QUANTITATIVE GENETICS OF ZOOPLANKTON LIFE-HISTORIES

Quantitative genetic techniques are powerful tools for use in understa nding the microevolutionary process. Because of their size, lifespan, and ease of culture, many zooplankton species are ideal for quantitati ve genetic approaches. As model systems, studies of zooplankton life h istories are becoming increasingly used for examination of the central paradigms of evolutionary theory. Two of the fundamental empirical qu estions that zooplankton quantitative genetics studies can answer are: 1) How much genetic variance exists in natural populations for life h istory traits? 2) What is the empirical evidence for trade-offs that p ermeate life history theory based on optimality approaches? A review o f existing data on Daphnia indicates substantial genetic variance for body size, clutch size, and age at first reproduction. Average broad-s ense heritabilities for these three characters across 19 populations o f 6 species are 0.31, 0.31, and 0.34, respectively. Although there is some discrepancy between the two pertinent studies that were designed to decompose the total genetic variance into its additive and non-addi tive components, a crude average seems to suggest that approximately 6 0% of the total genetic variance has an additive basis. The existing d ata are somewhat inconsistent with respect to presence/absence of trad e-offs (negative genetic correlations) among life history traits. A co mposite of the existing data seems to argue against the existence of s trong trade-offs between offspring size and offspring number, between present and future reproduction, and between developmental rate and fe cundity. However, there is some evidence for a shift toward more negat ive (less positive) covariances in more stressful environments (e.g., low food). Zooplankton will prove to be very useful in future study in several important areas of research, including the genetics and physi ology of aging, the importance of genotype-environment interaction for life history traits, and the evolution of phenotypic plasticity.