Morphology and life history of <i>Amphora commutata</i> (Bacillariophyta) II: Sexual reproduction and auxosporulation, with special reference to auxospore wall structures

In most diatoms, cell size decreases progressively during vegetative divisions and recovers after sexual reproduction, as a special zygote called the auxospore expands in volume and forms the maximum cell size for the species within it. Although auxospore structure shows significant diversity, including incunabular scales, bands, and strips, there is still limited information available on them compared to vegetative cells. This is mainly due to the short duration of the sexual phase in the diatoms’ life cycle compared to the vegetative phase and the challenge of observing these stages in natural samples. In this study, we provide a comprehensive account of sexual reproduction in Amphora commutata, where two cells pair via the girdle, each forming two gametes that fuse isogamously in the space between the gametangia. The auxospores expand parallel to each other and perpendicular to the long axes of the gametangia. Nuclear behaviour during sexual reproduction was revealed using Feulgen staining, which is uncommon in diatom research, showing an intense DNA-specific stain against a colourless background. The fine structure of the auxospore wall is also documented: the perizonium is composed of transverse and longitudinal perizonial bands, with incunabular scales that vary in shape from circular to highly elongated. The elongated scales possess a unique structure in this species, having a straight rib along their long axis, clearly differing from the ‘simple scales’ reported in many diatoms, which are circular or elliptical in outline with a ring-like annulus as a pattern centre.