Sexual reproduction: where and when?

Dense cell suspension of Cylindrocystis brebissonii

Image © Frantisek Hindak

Mating, conjugation and zygospore formation in Cylindrocystis brebissonii

Image © Frantisek Hindak

Zygospore of Closterium setaceum (a relative of Cl. kuetzingii)

However, of the vast majority of desmid species zygospores are only very incidentally reported or even quite unknown. For example, of Micrasterias truncata, by far the most common Micrasterias species in Europe, there is only one single record (Homfeld 1929).

Obviously, species-specific sexual potential is genetically determined. Apart from the fact that for conjugation often the simultaneous presence of physiologically different + and – cells is required, presumably in many species sexual potential in general is degenerated (Coesel 1974, Coesel & Teixeira 1974). Possibly this has to do with the environmental conditions under which most desmids use to flourish, i.e. peaty habitats that seldom or never dry out completely. In such an environment desmid cells may easily survive in vegetative state. Sexual reproduction, involving genetic recombination and attended by the formation of thick-walled, resistant zygospores, on the contrary, is particularly functional in instable habitats like wetlands that dry out thoroughly and for a lengthened time.

In this connection it is significant that European clones of Closterium ehrenbergii isolated from pools on clayey or sandy bottoms periodically drying out exhibited mating behaviour where clones of that same species isolated from peaty habitats could not be induced to any sexual activity (Coesel 1989). But even the fertility of the first mentioned group of European clones appeared strongly reduced as compared with that of many (sub)tropical strains of that same morphospecies (Coesel 1988, Ichimura et al. 1997).

A great deal of the (sub)tropical clones of Closterium ehrenbergii were isolated from paddy fields, known to dry out periodically.There are indications that zygospore formation in desmids in (sub)tropical regions is less rare than in temperate/boreal regions, maybe because of a greater difference between wet and dry seasons (Handke 1996).

Zygospore formation in the rare, filamentous species Matteola curvata, a (sub)tropical taxon from the American continents (Coesel 1997)


Image © Peter Coesel

The process of sexual reproduction