Inbreeding depression varies with investment in sex in a cyclic parthenogen

Background/Question/Methods The alternation of asexual and sexual reproduction in cyclical parthenogens can allow recessive mutations that accumulate during the parthenogenetic phase to be unmasked following sexual reproduction.  Having documented that the frequency and magnitude of sexual reproduction differs among populations and clones of the freshwater crustacean Daphnia pulicaria, we ask if this variation in investment in sex is predictive of the magnitude of inbreeding depression across populations.  Individuals in populations that invest less in sex will experience, on average, more asexual generations between bouts of sexual reproduction.  Longer periods of asexual reproduction are predicted to increase the accumulation of deleterious mutations within individuals, as well as reduce population-level genetic diversity via competition.  In five lake populations that span a gradient of sex investment, we first compared the viability of sexually produced eggs collected from the field (sire unknown) to those that were produced by mating mothers to their genetically-identical sons. 

Results/Conclusions

Inbreeding reduced the fraction of egg that developed on average 28% compared to the field-produced eggs.  Hatching fraction also differed among lakes.  For maternal genotypes that had hatching and survival of both the field-produced and laboratory-inbred daughters, we raised all three genotypes (mother, field-produced, lab-produced) on high food in the laboratory and estimated the fitness reduction of the sexually-produced offspring relative to the maternal genotype.  Inbred individuals had lower fitness than either the maternal genotype or their field-produced siblings.  Furthermore, we found that the magnitude of fitness reduction matched the rank order of sex investment in these lake populations:  inbreeding depression was highest in the low-sex population and lowest in the high-sex population.  There was less of a fitness reduction following sex in the field-produced daughters, suggesting that many of the mothers we collected from the field had mated with a genotype other than their own.