Density-dependent sex-biased development of macroptery in a water strider.

In wing-polymorphic insects, wing morphs differ not only in dispersal capability but also in life history traits because of trade-offs between flight capability and reproduction. When the fitness benefits and costs of producing wings differ between males and females, sex-specific trade-offs can result in sex differences in the frequency of long-winged individuals. Furthermore, the social environment during development affects sex differences in wing development, but few empirical tests of this phenomenon have been performed to date. Here, I used the wing-dimorphic water strider to test how rearing density and sex ratio affect the sex-specific development of long-winged dispersing morphs (i.e., sex-specific macroptery). I also used a full-sib, split-family breeding design to assess genetic effects on density-dependent, sex-specific macroptery. I reared water strider nymphs at either high or low densities and measured their wing development. I found that long-winged morphs developed more frequently in males than in females when individuals were reared in a high-density environment. However, the frequency of long-winged morphs was not biased according to sex when individuals were reared in a low-density environment. In addition, full-sib males and females showed similar macroptery incidence rates at low nymphal density, whereas the macroptery incidence rates differed between full-sib males and females at high nymphal density. Thus complex gene-by-environment-by-sex interactions may explain the density-specific levels of sex bias in macroptery, although this interpretation should be treated with some caution. Overall, my study provides empirical evidence for density-specific, sex-biased wing development. My findings suggest that social factors as well as abiotic factors can be important in determining sex-biased wing development in insects.