Heterosis and Outbreeding Depression in Interpopulation Crosses Spanning a Wide Range of Divergence

Abstract

The intertidal copepod Tigriopus californicus was used as a model organism to look at effects of crossing distance on fitness and to investigate the genetic mechanisms responsible. Crosses were conducted between 12 pairs of populations spanning a broad range of both geographic distance (5 m to 2007 km) and genetic distance (0.2% to 22.3% sequence divergence for a 606-bp segment of the mitochondrial COI gene). For each pair of populations, three fitness components (hatching number, survivorship number, and metamorphosis number) were measured in up to 16 cohorts including parentals, reciprocal F , F , F , and first-generation backcross hybrids. Comparisons of each set of cohorts allowed estimation of within- and between-locus gene interaction. Relative to parentals, F hybrids showed a trend toward increased fitness, with no correspondence with population divergence, and a decrease in variance, which in some cases correlated with population divergence. In sharp contrast, F hybrids had a decrease in fitness and an increase in variance that both corresponded to population divergence. Genetic interpretation of these patterns suggests that both the beneficial effects of dominance and the detrimental effects of breaking up coadaptation are magnified by increasing evolutionary distance between populations. Because there is no recombination in T. californicus females, effects of recombination can be assessed by comparing F hybrid males and females backcrossed to parentals. Both recombinant and nonrecombinant backcross hybrids showed a decline in fitness correlated with population divergence, indicating that segregation among chromosomes contributes to the breakup of coadaptation. Although there was no difference in mean fitness between the two backcross types, recombinational backcrosses showed greater variance for fitness than nonrecombinational backcrosses, suggesting that the breakup of parental gene ombinations within chromosomes has both beneficial and detrimental effects.

Is transplanting an effective means of preserving vegetation?

“…Selection against less adaptive hybrid genotypes would probably help to nullify the adverse effects of outbreeding (Storfer 1999), but poor progeny performances have been nevertheless documented in vertebrates and invertebrates as well as plants. The negative effects of interbreeding in the intertidal copepod, Tigriopsus californicus, were actually more pronounced in the F 2 than the F 1 (Edmands 1999), and the degree of debilitation increased with greater geographic and genetic distances between parental populations.…”

The roles of demography and genetics in the early stages of colonization

“…Outbreeding depression is usually attributed to the break-up of co-adapted gene complexes upon mating between divergent populations and may manifest in the F 1 [52] or more likely in later generations [53,54]. However, the effects of outbreeding may differ among traits and during different life stages [52,55], among genotypes and generations [56,57], thus its impact on colonization is hard to predict.…”

Mitonuclear interactions: evolutionary consequences over multiple biological scales

“…This has been most prominently illustrated by observations of intergenomic incompatibilities leading to mitochondrial dysfunction following experimental hybridization in the marine copepod, Tigriopus californicus. Such hybridization results in fitness breakdown in the F 2 generation, marked by decreased survivorship of larvae [62], slower development [63], reduced fecundity and viability [64], as well as decreased cyclooxygenase (COX) activity and ATP production [64][65][66]. The decrease in COX activity and ATP production is plausibly triggered by a set of mutations in the mitochondrial cytochrome c [67,68].…”