Developmental success of hybrids between two taxa of salmonid fishes with moderate structural gene divergence

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Abstract
The developmental rate and stability of Yellowstone cutthroat trout (Salmo clarki bouvieri) (Y), westslope cutthroat trout (Salmo clarki lewisi) (W), and their hybrids (female × male) were used to examine the evolutionary divergence of gene regulatory systems between these taxa. The Nei's structural gene divergence between these subspecies has been estimated as D = 0.34. Pure W × W developed significantly faster than Y × Y as detected by hatching time and degree of yolk sac resorption. In addition, W × W embryos had greater activities of malate dehydrogenase, lactate dehydrogenase, phosphoglucomutase, and glucose-6-phosphate isomerase from hatching to yolk sac resorption than Y × Y embryos. The two hybrids did not differ significantly from their maternal parental cross in hatching time. W × Y hybrids did, however, hatch significantly sooner than Y × W hybrids. There were no consistent differences between the parental crosses and hybrids in the amount and rate of increase in activity of the four enzymes. Hybrids had significantly greater meristic counts for two out of four characters than the parental crosses. Heterotic effects were observed in that hybrids were longer and showed a faster rate of yolk sac resorption than their maternal parental cross. Moreover, the Y × W hybrid had significantly greater developmental stability as measured by fluctuating asymmetry of four bilateral meristic characters than the Y × Y cross. The lack of perturbation in the development of hybrids indicates that despite their structural gene divergence, few gene regulatory incompatibilities affecting the developmental process exist between these cutthroat trout taxa.