The trade-off between egg size and fecundity in the bruchid beetle Callosobruchus maculatus : a quantitative genetic approach.

Female Callosobruchus maculatus have fixed reproductive resources to allocate to offspring. It is predicted that the option of investing in a few large eggs or many small eggs will result in a trade-off between egg size and fecundity in this species. The aim of this study was to examine this proposed trade-off and obtain empirical evidence for its significance in the life history of C. maculatus. Breeding and selection experiments were used to measure the genetic correlation between egg size and fecundity as evidence for the existence of the trade-off. Selection experiments also allowed egg size to be manipulated to investigate its intrinsic effects on offspring fitness.
Chapter 1 introduces life history theory and trade-off from the distinct but complementary perspectives of optimality theory and evolutionary genetics. Empirical methods that can be used to measure trade-offs are evaluated with respect to the egg size versus fecundity trade-off and the biology of C. maculatus. In Chapter 2 the general experimental methods are described.
Phenotypic correlations involving egg size, fecundity and other life history traits are measured in Chapter 3. Egg size is not phenotypically correlated with fecundity but varies with maternal emergence weight and age. Correlative effects of egg size on offspring fitness appear to be due to effects on offspring body weight and development rate.
The presence of genetic variation in egg size and fecundity are demonstrated in Chapter 4. In Chapter 5 the genetic correlation between egg size and fecundity is measured by means of a half sib breeding experiment. A negative genetic correlation of low precision was obtained. Chapter 6 provides a second estimate of the genetic correlation from an experiment in which egg size was increased and decreased by artificial selection. The correlated response in fecundity was downwards in both directions of selection which was not consistent with the proposed trade-off. The genetic correlation estimated was positive. Reasons for the unexpected response are discussed.
The effects of selection for egg size on other life history traits is examined in Chapter 7. Only development rate, fecundity and emergence weight in females had changed after selection.
Chapter 8 discusses what the quantitative experiments have revealed about the trade-off between egg size and fecundity and assesses the merits of the methodologies.