Sex allocation in a species with paternal genome elimination: the roles of crowding and female age in the mealybug Planococcus citri

Background: In species with paternal genome elimination, both sexes are diploid. However, in males the chromosomes inherited from the father are deactivated during early development and eliminated from the germ line. Sex allocation theory predicts that, all else being equal, females should bias their offspring sex ratio towards the sex that competes least with relatives. Organism: The mealybug Planococcus citri, a cosmopolitan pest on a wide range of agricultural and ornamental plant species. Hypothesis: In mealybugs, females compete locally for resources. To avoid competition among daughters, females should therefore produce a male-biased sex ratio when alone, ut a more equal sex ratio when together with other unrelated females. This will result in a rise of the number of female offspring with density. However, competition associated with population density might have different fitness effects for male and female offspring respectively, because females need more resources and have less opportunity to migrate compared with males, selecting for the opposite pattern of sex allocation. Methods: Measuring sex ratios in an experiment to manipulate the density a female experiences during two life stages. Results: Females that experienced high density as adults produced more male-biased sex ratios. In addition, the sex ratio females produced was strongly dependent on their age. Conclusion: Female mealybugs facultatively adjust their sex ratio, but in the direction opposite to that predicted by local resource competition, suggesting that sex-specific fitness consequences of density determine sex allocation in mealybugs. © 2010 Laura Ross