GENETICS OF LIFE-HISTORY IN DAPHNIA-MAGNA .2. PHENOTYPIC PLASTICITY

The phenotypic plasticities of life-history traits in 46 clones of the planktonic crustacean Daphnia magna were measured across two feeding conditions. Plasticity estimates for clutch sizes and offspring lengths of the first six clutches and for lengths of eight successive adult instars allowed us to compare plasticity between and within these three trait groups. Data were standardized (mean = 0, variance = 1) in each environment before analysis. The broad-sense heritability of plasticity of adult length increased from about 0 in the adolescent instar to 60 per cent in the 7th adult instar, while narrow-sense heritability was low for all instars. For clutch sizes, narrow- and broad-sense heritabilities were around 25 per cent. For offspring length, they were mostly close to zero. A comparison of three methods of quantifying plasticity showed that the heritabilities of trait differences across environments and the heritabilities based on genotype by environment interaction components were consistent with each other, but the later is always smaller. Cross-environment genetic correlations gave qualitatively different results.