Morphology, cytology and sexual reproduction in the aerophytic cave diatom Luticola dismutica (Bacillariophyceae)

Monoclonal cultures of the aerophytic cave diatom Luticola dismutica were studied and its frustule morphology, cytology and reproduction recorded. Luticola dismutica is a laterally asymmetrical, monoplastidic pennate diatom with imposed chloroplast division and nuclear behaviour of type 1.A sensu Mann & Stickle. Clones of L. dismutica decreased in cell size in culture until they have reached the sexual size range. Homothallic sexual reproduction and auxosporulation (type IB1a auxosporulation sensu Geitler) were induced in four sexualized clones. Gametangia paired via the girdle, two isogametes were formed per gametangium and hence two zygotes were produced per pair of gametangia. No surviving superfluous nuclei were observed in the gamete and zygote stages and no unfused haploid nuclei were seen in the auxospore stage; zygotes and expanded auxospores had only one nucleus. Auxospores expanded perpendicular to the apical axis of gametangia. Expanded auxospores and initial cells had a swollen central part, the linear-lanceolate outline shape of the vegetative valves was restored during the first divisions of the post-initial cells. Initial cells left the perizonium by a route unique to pennate diatoms, through a transverse rupture of the perizonium. The key cytological and reproductive characteristics reviewed in this paper indicate, that Luticola is more closely related to Placoneis and Dickieia, than to Navicula sensu stricto.