Heterogeneous selective pressures on egg-hatching time and the maintenance of its genetic variance in a Tetraneura gall-forming aphid

1. The performance of gall-forming aphids is largely dependent on the timing of egg hatch relative to host budburst. This study examined the mode of natural selection acting on egg-hatching time and the extent of its genetic variance in a Tetraneura aphid.
2. The budburst time of the primary host of the aphid varied greatly among individual trees. Egg hatch on a host tree was often asynchronous with host budburst.
3. Transplant experiments indicated that egg-hatching time was subject to heterogeneous selective pressures in heterogeneous host environments.
4. Hatching time was compared between half-sib families. Small nymphs in a half-sib family tended to hatch later, and this resulted in large within-family variance in hatching time.
5. Small nymphs that hatched late were likely to be selected out during the galling process. Thus, these nymphs may be produced not as bet-hedging but due to maternal effects. When such maladapted nymphs were not included in analysis, a significant amount of additive genetic variance was detected in hatching time.
6. Heterogeneous selection, coupled with density-dependent regulation of population on respective host trees, probably maintains additive genetic variance in this trait.