Zoospore ultrastructure in the green algaFriedmannia israelensis: An absolute configuration analysis

Abstract

Zoospore ultrastructure in the green algaFriedmannia israelensis has been studied in detail using serial section and absolute configuration analysis. The zoospores are naked, biflagellate and considerably flattened. In addition they exhibit an asymmetric external shape and an asymmetric distribution of major cell organelles. There is only a single contractile vacuole/cell. 3-dimensional reconstructions of mitochondrial profiles show the presence of a single branched mitochondrial reticulum/cell. The nucleus is pear-shaped and exhibits an anterior projection. In a concave depression of this projection lies the single dictyosome of the zoospore. The nuclear envelope presumably buds off small vesicles at the forming face of the dictyosome. The single chloroplast lacks an eyespot and pyrenoid. A single microbody is characteristically located posterior to the nucleus in close association with an elongated mitochondrial profile. The zoospores contain a peculiar system of three major ER-cisternae, of which an anterior one is linked to a mitochondrial profile. The flagellar apparatus has been studied in detail and its absolute configuration has been determined. The two basal bodies are displaced (in the 11/5 o'clock direction) and overlap at their proximal ends. They are interconnected by a principal non-striated capping plate and two smaller striated connecting fibres. The flagellar root system consists of a cruciate microtubular root system (4-2-4-2 or 5-2-5-2 system) and a greatly reduced system II fibre which connects one basal body with the nuclear membrane and runs along a smooth ER-cisterna. The 4- or 5-stranded roots are proximally associated with a terminal cap-like structure which consists of a number of parallel striations spaced 13 nm. Zoospores ofFriedmannia are structurally most similar to those of the Chaetophoralean green algaMicrothamnion. It is concluded that these two genera form a natural group and taxonomic implications are discussed. The absolute configuration analysis of the flagellar apparatus indicates thatFriedmannia has an Ulvaphycean-type flagellar apparatus. Phylogenetic aspects of the absolute configuration of the flagellar apparatus in green algae are discussed.