The present study on Brachionus plicatilus provides some new information on the biology of sexual reproduction in this species. It demonstrates the importance of density as inductive factor of mixis and the existence of intraspecific differences in the control of sexual reproduction. Furthermore, it helps to interpret the functional significance of density response in B. plicatilis. Finally, the study permits an approach to the analysis of the ecological significance of the evolution of stimuli for the transition from parthenogenetic to sexual reproduction.

The study is structured in two parts. In the first part, intraspecific variability due to both age and genotype in B. plicatilis is studied using a modified ultrasensitive silver-stain procedure to detect total proteins in polyacrilamide gels. Results from analysis of intraspecific genetic variability show a high level of interpopulational variation, contrasting with a low level of intrapopulational variation. Analysis of total-protein patterns related to age show the existence of age-dependent changes which permit the characterization of individual aging. Variability is great among isogenic individuals cultured in the same environment.

The aim of the second part, the main part of the study, is to determine the optimal timing of mixis in rotifer heterogenic cycles. This part focuses on the study of mixis induction, on related internal and external factors, and on implications of the sexual reproduction pattern in B. plicatilis. A dynamic model has been developed to theoretically address rotifer mixis. Results from simulations stress that the optimal timing for mixis is mainly related to the moment at which the habitat becomes unsuitable for population growth. Empirical results from the study manifest the effect of population density on mictic-female production and its modulation by both parental age and genotype. Experiments carried out to determine the density effect mode of action show that it may be attributed to the accumulation of some chemical substance released into the medium by the rotifers themselves. The detected unspecific chemical effect of crowding on mictic-female production in B. plicatilis suggests that the role of population density in mixis induction may be related to a strategy to escape through diapause from a deteriorating habitat. Finally, results from demographic and biochemical analysis show that sexual reproduction has important physiological implications: the aging process occurs earlier in mictic females than in amictic ones, probably as a result of differences in patterns of reproductive resource allocation between the two types of females.