Zygospore morphology in the conjugating green alga Spirotaenia diplohelica (Streptophyta, Zygnematophyceae, Mesotaeniaceae)

The zygospore of the saccoderm desmid Spirotaenia diplohelica is described for the first time. The spore is most peculiar, for surrounded by a series of transparent, conical projections. In that feature it very much resembles the zygospore of Spirotaenia truncata. Contrary to Spirotaenia condensata and Spirotaenia obscura, no twin zygospores are produced and walls of empty gametangial cells do not solve in an early stage but tend to stick to the zygospore for some time. In view of those essentially different modes of sexual reproduction it is not to be excluded that species in the traditional genus Spirotaenia belong to different phylogenetic cladess.