Description: Background: The microsporidian Nosema whitei is a natural parasite of the red flour beetle, Tribolium castaneum. The results of a previous study showed that, during co-evolution of the two species in the laboratory, host populations maintained elevated levels of heterozygosity. Hypothesis: Heterozygote advantage accounts for the maintenance of high levels of host heterozygosity during co-evolution with the parasite. Reduced heterozygosity of the beetle will lead to a decrease in resistance against parasite infection. Methods: In two experiments, we tested for (a) the effect of inbreeding and (b) the effect of inbreeding and outcrossing on the beetle’s resistance to infection, in relation to effects on egg hatching success, development time, and reproductive success. Results: Inbreeding reduced egg hatching success, prolonged development time, and resulted in lower reproductive success. Outcrossing shortened development time, while we did not find evidence for heterosis for reproductive success. We were unable to detect an effect of changes in heterozygosity on overall resistance to parasitism. The effect of inbreeding on development time did, however, influence parasite-induced mortality profiles: the prolonged development time, resulting from inbreeding, led to higher mortality in earlier developmental stages, but left the overall mortality rate unchanged. Hence, we conclude that heterozygosity is not a principal determinant of the beetle’s resistance to infection by N. whitei.

Description: Some studies have demonstrated that elevated CO2 concentrations in ocean waters negatively impact metabolism and development of marine fish. Particularly, early developmental stages are probably more susceptible to ocean acidification due to insufficient regulations of their acid-base balance. Transgenerational acclimation can be an important mechanism to mediate impacts of increased CO2 on marine species, yet very little is known about the potential of parental effects in teleosts. Therefore, transgenerational effects were investigated on life history in juvenile three-spined sticklebacks Gasterosteus aculeatus by acclimating parents (collected in April 2012, 55°03′N, 8°44′E) and offspring to ambient (~400 µatm) and elevated (~1,000 µatm) CO2 levels and measured parental fecundity as well as offspring survival, growth and otolith characteristics. Exposure to elevated CO2 concentrations led to an increase in clutch size in adults as well as increased juvenile survival and growth rates between 60 and 90 days post-hatch and enlarged otolith areas compared with fish from ambient CO2 concentrations. Moreover, transgenerational effects were observed in reduced survival and body size 30 days post-hatch as well as in enlarged otoliths at the end of the experiment, when fathers or both parents were acclimated to the high-CO2 environment. These results may suggest that elevated CO2 concentrations had rather positive effects on life-history traits of three-spined sticklebacks, but that parental acclimation can modify these effects without improving offspring fitness. Although the mechanistic basis of such transgenerational acclimation remains unclear, selective gradients within generations seem to determine the direction of transgenerational effects.