THE TRADE-OFF BETWEEN OFFSPRING SIZE AND NUMBER IN DAPHNIA-MAGNA - THE INFLUENCE OF GENETIC, ENVIRONMENTAL AND MATERNAL EFFECTS

Genetic and environmental effects of various maternal traits on newborn length were studies in the planktonic crustacean Daphnia magna. Newborn lengths within broods were normally distributed, but variance tended to decrease with mother age. Clutch number, mother length, mother age and the duration of the instar during which vitellogenesis (VID) of eggs takes place were positively correlated with newborn length, while food quantity was negatively correlated. In experiment-wise multiple regressions, with newborn length as dependent variable, only the effects of mother age and clutch number were non-significant, while clutch size and food quantity showed significant negative effects and mother length and VID showed significant positive effects. The residuals of these multiple regressions showed significant genetic (clonal) effects. Clutch-wise genetic and environmental correlations of mother length and VID with newborn length were mostly positive, but many were nonsignificant. Genetic correlations of newborn length with clutch size were negative under low food conditions and around zero under high food conditions. Using partial correlations with mother length kept constant, all trade-off correlation coefficients became negative. I argue that non-negative correlation coefficients of the offspring size-number trade-off can to a large extent be attributed to variation in mother length.