CHIMERIC CONUNDRA - ARE NUCLEOMORPHS AND CHROMISTS MONOPHYLETIC OR POLYPHYLETIC

All algae with chloroplasts located not freely in the cytosol, but ins ide two extra membranes, probably arose chimerically by the permanent fusion of two different eukaryote cells: a protozoan host and a eukary otic algal symbiont. Two such groups, cryptomonads (phylum Cryptista) and Chlorarachniophyta, still retain a DNA-containing relic of the nuc leus of the algal endosymbiont, known as the nucleomorph, as well as t he host nucleus. These two phyla were traditionally assumed to have ob tained their chloroplasts separately by two independent symbioses. We have sequenced the nuclear and the nucleomorph 18S rRNA genes of the n onphotosynthetic cryptomonad Chilomonas paramecium. Our phylogenetic a nalysis suggests that cryptomonad and chlorarachniophyte nucleomorphs may be related to each other and raises the possibility that both phyl a may have diverged from a common ancestral chimeric cell that origina ted by a single endosymbiosis involving an algal endosymbiont related to the ancestor of red algae. But, because of the instability of the m olecular trees when different taxa are added, there is insufficient ev idence to overturn the traditional view that Chlorarachnion nucleomorp hs evolved separately from a relative of green algae. The four phyla t hat contain chromophyte algae (those with chlorophyll c-i.e., Cryptist a, Heterokonta, Haptophyta, Dinozoa) are distantly related to each oth er and to Chlorarachniophyta on our trees. However, all of the photosy nthetic taxa within each of these four phyla radiate from each other v ery substantially after the radiation of the four phyla themselves. Th is favors the view that the common ancestor of these four phyla was no t photosynthetic and that chloroplasts were implanted separately into each much more recently. This probable polyphyly of the chromophyte al gae, if confirmed, would make it desirable to treat Cryptista, Heterok onta, and Haptophyta as separate kingdoms, rather than to group them t ogether in the single kingdom Chromista.