DESCRIPTION OF THE SUPERGROUP ARCHAEPLASTIDA (ADL ET AL. 2005)

EUKARYA> ARCHAEPLASTIDA

Archaeplastida (ar-ka-PLAS-ti-da) is derived from two Greek roots that mean “ancient” (arkaios -αρχαίος) and “molded” (plastos -πλαστός). The old view of a plastid was that it was variable malleable in shape. The reference is to this supergroup having the chloroplasts that came from a primary endosymbiosis with a cyanobacterium.

INTRODUCTION TO THE SUPERGROUP ARCHAEPLASTIDA Photosynthesis seems to have evolved from a single endosymbiotic event between a eukaryote and a cyanobacterium, a concept first proposed by Cavalier-Smith (1982). Those taxa in this supergroup (Viridiplantae, Rhodoplantae, and Glaucophyta) are the lineal descendants of that primary endosymbiosis (Cavalier-Smith, 2003), and all other photosynthetic eukaryotic taxa are derived from the secondary enslavement of a primary photosynthetic endosymbiont (Cavalier-Smith, 2002 and Keeling, 2004). In recognition of this as a primary endosymbiotic group, Adl et al. (2005) applied the name Archaeplastida (ancient plastids) to them. In addition, members of this supergroup also in a general sense are called plants. The non-plant chlorophyll a+b taxa (e.g. photosynthetic euglenoids) enslaved a chlorophyte while the chlorophyll a+c taxa (heterokonts, haptophytes, and cryptophytes) enslaved a rhodophyte. The photosynthetic dinoflagellates seem to be tertiary photobionts and enslaved a haptophyte (Morden and Sherwood, 2002). This unexpected scenario emerged from molecular phylogenetic work and flew in the face of conventional thought about the relative ease of photosynthetic endosymbiosis that had been promulgated through the application of Serial Endosymbiosis Theory (e.g. Margulis and Schwartz, 1982, 1988, 1998). Tomitani et al. (1999) provide convincing evidence that the original cyanobacterium had phycobillisomes and chlorophyll b, and Yoon et al. (2004) estimate that the symbiotic event occurred more than 1.5 billion years ago by combined molecular-paleontological methods. Also, Moreira et al. (2000) said that the sister taxa Viridiplantae and Rhodoplantae, descendants of the glaucophyte condition, lost the peptidoglycan of the cyanobacterium; then, the Viridiplantae lost the phycobillisomes while the Rhodoplantae lost chlorophyll b. Moreira et al. (2000) and recent supertree analyses (Baldauf, 2003; Keeling, 2004; Palmer et al., 2004) confirm the monophyly of the three groups. Cavalier-Smith (2002, 2003) interprets the Rhodophytes and Viridiplantae as sister groups while the Glaucophytes are outgroups within the Plant clade. All anaylses do not tell the same story, though. The supertree analysis of Nikolaev et al. (2004) shows the Viridiplantae and Rhodophyta are sisters, but the taxa of Glaucophyta emerge within the Centrohelid-Glaucophyte-Cryptophyte clade. The systematics of the Viridiplantae and the Rhodoplantae have been studied for more than a century and still are not completely stable. Recent reviews of the Viridiplantae (Lewis and McCourt (2004) and of the Rhodoplantae (Saunders and Hommersand, 2004) demonstrate that they are stabilizing, though. On the other hand, the glaucophytes are very problematic and may simply be the last remnants of a once diverse group. In this system, the glaucophytes are given uncertain status within the Rhodoplantae. See Table 1 for comparisons between the highest subtaxa of the Rhodoplantae and Viridiplantae together with the problematic Glaucophyta according to five characters: flagella, cell covering, chloroplast, pigments, and storage product.

TABLE 1. Comparisons between the Kingdoms of the Supergroup Archaeplastida. This table gives the states of five important characters (flagella, cell covering, chloroplast, pigments, and storage products) for the major groups within the kingdoms Rhodoplantae and Viridiplantae. The Glaucophytes are problematic and are placed within the Rhodoplantae as incertae sedis.
CHARACTERS	INCERTAE SEDIS	KINGDOM RHODOPLANTAE		KINGDOM VIRIDIPLANTAE
	GLAUCOPHYTES	CYANIDIALES	RHODOPHYTES	PRASINOPHYTES	CHLOROPHYTES	STREPTOBIONTS
FLAGELLA	2 unequal; with scales or hairs. subapical	no	no	2 equal (or unequal) with scales or hairs. subapical.	2 equal. apical (in most)	2 equal; subapical.
CELL COVERING	alveolate	thick walls that are proteinaceous	walls of xylan and mannan microfibrils covered with much gelatinous material	naked or cellulosic scales	cellulosic wall	cellulosic wall
CHLOROPLAST	thyllakoids in concentric layers around periphery, but not stacked; endosymbiont retains murein in its wall	thyllakoids in concentric layers around periphery, but not stacked	thyllakoids in concentric layers around periphery, but not stacked	Stacked thyllakoids; eyespot, when present, is always enclosed within the chloroplast.	Stacked thyllakoids; eyespot, when present, is always enclosed within the chloroplast	Stacked thyllakoids; eyespot, when present, is always enclosed within the chloroplast
PIGMENTS	chlorophyll a only; carotenes, xanthophylls, and phycobillins in phycobillisomes	chlorophyll a only; carotenes, xanthophylls, and phycobillins in phycobillisomes	chlorophyll a only; carotenes, xanthophylls, and phycobillins in phycobillisomes	chlorophylls a and b; carotenes, xanthophylls.	chlorophylls a and b; carotenes, xanthophylls.	chlorophylls a and b; carotenes, xanthophylls.
STORAGE PRODUCT	polysaccharide is starch which is formed outside of the chloroplast	storage product similar to glycogen (floridean starch in one); formed outside of the chloroplast	polysaccharide is floridean starch	variable; sometimes true starch	polysaccharide is true starch	polysaccharide is true starch

FURTHER READING: INTRODUCTION TO THE DOMAIN EUKARYA

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By Jack R. Holt. Last revised: 03/17/2013