Comment on "The evolution of modern eukaryotic phytoplankton".

Falkowski et al. (1) reviewed the evidence that three disparate groups of algae— dinoflagellates, diatoms, and coccolitho-phores, each with plastids derived from red algae by secondary endosymbiosis—have come to dominate the oceans_ flora over the past 250 million years and speculated about the forces responsible for this domination. Central to this speculation is the Bportable plastid hypothesis[ (1, 2), which posits that the likelihood with which plastids will be transferred between eukaryotes by secondary endosymbiosis is directly related to the number of genes in their genomes. The more genes, the argument contends, the more portable the plastid. This hypothesis rests on three claims: (i) red algal plastids retain more genes than do green algal plastids; (ii) gene transfer from the (primary) endosymbiont nucleus to the (secondary) host nucleus is rare; and (iii) red algae have been acquired by secondary endosymbiosis more often than have green algae. Although the limited number of red algal plastids examined to date do have more genes (3), claims (ii) and (iii) are not consistent with the available data, thus rendering the hypothesis effectively unsupported. Although red algal plastids may contain more genes than those of green algae, this difference pales against the nuclear contribution to plastid function. All plastid genomes encode only a small fraction of the proteins needed for plastid function—at most È10 to 20%, and only È1% in the case of dino-flagellates. The vast majority of plastid proteins are encoded by nuclear genes; most of these genes are derived from the plastid but have been transferred to the nuclear TECHNICAL COMMENT Fig. 1. Algal evolution and the origin and spread of plastids by endosymbiosis. At the top is the single origin of plastids by primary endosymbiosis between a cyanobacterium and a eukaryotic host. This endosymbiont was reduced and integrated, and part of this process involved the transfer of hundreds of genes from the cyanobacterium/plastid to the eukaryotic host nucleus (red arrow). Glaucophytes, red algae, and green algae all descended from this fully integrated partnership. Next, plastids spread to other eukaryotic groups by secondary endosymbiosis (middle). Green algae were most likely involved in two independent events, giving rise to euglenids (turquoise) and chlor-arachniophytes (orange). A single endosymbiosis involving a red alga probably gave rise to the chromalveolates (yellow); this group is supported by several molecular characters and gene trees (plotted on the figure). Plastids have apparently been lost in ciliates and Cryptosporidium (and perhaps …