OCCURRENCE AND PHYLOGENETIC SIGNIFICANCE OF “SPECIAL WALLS” AT MEIOSPOROGENESIS IN COLEOCHAETE

Within germinating zygotes of Coleochaetepulvinata, meiospores are individually surrounded by chamber walls which are ultrastructurally and chemically different from vegetative cell walls of the same species. Meiospore chamber walls exhibit the staining reactions typical of callose. They thus resemble the "special walls" present during sporogenesis in embryophytes. Their presence suggests that the charophycean green algal ancestors of land plants may have possessed spore development preadaptations influential in the evolution of walled spores, an important plant adaptation to terrestrial life. FOR OVER A CENTURY, the green alga Coleochaete has been regarded as a relative of land plants, and during the last 15 years, modem ultrastructural and biochemical evidence in support of such a relationship has rapidly been accumulating. In 1971, McBride found a multilayered structure (MLS) in zoospores of Coleochaete scutata, which was similar to MLSs known to occur in spermatozoids of land plants. Pickett-Heaps and Marchant (1972) confirmed this discovery, and also found a phragmoplast similar to that of land plants in dividing cells of C. scutata (Marchant and Pickett-Heaps, 1973). Since then, MLSs have been found in spermatozoids of three species of Coleochaete (Graham and McBride, 1979; Graham and Wedemayer, 1984), in zoospores of C. pulvinata (Sluiman, 1983) and in meiospores of C. pulvinata (Graham and Taylor, in press). Glycolate oxidase, an enzyme characteristic of land plant peroxisomes, but not of noncharophycean algal microbodies, was found in Coleochaete by Frederick, Gruber, and Tolbert (1973), while Marchant and Pickett-Heaps (1973) demonstrated the ultrastructural presence of a land plant-like peroxisome in C. scutata. Some species of Coleochaete have evolved apical cell division (Hagemann, 1978), a parenchymatous growth habit (Graham, 1982), and multicellular antheridia (Wesley, 1930; Graham, 1984). The evolutionary development of these latter three characters is thought to illustrate a way in which the charophycean ' Received for publication 18 September 1985; revision accepted 8 January 1986. ThisresearchwassupportedbyNSFgrantsPCM8308441 and BSR 8306909. The use of Dr. R. Evert's TEM facility is gratefully acknowledged. We also thank Dr. L. Wilcox for substantial technical assistance, R. Spear and Dr. W. Russin for advice and assistance with fluorescence microscopy, and Dr. R. Patton for use of facilities. ancestors of land plants could also have evolved these features. Coleochaete also provides a model for the origin of the land plant sporophyte (Graham, 1984, 1985). Thus, it seems likely that Coleochaete may be the source of further clues to the origin of other land plant characters. Among green algae, Coleochaete is unique in having zygotes which become multicellular during germination (Pringsheim, 1860). At first it was believed that these multicellular zygotes represented early stages in the origin of a sporophyte generation (Bower, 1908). However, Allen (1905) and Hopkins and McBride (1976) demonstrated that the first division of the zygote was a meiotic one, so that the resulting cells, which all differentiate into meiospores, are haploid. Nevertheless, the germinating zygotes of Coleochaete are intriguing because within them, each meiospore develops within a chamber wall. These chamber walls are different from ordinary vegetative cell walls in their almost complete lack of plasmodesmata (Graham and Taylor, in press). They reminded us of the "special walls" which typically surround spore mother cells (sporocytes), or members of the spore tetrad formed by meiotic division of the sporocyte in embryophytes (Esau, 1965; Doyle, 1970; Bold, Alexopoulos, and Delevoryas, 1980). Special walls are thought to function during sporogenesis to isolate developing spores from the influences of neighboring cells and to serve as a template for deposition of more permanent spore walls. Generally, the special wall consists at least partially of callose, a 0-1,3-glucan (Esau, 1965; Doyle, 1970). Consequently, we tested the chamber walls surrounding meiospores of Coleochaete pulvinata for the presence of callose. This investigation is part of a larger study designed to evaluate the evolutionary gap be-