Gary L. Floyd

MITOSIS, CYTOKINESIS, THE DISTRIBUTION OF PLASMODESMATA, AND OTHER CYTOLOGICAL CHARACTERISTICS IN THE ULOTRICHALES, ULVALES, AND CHAETOPHORALES: PHYLOGENETIC AND TAXONOMIC CONSIDERATIONS1

The results of previous research and a present survey of some of the cytological characteristics of 18 additional genera and 34 additional species are presented and discussed from the viewpoint of phylogenetic and taxonomic significance. A preliminary attempt is made to place these, algae, in 4 orders on the basis of comparative cytology, and in particular on the basis of variation in mitosis and cytokinesis and the distribution of plasmodesmata. Consideration is given to the evolution of the phragmoplast and to the hypothesis that the Chaetophorales are related to the ancestry of land plants.

CELLULAR ORGANIZATION, MITOSIS, AND CYTOKINESIS IN THE ULOTRICHALEAN ALGA, KLEBSORMIDIUM1,2,3

Mitosis, cytokinesis, and cellular organization during interphase are described. Interphase cells possess a single microbody‐like organdie which occurs between, and is appressed to, the chloroplast and nucleus. The microbody‐like organelle divides during mitosis and. the division of the chloroplast. Anaphase is unusual in that chromosome‐to‐spindle pole distance remains constant even though the 2 groups of chromosomes become widely separated. When anaphase is half completed, a vacuole forms in the interzonal region and appears to be involved in further separation of the chromosomes. Vacuolar development is also involved with events of early interphase. The cytology of K. flaccidum is compared to those of Ulothrix fimbriata and Stigeoclonium helveticum. The comparison, does not support the present classification of the 3 species, and indicates the value of comparative fine structural studies in the classification of ulotrichalean algae.

COMPARATIVE CYTOLOGY OF ULOTHRIX AND STIGEOCLONIUM1

This investigation describes the cytology of the ulotrichalean genera Ulothrix and Stigeoclonium. Cellular organization is similar to the degree that interphase cells of the 2 genera cannot be distinguished with certainly. In Stigeoclonium, the nuclear envelope becomes disrupted at the end of prophase, and centrioles enter the nucleoplasm. At metaphase the nuclear envelope is again intact, and some of the spindle tubules appear to be contiguous with the nuclear envelope. The spindle in Ulothrix is essentially open with, no attachment of spindle tubules to the nuclear envelope and with, centrioles on the spindle‐cytoplasm interface at the spindle poles. Spindle poles are blunt in Stigeoclonium and pointed in Ulothrix. Cytokinesis is by cell plate formation in both genera, but there is no phragmoplast.

Polyphyly of tetrasporalean green algae inferred from nuclear small-subunit ribosomal DNA

Ultrastructural studies of tetrasporalean green algae have suggested the order is polyphyletic. These features, including the absolute orientation of the flagellar apparatus and the bi‐ versus quadriflagellated motile cell morphology, suggest that Chaetopeltis as well as a number of others, may be ancestral to a group that includes Tetraspora. In this study, we examine the phylogenetic relationships of selected tetrasporalean taxa based on analysis of 18S ribosomal RNA gene sequences. Results show that the tetrasporalean taxa are polyphyletic. Biflagellated genera group with biflagellated volvocalean taxa, whereas the quadriflagellated species compose a distinct monophyletic clade not closely related to the biflagellated taxa. In addition, tetrasporalean taxa group with other chlorophycean algal species with similar flagellar apparatus absolute orientation, but the quadriflagellated Tetrasporales do not appear to be ancestral to the entire Chlorophyceae. These results are concordant with previous conclusions drawn from ultrastructural data and further confirm the utility of (small‐subunit) ribosomal RNA gene sequences to discern green algal evolutionary relationships.

CYTOKINESIS AND PLASMODESMATA IN ULOTHRIX1

Cytokinesis essentially similar to that of vascular plants occurs in Ulothrix, an unbranched filamentous green alga. Plasmodesmata, similar to those of vascular plants, but different from those of many other algae, are also present. Cell plate formation and plasmodesmata also occur in Stigeoclonium, a branched green alga.

ORIGINS AND AFFINITIES OF THE FILAMENTOUS GREEN ALGAL ORDERS CHAETOPHORALES AND OEDOGONIALES BASED ON 18S rRNA GENE SEQUENCES

The order Chaetophorales includes filamentous green algae whose taxonomic relationships to other chlorophycean orders is uncertain. Chaetophoralean taxa include filamentous species which are both branched and unbranched. Ultrastructural studies of zoospores have revealed similar flagellar apparatuses in a number of genera, including Uronema, Stigeoclonium, and Fritschiella, suggesting a close phylogenetic relationship among these taxa. The order Oedogoniales represents a second group of branched and unbranched filamentous green algae whose relationships to other chlorophycean orders also has been unclear. A possible close relationship between the Chaetophorales and Oedogoniales has been suggested. Using DNA sequences from the small‐subunit ribosomal RNA gene (SSU rRNA) of several members of each order, we have examined the monophyly of the Chaetophorales and Oedogoniales, as well as the nature of their relationship to other chlorophycean orders. Our results show that chaetophoralean and oedogonialean taxa form separate monophyletic groups. Results also suggest that the two orders are not closely related to each other.