J. Emlen, M. Hoy, G. Sanders, C. Chambers, J. Steinbacher, C. Galitsky, and A. Newman assisted in PIT tagging and fish care. K. Orekoya, R. Bice, N. Menezes, and J. Nutter (students at Berry College) helped with photographing and digitizing the images. Cathy Chamberlin-Graham assisted with the literature review. Paul Sampson and Shirley Ren provided key statistical advice. The PIT tags were generously loaned by the U.S. Army Corps of Engineers and H. Balbach. We thank G. Winans and O. Johnson and two anonymous reviewers for providing comments that improved the manuscript. Berry College provided release time for J.H.G.; all other funding was provided by the U.S. Geological Survey. Use of trade names is for the convenience of the reader and does not constitute an endorsement of products over others that may be suitable.