Factors regulating transitions among life cycle phases in the marine pennate diatom Pseudo-nitzschia multistriata

Abstract

The marine pennate diatom Pseudo-nitzschia multistriata has a heterothallic life cycle and the two mating types ('+' and '-') need to be in contact to allow sexual reproduction to occur. In this thesis, results 'are presented of an investigation using con focal and time lapse microscopy aimed at describing the different stages of the life cycle of this species. Results are also presented of various experiments aimed at defining the endogenous and exogenous factors that induce sexual, reproduction. Parental strains need to reach a certain cell density before undergoing sexual reproduction and an inverse correlation was observed between the inoculum size and the time required for the induction of sexual reproduction. This suggests that sexual reproduction is a density-dependent event, regulated by a chemical mediator. Support for this latter hypothesis was provided by experiments on single parental strains which underwent gametogenesis and formation of gametes when exposed to culture medium conditioned by the growth of the opposite mating type. Other support was provided by the fact that sexual reproduction was considerably reduced in cultures incubated in mixed conditions versus still ones. In the latter experimental setting, cells could settle to the bottom, therefore increasing their density and facilitating the onset of the sexual phase. Evidence was also obtained to support the hypothesis that chemical compounds produced during sexual reproduction might reduce the growth of parental strains. The results obtained in the project have provided novel insights into the mechanisms that regulate the life cycle of this planktonic diatom, showing that the transition from vegetative to sexual phase is regulated by chemical mediators. This information represents an experimental background that will allow future studies, aimed at the characterization of the chemical signalling molecules and/or at molecular investigation, to characterize the genes involved in the different life phases.