Biochemistry and regulation of the heteromorphic life cycle of the green alga Derbesia-Halicystis

Abstract

During the life cycle of the green marine alga Derbesia marina-Halicystix ovalis, a diploid filamentous sporophyte called Derbesia alternates with a haploid spherical gametophyte called Halicystis. Beside this wild type a so-called mutant and a parthenogenetic enogenetic form stand at our disposal. The mutant is a haploid form with a Derbesia-like morphology. The morphology of the parthenogenetic form is also very similar to that of Derbesia. Biochemically sporophyte and gametophyte are characterized by a different cell-wall composition and enzyme pattern. The main polysaccharide of the cell wall of the sporophyte (Derbesia) is a mannan, whereas the cell wall of the gametophyte is built up mainly of glucose- and xylose-containing polysaccharides. The cell wall composition of the mutant is nearly identical with that of the Derbesia wild type. On the other hand the parthogenetic form lies to a certain degree between Derbesia and Halicystis with respect to its cell wall composition. Differences in enzyme pattern exist especially in the enzymes of GDPM- and mannan synthesis. These enzymes are present in high activities in Derbesia, the mutant, and the parthogenetic form but are absent or present only with very small activities in Halicystis. From these results we conclude that the differences in morphology, cell wall composition and enzyme pattern between the sporophyte (Derbesia) and the gametophyte (Halicystis) are brought about by a relatively stable pattern of gene activity which is changed only during the formation or the out-growth of the zoospores and the formation or fusion of gametes (see discussion).