Phycological Research 1996; 44: 107-108

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Research note

Parietochloris incisa comb. nov. (Trebouxiophyceae, Ch lorophyta)

Shin Watanabe,' Seishiro HirabayashiI2 Sammy Boussiba,Z Zvi Cohen,2 Avigad Vonshak' and Amos Richmond2 'Department of Biology, Faculty of Education, Toyama University, Toyama 930, Japan, 2Microalgal Biotechnology, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boker Campus 84993, Israel

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SUMMARY A coccoid green alga, Myrmecia incisa Reisigl, was iso- lated from the soil of Mt Tateyama, Japan. Electronmicroscopy revealed that the organism has py- renoids sparsely covered with starch segments and tra- versed by many parallel thylakoid membranes, and zoo- spores with counterclockwise basal body orientation. Due to the presence of these features, we have pro- posed a reclassification of M. incisa into the genus Par- iefochloris, Trebouxiophyceae.

Key words: Chlorococcales, Myrmecia, Parietuchloris, pyrenoid, soil algae, Trebouxiophyceae. ...................................................................................

Reisigl (1964) described many green soil algae from alpine regions, including a coccoid organism, Myrrnecia incisa Reisigl. The genus Myrrnecia diagnostically has a partially thickened cell wall and a parietal chloroplast without a py- renoid, and reproduces by means of naked zoospore forma- tion. Recently a coccoid green alga was obtained from a soil sample of Mt Tateyama in Toyama Prefecture, Japan, and its taxonomic position was investigated by light and electron- microscopy using methods employed by other researchers (Watanabe and Floyd 1989). Light microscope study showed it to be identical to M. incisa, while transmission electron- microscopic investigation revealed the presence of a pyrenoid in the chloroplasts. We therefore propose that this species be reclassified into a different genus, and a new classification is given in this research note.

Vegetative cells in the Mt Tateyama isolate were spher- ical, ovoid and 10-15 p m in diameter. Cell wall thickness was even in spherical cells, however, it was often partially thickened at the narrower portion of ovoid cells. Mature cells were uninucleate. The chloroplast was parietal and deeply incised in several parts, showing various morphol- ogies among individuals. Light microscope investigation revealed ellipsoidal or broad, crescent-shaped regions in the chloroplast which were relatively pale in color. A region similar to this is illustrated in Reisigl (19641, although he mentioned an absence of pyrenoids. Electronmicroscopic

study showed pyrenoids traversed by many parallel thyla- koid membranes (Fig. 1). Some starch segments were 10- cated around the pyrenoid matrix, but starch accumulation was usually found to be poor. Unless staining techniques are employed, pyrenoid recognition under the light micro- scope usually depends on the accumulation of starch sheath around the pyrenoid matrix. In particular, when starches are scarce, as in M. incisa, the pyrenoid might easily be overlooked. Taxonomically, M. incisa, in which pyrenoids were found using transmission electron micros- copy, should be excluded from the Myrmecia genus.

Zoospores were not abundantly produced by culture of the isolate. Electronmicroscopic studies of zoospores that remained in the mother cell showed that the cell wall did not cover the cell body, and that the basal bodies were arranged in a counterclockwise orientation.

The cytological features of the flagellar apparatus archi- tecture are regarded as diagnostic at the higher ranks of green algal systematics (for review see Mattox and Stewart 1984). The counterclockwise orientation of basal bodies is distributed in motile cells of the Ulvophyceae (Mattox and Stewart 1984), or the Trebouxiophyceae (Friedl 19951, which was proposed to include the organisms pre- viously accommodated in the order Microthamniales (Mel- konian 1990). So far the algal group that was classified in the Trebouxiophyceae has been partly included in the Pleurastrophyceae (Mattox and Stewart 1984). However, it was recently demonstrated that the type species of Pleu- rastrurn representing the latter class belong to the Chlo- rophyceae with clockwise basal body orientation (Friedl and Zeltner 1994). Ecologically, most organisms of the UI- vophyceae are marine inhabitants and those of the Tre- bouxiophyceae are terrestrial or lichen phycobionts. Ultra- structural and ecological features, therefore, allow for accommodation of this isolate in the Trebouxiophyceae.

In the Trebouxiophyceae there are several coccoid unicel- lular genera, including Trebouxia (Melkonian and Peveling 19881, Parietochloris (Watanabe and Floyd 1989; Deason et a/. 19911, Fusochloris (Floyd eta/. 1993) and Kenfrosphaera ...................................................................................

Communicating editor: T. Horiguchi. Received 5 October 1995; accepted 31 January 1996

108 Watanabe et a/ .

REFERENCES Deason, T. R., Silva, P. C., Watanabe, S. and Floyd, G. L.

1991. Taxonomic status of the species of the green algal genus Neochloris. PI. Syst. E d . 177: 213-19.

Floyd, G. L., Watanabe, S. and Deason, T. R. 1993. Compar- ative ultrastructure of the zoospores of eight species of Characium (Chlorophyta). Arch. Protistenkd. 143: 63-73.

Friedl, T. 1995. Inferring taxonomic positions and testing ge- nus level assignments in coccoid green lichen algae: A phy- logenetic analysis of 18s ribosomal RNA sequences from Dictyochloropsis reticulata and from members of the genus Myrmecia (Chlorophyta, Trebouxiophyceae cl. nov.). J. Phy-

Friedl, T. and Zeltner, C. 1994. Assessing the relationships of some coccoid green lichen algae and the Microthamni- ales (Chlorophyta) with 18s ribosomal RNA gene sequence comparisons. J. Phycol. 30: 500-6.

Mattox, K. R. and Stewart, K. D. 1984. Classification of the green algae: A concept based on comparative cytology. In Irvine, D. E. G. and John, D. M. (Eds) The Systematics of the Green Algae. Academic Press, London, pp. 29-72.

Melkonian, M. 1990. Microthamniales. In Margulis, L., Cor- Iiss, J. O., Melkonian, M. and Chapman, D. J. (Eds) Hand- book of Protoctista, Jones & Bartlett Publishers, Boston, pp. 652-4.

Melkonian, M. and Peveling, E. 1988. Zoospore ultrastructure in species of Trebouxia and Pseudotrebouxia (Chlorophyta).

Reisigl, H. 1964. Zur Systematik und okologie alpiner Bo- denalgen. &err. Bot. Zeitschr. 111: 402-99.

Watanabe, S. and Floyd, G. L. 1989. Comparative ultrastruc- ture of the zoospores of nine species of Neochloris (Chlo- rophyta). PI. Syst. Evol. 168: 195-219.

Watanabe, S. and Floyd, G. L. 1994. Ultrastructure of the flagellar apparatus of the zoospores of the irregularly shaped coccoid green algae, Chlorochytrium Iemnae and Kentrosphaera gibberosa (Chlorophyta). Nova Hedwigia 59: 1-11.

COI. 31: 632-9.

PI. S y ~ t . E d . 158: 183-210.

Fig. 1. Electron micrograph of vegetative cell of Myrmecia inci- sa. Note pyrenoid matrix traversed by parallel thylakoid rnern- branes. P, pyrenoid; C, chloroplast. Bar = 1 prn.

(Watanabe and Floyd 1994). Among these, Parietochloris is considered to be a good candidate for a new genus in which to reclassify M. incisa due to distinguishing characteristics, such as spherical vegetative cells, a non-attaching habit and parietal chloroplasts with pyrenoids. The partially thickened cell wall commonly found in the genus Myrmecia is a distin- guishing feature of species of Parietochloris but could not be used as the basis for establishment of another genus. Three species have been included in Parietochloris (Deason et a/. 19911, but M. incisa differs from these in having a smaller cell size, frequent, uneven thickening of the cell wall, and ellipsoidal or broad, crescent-shaped pyrenoids surrounded with a few starch segments.

The following taxonomic treatment is proposed:

Parietochloris incisa (Reisigl) Watanabe comb. nov.

Basionym: Myrmecia incisa Reisigl. osterr. Bot. Zeitschr. 111 (1964); 457-460, figs. 20, 21.