VARIATION AND PLASTICITY OF BIOMASS ALLOCATION IN DAPHNIA

1. For organisms with indeterminate growth a trade-off between growth and reproduction is expected. The detection of this trade-off depends not only on the covariance between these two traits, but also on the v ariation of their sum, the total production (TP = growth + reproductio n). The smaller the relative variation in TP, the more likely it is to detect the trade-off. To investigate this trade-off, we studied bioma ss allocation and variation of 56 clones of Daphnia magna from two pop ulations under two food conditions. 2. Broad-sense heritabilities of c lutch mass, growth, TP and the allocation ratio (R = clutch mass/TP) w ere higher in rich than in poor food conditions. Growth and R showed n o genetic variation at the low food level. 3. No negative correlations were found for the expected trade-off between biomass allocated to re production and to growth. Correction for the time period between two r eproductive events and for body length, both of which might influence TP, did not influence this result. However, some negative correlations were found between current reproduction and growth during the subsequ ent instar. 4. Two-way ANOVA with R as the dependent variable show sig nificant food and population effects for some instars. Including lengt h as a covariable in this two-way ANOVA revealed significant effects f or the covariable and for food level in all instars. Population effect s and population x food interactions were significant in all but two a dult instars, indicating that body size does not constrain genetic var iation for biomass allocation, and that the food level plays an import ant role in detecting population effects.