What is the fitness outcome of crossing unrelated fish populations? A meta-analysis and an evaluation of future research directions

Outbreeding has been shown to decrease fitness in a variety of species, including several species of fish. An understanding of the general outcomes following outbreeding is required in order to understand the consequences of conservation-related actions and hybridization in aquaculture. A meta-analysis was conducted on outbreeding studies in fishes using 670 comparisons between parent populations and their hybrid progeny. Five hundred and seventy-six comparisons involved first generation hybrids (F1), and a much smaller number (94) involved second generation hybrids (F2). The overall response to outbreeding in the F1 and F2 generations was positive and significant (F1: d*(i+) = 0.425 +/- 0.121; F2: d *(i+) = 0.548 +/- 0.295, where d*(i+) is the effect size of studies within generations); however, responses differed when studies were separated into groups describing the experimental environment, taxon, or trait. Findings may be biased by a few studies of large effect. Genetic distance explained little of the variance in effect sizes across studies. Results of the meta-analysis suggest that there is no reliable predictor for the effects of outbreeding in fishes, although inconsistencies in experimental design were noted across studies. Future research should include comparisons in both parental environments in order to detect the underlying mechanisms of outbreeding depression, and should focus on measurement of equivalent sets of fitness-related traits. Experimental design should permit estimation of genetic distances based on quantitative traits, which may in turn be meaningful predictors of the outcomes of outbreeding depression. Concerted and consistent research in this area will provide information of relevance to conservation, aquaculture and evolutionary studies.