GENETICS OF LIFE-HISTORY IN DAPHNIA-MAGNA .1. HERITABILITIES AT 2 FOOD LEVELS

Broad- and narrow-sense heritabilities for several life-history traits were estimated from 23 mother-daughter pairs of Daphnia magna at two food levels. Sexually produced daughter clones were obtained from a field collection of ephippial females (mothers) and the subsequent hatching of their ephippial eggs (daughters). Mother and daughter clones were maintained by parthenogenetic reproduction. Heritabilities of adult body-length of eight successive instars were the highest estimates of all in good food, but the lowest of all in poor food. For clutch size and body-length of offspring from the first six clutches, narrow- and broad-sense heritabilities were about equal and lower in poor food than in good food. The amount of genetic variation present would allow a response to selection on clutch size and offspring length in both environments but adult length only in good food. For age at maturity we found no additive genetic variance. We found no difference in broad-sense heritabilities between mother clones, representing the last generation after a period of asexual reproduction, and their sexually produced offspring. This suggests that genetic variance does not increase after one sexual generation or that it was not reduced before. Differences in heritabilities between environments are discussed with reference to the enlarged phenotypic variances that result from variation in juvenile instar number. Targeted growth could explain the pronounced differences in the heritabilities of adult length between environments.