Developmental studies in trebouxioid algae and taxonomical consequences

Abstract

In the genusTrebouxia (incl.Pseudotrebouxia) two sorts of nonmotile reproductive cells exist: autospores and aplanospores. In subg.Trebouxia small mother cells give rise to 4, 8, or 16 autospores, while comparatively large mother cells develop into zoosporangia or, if the release of zoospores is arrested, into aplanosporangia. Both zoo- and aplanosporangia contain (32) 64 or 128 daughter cells. The transformation of trophic cells into zoo-/ aplanosporangia starts with the formation of a local thickening of the cell wall that marks the prospective opening, and (in most species) with the disappearance of the pyrenoids; sooner or later strong starch deposition can be observed. In subg.Eleutherococcus autospores do not occur; zoo- and aplanosporangia are formed essentially in the same way as in subg.Trebouxia. Differences occur between the form and position of chloroplasts during successive divisions: flattened and parietal in subg.Eleutherococcus, not flattened and ± central in subg.Trebouxia. InEleutherococcus, besides large cells also relatively small cells may produce zoo- or aplanospores.—Dictyosomes could be observed in the living state in representatives of subg.Eleutherococcus under optimal conditions. In trophic cells they are arranged in a group surrounding a hyaline area at the side of the nucleus. In young uninuclear sporangia they are positioned between the nucleus and the local thickening of the cell wall. In somewhat older sporangia they occupy mainly those parts of the nuclear surface which is turned towards the cytoplasmatic cleavage furrow. In subg.Trebouxia dictyosomes could not be observed by light-microscopy. In several species the chloroplast lobation (observed under optimal conditions) differs from that described in the literature.