Update on Evolution
Algal Phylogeny and the Origin of Land Plants1
Debashish Bhattacharya* and Linda Medlin Department of Biological Sciences, Biology Building, University of Iowa, Iowa City, Iowa 52242–1324 (D.B.); and the Alfred Wegener Institute for Polar and Marine Research, Am Handelshaven 12, 27570 Bremerhaven, Germany (L.M.)
The green algae and landplants form a monophyletic lineage (the chlorophytes) that contains both protistan and higher taxa (Graham, 1996). An important issue regarding the evolution of this green lineage that still remains in question is the identity of the green algal (i.e. flagellate) ancestor of land plants. Modern molecular phylogenetic data provide the framework for reconstructing this evolutionary history and forasking deeper questions about the origin of the genetic inventions that have played a role in the radiation of the green lineage, a group that contains nearly all levels of vegetative morphology, from single cells to filaments to well-organized colonies to complex terrestrial plants. The green lineage is, however, only one example of photosynthetic taxa that have successfully colonized our planet.A much greater diversity of plastid-containing organisms is defined by the various other forms of algae. The algae include the green algal relatives of land plants and a diverse collection of single-celled and multicellular taxa such as the heterokonts, rhodophytes (red algae), cryptophytes, chlorarachniophytes, dinoflagellates, and haptophytes. Understanding the interrelationships and origins ofthese lineages is an interesting problem in evolutionary biology, not only because the algae contain the dominant primary producers on this planet, but also because uncovering the ancestry of their plastids offers the possibility to gain insights into the many facets of endosymbiosis, such as endosymbiont genome reduction and gene transfer to the host nucleus (Gilson and McFadden, 1996). Presentknowledge argues overwhelmingly for a cyanobacterial origin of all algal plastids, with stable incorporation of many of the endosymbiont’s genes in the host genome (Bhattacharya and Medlin, 1995). These and other recent data concerning the origins of algae and their plastids form a starting point from which the origin of the green lineage can be better understood. a bewildering array of cellmorphologies and life cycles and live in a multitude of habitats. The major lineages of the algae are the Chlorophyta (green algae), Rhodophyta (red algae), Glaucocystophyta, Euglenophyta, Chlorarachniophyta, Heterokonta, Haptophyta, Cryptophyta, and the dinoflagellates (within the Alveolata). The latter four groups have been loosely termed the chromophyte algae because they contain chlorophyll a and cand various xanthophylls that make them appear yellow or brown. The algae include not only the world’s largest protists, the kelps (Macrocystis spp. in the Heterokonta, which may be up to 30 m in length), but also many bacteria-sized (1–5 m) coccoid taxa (e.g. Chlorella spp. and Micromonas spp. in the Chlorophyta and Pelagomonas spp. in the Heterokonta). Many tiny, single-celled algae live withina complex exoskeleton made of CaCO3 or silica, which accumulates over time in deep sea deposits in the world’s oceans (coccolithophorids, diatoms). The fossil remains of these algae are routinely used for paleoclimatic reconstructions and to predict climate change. Algae have played critical roles in ecological studies of aquatic (e.g. kelp forests in northern California) and terrestrialecosystems and have been used as model protists in physiological and biochemical studies (e.g. Chlamydomonas reinhardtii in the Chlorophyta) and have been the cause of many fundamental questions in biology because of their diverse and complex life histories. In addition, algae have had a long history in the food (e.g. nori, wakame) and drug (e.g. agar-agar, carrageenan, alginic acid) industries. The...