Algae form a diverse polyphyletic group of uni- and multicellular organisms. Traditional taxonomy divides algae into red, brown, and green lineages based on their plastid pigment composition. More recently algae have been divided into two main groups based on the origin of their plastids: primary and secondary algae. The primary algae, or archaeplastida, are comprised of three monophyletic groups: green algae (including land plants), red algae, and glaucophytes, all of which arose from a single endosymbiotic event between a cyanobacterium and a flagellate protist. Secondary algae result from the engulfment of a primary alga by another protist; they include seven groups of single-celled organisms, two having arisen from symbiosis with a green archaeplastid alga (Euglenozoans, Chlorarachniophytes) and the other five having arisen from one or more endosymbiotic events between a red alga and another protist (Cryptophytes, Haptophytes, Dinoflagel-lates, Heterokonts, Apicomplexans) (for reviews see Bhattacharya et al. 2003; Palmer 2003). While primary algal plastids are surrounded by only two membranes (the outer membrane of the cyanobacterium and a host endosomal membrane), secondary algal plastids are surrounded by three or four membranes and sometimes even contain nucleomorph, a relict of the primary alga nucleus (Chlorarachniophytes, Cryptophytes). Each endosymbiosis is followed by massive gene transfer from the endosymbiont plastid/nucleomorph genome into the host nuclear genome and an introduction of signal peptides (or composite signal peptides) allowing trafficking of plastid proteins from cytoplasm back to the plastids.

Some algal species have been model organisms for decades (Chlamy-domonas reinhardtii, Euglena gracilis, Scenedesmus quadricauda, Volvox car-

teri). Others have more recently drawn scientists' attention (Cyanidioschyzon merolae, Ostreococcus tauri, Thalassiosira pseudonana). Here, I will discuss the merits of a handful of algal species as models for the study of the cell cycle. I will focus on Chlamydomonas reinhardtii, the most popular algal model organism used mainly for studies of photosynthesis and flagellar movement but also cell growth and cell cycle regulation. I will also point out the merits of other algal models for the study of the cell cycle.

Scenedesmus quadricauda and Ostreococcus tauri, like Chlamydomonas reinhardtii, belong to the green algae and Cyanidioschyzon merolae is a primary red alga; all therefore represent archaeplastid algae. Chlamydomonas reinhardtii and Scenedesmus quadricauda are closely related to one another and are also the closest relatives to land plants among the foursome; Ostreococcus tauri is evolutionarily placed at the base of the green algae (see below). Cyanidioschyzon merolae is a primary red alga that therefore diverged from the other three models earlier in archaeplastidian evolution. While the older models Chlamydomonas reinhardtii and Scenedesmus quadricauda were chosen based on their abundance in nature, the two recent models Ostreococcus tauri and Cyanidioschyzon merolae were chosen due to their evolutionary position and their small genomes that could be easily sequenced.

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