Reproduction in Scenedesmus

Algae > Volume 11(2); 1996 > Article

Algae 1996;11(2): 183-201.

Francis R. Trainor

Ecology and Evolutionary, Biology University of Connecticut

ABSTRACT

Scenedesmus species are widely distributed in fresh waters throughout the world. Two basic types would include those with fusiform or spindle shaped cells with those possessing oblong-ovate cells; the latter are often spiny. Inasmuch as the basic form is a simple colony, often consisting of but four cells, one is tempted to believed that we can easily understand Scenedesmus reproduction, as well as to identify specimens encountered. Colonies reproduce asexually by successive divisions of the protoplast within the parent cell wall, and when progeny are released, the parent wall remains. Young spiny colonies have all of their ornamentation upon their release. Daughter colony may be morphologically identical to the parent, or they may exhibit remarkable phenotypic plasticity. The plasticity is best known within spine-bearing species. Inasmuch as environmental conditions determine what cytological features are displayed, the individual coenobial types for each genotye are termed ecomorphs. Since this plasticity would include production of spiny unicells, there are obvious taxonomic complications. In carefully controlled laboratory experimentation, an ordered sequence of development has been detected, with spines produced at predetermined sites. The most spiny morphs, whether unicells or colonies, develop when conditions are optimal. These ecomorphs, which easily remain in the plankton, are recorded early in log growth. In nature one expect to observe them in the spring. Spineless coenobia, lacking appendages which provide frictional resistance to setting, are detected late in the season or when growth conditions are not optimal. Scenedesmus sexual reproduction has been reported only in the laboratory, although flagellated cells (gametes) have been observed in outdoor fish ponds. Both spiny and spine-less species may reproduce sexually, but species such as S. obliqus(Turp.) Kutz. provide the most favorable material. Gametes are without walls, biflagellated, photosynthetic and heterothallic, however, thus far they have not been produced in large numbers. Once active, gametes clump and pair; fusing gametes soon form a quadriflagellated zygote. The latter enlarges, produces a thick smooth wall and upon germination numerous(up to 32) single cells result. Presumably the mating types are represented by half that number.

Key words: asexual reproduction, ecomorph, ordered sequence of development, phenotypic plasticity, Scenedesmus, sexual reproduction, spine site