PHENOTYPIC PLASTICITY AND CONSTANCY OF LIFE-HISTORY TRAITS IN LABORATORY CLONES OF DAPHNIA-MAGNA STRAUS - EFFECTS OF NEONATAL LENGTH

1. The phenotypic constancy of four laboratory Daphnia magna clones in fitness-related life-history traits, such as age and clutch size at m aturity, was studied among consecutive experimental runs in differing food environments. 2, A significant part of the observed clonal and ge netic-by-environmental variation in age and clutch size at maturity wa s explained by experimentally uncontrollable variations in neonatal bo dy length. 3, Despite food availability, neonatal length determined th e number of instars invested to maturity and thus maturation age. Clon al differences in neonatal length and thus in maturation instar occurr ence across environments explained most of the clonal variability obse rved in maturation age. 4. Although interclonal differences in clutch size existed, most of the phenotypic plasticity observed for clutch si ze was mediated by clonal differences in neonatal length. 5. Clonal di fferences in neonatal length and in the occurrence of maturation insta rs across environments dramatically affected the body length of instar IM-2 where provisioning of eggs take place. Since clutch size is dete rmined from clutch mass and clutch mass was strongly related to the bo dy length of instar IM-2, clonal differences across environments in bo dy length of instar IM-2 mirrored clonal differences across environmen ts in clutch size. 6, The results reported in the present study show t hat maternally mediated traits such as neonatal length affect how geno types respond to different environmental selection regimes (genetic-by -environmental interaction). Future research needs to focus on the eff ects of neonatal length on the heritability or genetic variation of th e reaction norms, since prediction of the response to selection is a k ey research objective in quantitative genetic studies.