Egg Size Variation in Aedes aegypti: Relationship to Body Size and Other Variables

Abstract

Egg size variation among female mosquitoes (Aedes aegypti) is documented. Females hatching from large eggs grow faster, attain a larger adult size, take larger blood meals, lay more and larger eggs than females hatching from small eggs. This cycle is self-perpetuating; offspring of large females have a high probability of attaining a large size. Three factors oppose selection for large egg size by this apparently deterministic cycle: (1) Smaller and slower-growing females may also produce large eggs under some circumstances; (2) genetic differences among sibships are more important than differences in egg size; (3) there is no effect of body size on the reproductive success of males, so males pass on genetic material which has not been size-selected. It does not appear that there is an optimal egg size within the limits of observed egg sizes. INTRODUCTION Wilbur (1977) investigated the interrelationship between propagule size and number for two genera of long-lived iteroparous organisms adapted to uncertain reproductive success. Although these two groups (milkweed plants and salamanders) are taxonomically distant, the species considered are similar in that the total energy allotted to reproduction is constant. Variation among species within each group occurs in the way that the total energy is divided among individual offspring. In this article I look at the relationship between propagule (egg) size and number for an insect which differs from the previous examples in that the total energy invested is not constant. The female of the yellow fever mosquito Aedes aegypti L. takes at least one blood meal in order to mature each clutch of eggs. Blood meal mass varies (see Steinwascher, 1982, and references cited therein); for this reason, energy investment is not constant among females or among clutches for a single female. The number of eggs an individual female will lay in the first ovarian cycle depends on its body mass (Steinwascher, 1982), the mass and number of blood meals (Lea et al., 1978; Feinsod and Spielman, 1980; Steinwascher, 1982) and the blood source (Woke, 1937; Bennett, 1970). Furthermore, among eggs collected from a colony of Aedes aegypti, there are visible differences in size. In order to determine the interrelationships among egg size, number of eggs and the probabilities of reproductive success for this short-lived, iteroparous invertebrate, I did four sets of experiments. METHODS Variation in egg size. -A cohort of pupae, part of the seventh generation after colonization, was allowed to emerge in a screened cage (17 x 17 x 35 cm) with 10% sugar water available. The original individuals for the colony were collected from tires in Dade Co., Florida. Week-old females were removed from their emergence cage, anesthetized and weighed to the nearest .01 mg. They were held separately overnight without sugar water and each was allowed access to a mouse the next day. Engorged females were immediately anesthetized and weighed to the nearest .01 mg. Each was placed in a separate cylindrical paper cage (diam = 17 cm, height = 18 cm) with a soaked raisin (as a sugar source) and an oviposition vial containing 10 ml distilled water and lined with paper towelling. After 2 weeks, females were dissected and University of Florida, Institute of Food and Agricultural Sciences, Journal Series No. 4522.