Algal Phylogeny and the Origin of Land Plants

The green algae and land plants 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 for asking 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 of these 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). Present knowledge 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.