A solution to a sex ratio puzzle in Melittobia wasps

The puzzling sex ratio behavior of Melittobia wasps has long posed one of the greatest questions in the field of sex allocation. Laboratory experiments have found that, in contrast to the predictions of theory and the behavior of numerous other organisms, Melittobia females do not produce fewer female-biased offspring sex ratios when more females lay eggs on a patch. We solve this puzzle by showing that, in nature, females of Melittobia australica have a sophisticated sex ratio behavior, in which their strategy also depends on whether they have dispersed from the patch where they emerged. When females have not dispersed, they lay eggs with close relatives, which keeps local mate competition high even with multiple females, and therefore, they are selected to produce consistently female-biased sex ratios. Laboratory experiments mimic these conditions. In contrast, when females disperse, they interact with nonrelatives, and thus adjust their sex ratio depending on the number of females laying eggs. Consequently, females appear to use dispersal status as an indirect cue of relatedness and whether they should adjust their sex ratio in response to the number of females laying eggs on the patch.

Solving the sex ratio scandal in Melittobia wasps

The scandalous sex ratio behaviour of Melittobia wasps has long posed one of the greatest problems for the field of sex allocation. In contrast to the predictions of theory, and the behaviour of numerous other organisms, laboratory experiments have found that Melittobia females do not produce less female-biased offspring sex ratios when more females lay eggs on a patch. We resolve this scandal, by showing that, in nature, females of M. australica have sophisticated sex ratio behaviour, where their strategy also depends upon whether they have dispersed from the patch where they emerged. When females have not dispersed, they will be laying eggs with close relatives, which keeps local mate competition high, even with multiple females, and so they are selected to produce consistently female-biased sex ratios. Laboratory experiments mimic these conditions. In contrast, when females disperse, they will be interacting with non-relatives, and so they adjust their sex ratio depending upon the number of females laying eggs. Consequently, females appear to use dispersal status as an indirect cue of relatedness, and whether they should adjust their sex ratio in response to the number of females laying eggs on the patch.