Coevolved genetic interactions within populations can be disrupted by hybridization resulting in loss of fitness in hybrid individuals (i.e., hybrid breakdown). However, the extent to which variation in fitness-related traits among hybrids is inherited across generations remains unclear, and variation in these traits may be sex-specific in hybrids due to differential effects of genetic incompatibilities in females and males. Here we present two experiments investigating variation in developmental rate among reciprocal inter-population hybrids of the intertidal copepod Tigriopus californicus. Developmental rate is a fitness-related trait in this species that is affected by interactions between mitochondrial-encoded and nuclear-encoded genes in hybrids that result in variation in mitochondrial ATP synthesis capacities. First, we show that F₂-hybrid developmental rate is equivalent in two reciprocal crosses and is unaffected by sex, suggesting that breakdown of developmental rate is likely experienced equally by females and males. Second, we demonstrate that variation in developmental rate among F₃ hybrids is heritable; times to copepodid metamorphosis of F₄ offspring of fast-developing F₃ parents (12.25 ± 0.05 d, μ ± SEM) were significantly faster than those of F₄ offspring of slow-developing parents (14.58 ± 0.05 d). Third, we find that ATP synthesis rates in these F₄ hybrids are unaffected by the developmental rates of their parents, but that mitochondria from females synthesize ATP at faster rates than mitochondria from males. Taken together, these results suggest that sex-specific effects vary among fitness-related traits in these hybrids and that effects likely associated with hybrid breakdown display substantial inheritance across hybrid generations.

Developmental rate data – collected by daily monitoring of larval development (naupliar and copepodid) of individual Tigriopus californicus until either stage 1 copepodid metamorphosis was observed or adulthood was reached.

ATP synthesis rate data – mitochondria isolated by differential centrifugation followed by in vitro assays of maximal ATP synthesis rate with saturating concentrations of substrates for either electron transport complex I or II.

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