Heterothallic sexual reproduction in the model diatom <i>Cylindrotheca</i>

Cylindrotheca is one of the main model diatoms for ecophysiological and silicification research and is among the few diatoms for which a transformation protocol is available. Knowledge of its life cycle is not available, however, although sexual reproduction has been described for several related genera. In this study, 16 Cylindrotheca closterium strains from a single rbcL lineage were used to describe the life cycle of this marine diatom, including the sexual process and mating system. Similar to other Bacillariaceae, sexual reproduction was induced by the presence of a suitable mating partner, with two gametes produced per gametangium, resulting in two auxospores per gametangial pair. Differences with other Bacillariaceae include details of cell pairing, gamete behaviour, auxospore orientation and chloroplast configuration, and perizonium structure. The mating system is heterothallic, since strains fell into two mating type groups, with several strains of one mating type occasionally displaying intraclonal auxosporulation. Initial cell lengths were on average 95–100 µm, the sexual size threshold was at least 66 µm, and the minimal viable cell length c. 11 µm. Sexual reproduction could be synchronized by dark conditions, which allowed us to determine that the whole sexual process is completed in less than 24 h. Furthermore, large percentages of cells at defined sexual stages can easily be obtained and the possibility to experimentally manipulate the life cycle provides a valuable tool for future research on all aspects of the biology of this model diatom