EFFECTS OF FOOD, GENOTYPE, AND MATERNAL SIZE AND AGE ON OFFSPRING INVESTMENT IN DAPHNIA-MAGNA

This laboratory study of Daphnia magna had two major aims: (1) to examine the effect of food quantity on offspring size and number and how this effect varies with genotype and maternal size, age, and metabolic demand; and (2) to examine the relationship of investment per offspring to total reproductive investment. Differences in the response of offspring size to food quantity between the two clones in this study and between Daphnia studies in the literature are explained by a nonlinear model that explicitly considers the effect of food quantity relative to the metabolic demands of the mother. According to this model, offspring size covaries positively with food quantity/maternal demand at very low food levels because of reproductive constraints, but it covaries negatively with food quantity/maternal demand at middle food levels as an adaptive response, and it remains constant at very high food levels because of a lower limit on viable offspring size. Effects of genotype (clone) and ration level on trade-offs between egg size and number and on the maternal size-dependence of egg mass, brood mass, and brood size are described. Strong genotype-environment interactions are evident for egg size and number. Trade-offs between egg size and number are seen at high and low ration levels, but only in second (or third) to sixth broods, and not in first broods, which had relatively high variation in total mass. Results of this study that contradict predictions or assumptions of optimal offspring investment theory include: (1) at both ration levels both egg size and number were strongly positively correlated with brood mass, independently of maternal mass, (2) although, at low ration, variability of brood mass usually exceeded the variability of egg mass, as expected, the opposite was true at high ration, and (3) the interclonal difference in offspring size was not correlated with survival ability under starvation. Offspring size covaried positively with maternal size and age, partly independently of culture conditions. This relationship is explained as the result of selection in growing parthenogenetic populations for increased fecundity in young small females, which are constrained to produce relatively small eggs because of spatial limitations of the brood pouch.