The dynamics of sexual phase in the marine diatom Pseudo‐nitzschia multistriata (Bacillariophyceae)

Abstract

Sexual reproduction represents a fundamental phase in the life cycle of diatoms, linked to both the production of genotypic diversity and the formation of large-sized initial cells. Only cells below a certain size threshold can be sexualized, but various environmental factors can modulate the success of sexual reproduction. We investigated the role of cell density and physiological conditions of parental strains in affecting the success and timing of sexual reproduction in the marine heterothallic diatom Pseudo-nitzschia multistriata. We also studied the dynamics of the sexual phase in still conditions allowing cell sedimentation and in gently mixed conditions that keep cells in suspension. Our results showed that successful sexual reproduction can only be achieved when crossing parental strains in the exponential growth phase. Evidence was provided for the fact that sexual reproduction is a density-dependent event and requires a threshold cell concentration to start, although this might vary considerably amongst strains. Moreover, the onset of the sexual phase was coupled to a marked reduction in growth of the vegetative parental cells. The crosses carried out in physically mixed conditions produced a significantly reduced number of sexual stages as compared to crosses in still conditions, showing that mixing impairs sexualization. The results of our experiments suggest that the signaling that triggers the sexual phase is favored when cells can accumulate, reducing the distance between them and facilitating contacts and/or the perception of chemical cues. Information on the progression of the sexual phase in laboratory conditions help understanding the conditions at which sex occurs in the natural environment.

Finding a partner in the ocean: molecular and evolutionary bases of the response to sexual cues in a planktonic diatom

“…In a cross, both MTs behave similarly (most of the cells arrest their growth and only a small fraction, c . 20%, undergo meiosis; Scalco et al ., 2014), and the gametes produced are morphologically indistinguishable (Fig. 1).…”

“…4b). Previous observations over a longer period of time (14 d, Scalco et al ., 2014) have revealed a marked decrease in growth of P. multistriata cultures undergoing sexual reproduction. The occurrence of growth arrest concomitantly with sexual reproduction is known in yeast, where the cell cycle is arrested in G1 as a consequence of pheromone signalling (Wilkinson & Pringle, 1974).…”

“…1). Sexual reproduction requires a threshold cell concentration to start, suggesting that chemical signalling is needed to allow the induction of the sexual phase in this species (Scalco et al ., 2014). Diffusible chemical cues have been shown to be responsible for a multi‐step sexualization process, and two sex pheromones have been characterized for the benthic diatom Seminavis robusta (Gillard et al ., 2013; Moeys et al ., 2016).…”

Isolation by Time During an Arctic Phytoplankton Spring Bloom

“…Also, sexual reproduction in diatoms, which produces novel genotypes and, therefore genetic diversity, requires relatively high cell concentrations to initiate (Scalco et al. ). Bloom formation thus may be an important phytoplankton strategy.…”

The diatom molecular toolkit to handle nitrogen uptake

“…Interestingly, in the sexually reproducing diatom P. multistriata, growth can be arrested by specific signals during mating even in nutrient replete conditions (Scalco et al, 2014).…”