Division Chlorarachniophyta
Chlorarachniophyte algae contain a complex, multi-membraned chloroplast derived from the endosymbiosis of a eukaryotic alga. Phylogenetic trees indicate that the host is closely related to filose amoebae and sarcomonads whereas the endosymbiont is most cl
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-Plant-.--SystematIcs and Evolution
© Springer-Verlag 1997
Division Chlorarachniophyta GEOFFREY I. McFADDEN, PAUL R. GILSON, and CLAUDIA J. B. HOFMANN Key words: Chlorarachniophyta, Chlorarachnion. - Secondary endosymbiosis, introns, operons, nucleomorph, plastid, chloroplast. Abstract: Chlorarachniophyte algae contain a complex, multi-membraned chloroplast derived from the endosymbiosis of a eukaryotic alga. Phylogenetic trees indicate that the host is closely related to filose amoebae and sarcomonads whereas the endosymbiont is most closely related to green algae. The endosymbiont is greatly reduced retaining only the plastid, plasmamembrane, a modicum of cytoplasm, and the nucleus. The vestigial nucleus of the endosymbiont, called the nucleomorph, contains three small linear chromosomes with a haploid genome size of 380 kb and is the smallest known eukaryotic genome. The overall gene organisation of the nucleomorph genome is extraordinarily compact making this a unique model for eukaryotic genomics. Chlorarachnion reptans GEITLER, the type species of the division Chlorarachniophyta, was described by LOTHAR GEITLER (1930) from Las Palmas in the Canary Islands. The cells are amoeboid with pseudopodia interconnecting as many as 150 cells into a reticuloplasmodial continuum (GEITLER 1930). The reticulopodia entrap bacteria, flagellates, and eukaryotic algae which are transported to the cell and digested (GEITLER 1930). Currently, there are four genera and six species described (ISHIDA 1994, ISHIDA & HARA 1994, HARA & al. 1992), but one genus, Cryptochlora, is not yet characterised electron microscopically (CALDERON-SAENZ & SCHNETTER 1987, 1989; BEUTLICH & SCHNETTER 1993) and several undescribed species await full characterisation. Distribution is worldwide, concentrated in moderately tropical zones, but biogeographical information is incomplete. A sexual cycie is reported (ISHIDA 1994, GRELL 1990, BEUTLICH & SCHNETTER 1993) but no details of meiosis or ploidy levels for various phases have been established, so proof of true sexuality is wanting. An extraordinary feature of Chlorarachnion is the presence of grass-green plastids with a pyrenoid. The combination of green plastids and reticuloplasmodial habit presented GEITLER (1930) with a systematic dilemma and he considered Chlorarachnion could be either a rhizopodial euglenoid or a heterokont. For the following 37 years Chlorarachnion languished in obscurity until RICHARD NORRIS (1967) isolated it from the Gulf of California in Mexico. Subsequently, HIBBERD & NORRIS (1984) undertook electron microscopical examination of Chlorarachnion reptans. On the basis of the very unusual nature of the plastids, they erected a new division - division Chlorarachniophyta (alternatively phylum Chlorarachnida,
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1990). The plastids, which number between five and seven in the amoeboid cells of Chlorarachnion reptans, are surrounded by four membranes. Between the inner and outer pairs of chloroplast membranes is a small volume of cytoplasm containing part
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