Genetic and Microscopic Evidence for Sexual Reproduction in the Centric Diatom Skeletonema marinoi

This study provides microscopic and molecular evidence for sexual reproduction in the homothallic centric diatom Skeletonema marinoi isolated from the Baltic Sea. The species is capable of restoring cell size asexually through an auxospore-like stage. However, cells were sexualized after shifting strains from low (6 PSU) to high (16 PSU) salinity. We observed flagellate male gametes and oogonia, with diameters of 3-4 and 3.2-6.3 μm, respectively. Fertilization took place followed by the formation of round auxospores surrounded by thin siliceous incunabular scales. Auxosporulation was synchronized, and a maximum of auxospores was detected on day three following the salinity shift. The proportion of auxospores to vegetative cells ranged from 0.02 to 0.18. There was a significant correlation between auxosporulation success and inoculum cell density. At lower cell concentration (5,000 cells ml-1), proportionally fewer auxospores were formed. Auxospores were formed in single strains and in crosses of strains. The proportion of auxospores differed significantly among strains and crosses of strains. Additionally, we isolated single auxospores, obtained F1 strains and performed microsatellite based pedigree analysis of parental generations and their offspring. We proved that the auxospores were formed sexually, either by inter- or by intra-strain fertilization.