ENVIRONMENTAL AND GENETIC-CONTROL OF SEXUAL REPRODUCTION IN DAPHNIA

Sexual reproduction in cyclically parthenogenetic Daphnia species is a critical life history component. To investigate the nature of variati on of the intensity of sexual reproduction (ISR) in Daphnia, and to me asure the relative importance of different aspects of the variation, 3 0 random clones from a Daphnia pulicaria population were tested in a f actorial design involving five photoperiods and two food concentration s. The results revealed that the ISR, indexed by the number of resting eggs produced, is influenced significantly by both the environmental conditions employed (photoperiod, food) and genetic factors (clonal ef fects within single environments and clone-photo-food interaction acro ss environments). At the population level, a critical photoperiod exis ts (14 h light/day) for sexual reproduction to be initiated. There are significant differences among the estimates of the genetic variation for the ISR across some environments, signifying that the measurement of genetic variability in one environment has limited value for inferr ing the magnitude of genetic variability in other environments. There is highly significant genotype-environment (G x E) interaction, accoun ting for about 37.5 per cent of the total variance of the ISR. The res ults are discussed in the context of the population's adaptation to th e ecological surroundings and the maintenance of genetic variability f or the ISR in a particular environment.