Egg size and reproductive allocation in the pitcherplant mosquito Wyeomyia smithii (Diptera: Culicidae)

Reproductive consequences of female size in haematophagous ectoparasites

To test relationships between maternal size, egg size and size of new offspring, we studied (a) the effect of maternal size on egg size and number, and maternal survival after oviposition and (b) the effect of egg size on the duration of development and new imago size in three flea species (Xenopsylla ramesis, Synosternus cleopatrae, Parapulex chephrenis) with varying host and habitat specificity. In general, the number and size of eggs as well as total egg volume appeared to be independent of maternal body size. There was no trade-off between egg number and size. However, female body size was related to post-oviposition survival, with larger females surviving longer after oviposition than smaller females. In addition, females that produced more eggs died faster after oviposition in X. ramesis but not in the two other species. There were no significant size differences between eggs that developed into new imagoes and eggs that did not survive. Survivorship of male and female eggs did not differ; however, new adult females were significantly larger than new adult males. Female, but not male, new imagoes exhibited a significant positive relationship between egg size and duration of preimaginal development in all three species, with larger eggs developing faster than smaller eggs. In X. ramesis and S. cleopatrae, faster developing eggs also developed into larger new imagoes. We conclude that these patterns were largely consistent among the three flea species, suggesting that they result from the same mechanisms and are weakly affected by the ecological specialization of a given species.

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.

Evolutionary ecology of progeny size in arthropods

Most models of optimal progeny size assume that there is a trade-off between progeny size and number, and that progeny fitness increases with increasing investment per young. We find that both assumptions are supported by empirical studies but that the trade-off is less apparent when organisms are iteroparous, use adult-acquired resources for reproduction, or provide parental care. We then review patterns of variation in progeny size among species, among populations within species, among individuals within populations, and among progeny produced by a single female. We argue that much of the variation in progeny size among species, and among populations within species, is likely due to variation in natural selection. However, few studies have manipulated progeny environments and demonstrated that the relationship between progeny size and fitness actually differs among environments, and fewer still have demonstrated why selection favors different sized progeny in different environments. We argue that much of the variation in progeny size among females within populations, and among progeny produced by a single female, is probably nonadaptive. However, some species of arthropods exhibit plasticity in progeny size in response to several environmental factors, and much of this plasticity is likely adaptive. We conclude that advances in theory have substantially outpaced empirical data. We hope that this review will stimulate researchers to examine the specific factors that result in variation in selection on progeny size within and among populations, and how this variation in selection influences the evolution of the patterns we observe.