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SUPERGROUP PLANTA

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, Rhodophytae, 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).  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 Rhodophytae, descendants of the glaucophyte condition, lost the peptidoglycan of the cyanobacterium; then, the Viridiplantae lost the phycobillisomes while the Rhodophytae 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 Rhodophytae 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 Rhodophytae (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 Rhodophytae.

[Domain Eukarya]

For further information, go to the Evolution of the Eukaryotic Supertree.

REFERENCES.

This page is maintained by Jack R. Holt & Carlos A. Iudica: 04/22/2008