ABSTRACT
Fewer F2 hybrids between even-and odd-bloodline pink salmon (Oncorhynchus gorbuscha), which are lines that are genetically isolated by their strict two-year life cycle, survived than did F2 controls, indicating outbreeding depression. Cryopreserved sperm of 40 broodyear 1990 males and of 40 broodyear 1991 males fertilized equal subsamples of eggs from 40 broodyear 1992 females. Return rates of F1 hybrids (1.73%) and controls (1.63/%) in 1994 did not differ significantly (P=0.30). F2 hybrid and control were made from 40 males and 40 females selected at random from each return group. Offspring were differentially marked and released. In 1996, returns differed significantly (P=0.011) between hybrids (n=34, 0.34%) and controls (n=44, 0.42%). The low rate of return of the control fish was similar to the measured return of a much larger group of tagged Auke Creek pink salmon, and probably not an artifact of the experiment. Although no increase in fluctuating asymmetry of paired meristic counts of hybrids exceed measurements of controls, suggesting heterosis for those traits. The observations of decreased survival in F2 hybrids confirm previous work [Gharrett, A.J., Smoker, W.W., 1991). Two generations of hybrids between even and odd-year pink salmon. Canadian Journal of Fisheries and aquatic Science 48(9) 1744-1749]. Although divergence between pink salmon broodlines is large and outbreeding depression might be expected in such unlikely hybrids, the results document the occurrence of outbreeding depression in salmon and signal caution in making management and aquacultural decisions that may create the possibility of outbreeding depression in self-sustaining or cultured populations.
QUOTES FROM TEXT:
One risk of crossing genetically different strains is reduced average fitness (productivity) of F1 or later generation hybrids. This phenomenon is called outbreeding depression. Outbreeding depression can result from additive genetic effects of hybridization, nonadditive effects, or both. Additive effects may produce hybrids that are intermediate in phenotype to both parental types and that have reduced fitness in either parentās environment. Outbreeding depression resulting from additive genetic effects would be expected to appear in first generation hybrids (F1), like the depression observed in tolerance to DDT in F1 mosquitoes or the increased susceptibility of F1 coho salmon to the parasite Ceratomyxa shasta. Outbreeding depression can also occur as a result of non-additive genetic effects. Many complex polygenic traits coevolve as a result of local selection regime(s) and random drift. Distinct populations with unique coadapted allele complements can occur even in the presence of gene flow. If epistatic interactions are integral to coadapted, introgression of an Īalienā genotype can disrupt the coadapted gene complex. Resultant outbreeding depression may not occur until the second or later generations when assortment of alleles at different loci takes place.
In fisheries management and aquaculture, our primary concern is that reduced fitness will translate into decreased productivity in wild and culture populations. Delay of the effect until the second generation makes outbreeding depression especially insidious for longer lived species. For some Pacific salmon species, it could take more than a decade for outbreeding depression to be manifest. Reconstruction of a coadapted genome would require at least several generations at reduced productivity, depending on the number of loci involved and their linkages.
ćF1 hybrids and controls returned in 1996. The return dates for hybrids and controls were similar. Survival of F2 hybrid fish (0.24%) was significantly (P=0.011) lower than survival of control fish (0.42%). Both were lower than survivals of wild fish counted past the weir (2.49%).
The genetic divergence between the two populations of pink salmon that we hybridized was large. We consciously chose these diverse populations for this experiment to maximize the chances of detecting an effect because an experiment that fails to demonstrate a real effect often is interpreted as no effect, rather than an inconclusive result from a test that had low power. Although extrapolation of these results is not straightforward, we have demonstrated that outbreeding depression is possible in salmonids. Research to evaluate the potential of outbreeding depression with populations less distinct genetically will require time. In the interim, these results suggest that persistent introgression of non-native genetic material has the potential to erode production from naturally self-sustaining populations, and recovery may be very slow. Such erosion may occur when genes of hatchery fish introgression into local wild populations, particularly when hatchery stocks are derived from sources remote from the local population.