Commentary Second - hand chloroplasts and the case of the disappearing nucleus Jeffrey

The basic facts of chloroplast and mitochondrial endosymbiosis are by now the stuff of textbooks. Early in eukaryotic evolution, an anaerobic protist engulfed a respiring bacterium, probably an a-proteobacterium (1, 2), and the resulting partnership between symbiont (bacterium turned mitochondrion) and host gave rise to most, perhaps all, extant eukaryotes. Subsequently, a respiring eukaryote acquired a second bacterium, unquestionably a cyanobacterium (3, 4), which became the chloroplast, thereby endowing a subset of eukaryotes with photosynthetic powers. Massive relocation of gene function from organelle to nucleus ensued, by both direct transfer and indirect substitution, so that the great majority of organellar proteins are now encoded by nuclear genes (1, 2, 3, 5). Current evidence (1-4) suggests that all chloroplasts probably trace back to a single primary (prokaryotic/eukaryotic) endosymbiont event, as do all mitochondria. Less well appreciated is the fascinating realm of secondary endosymbiosis, in which one eukaryote engulfs another eukaryote and permanently retains part of its prey as a degenerate endosymbiont. Although no eukaryotes are known to have obtained their mitochondria secondarily, a rich diversity of eukaryotes have acquired their plastids via a multitude of independent, but strikingly parallel, secondary symbioses (Fig. 1). Two papers in this issue investigate the phylogeny and molecular biology of chlorarachniophytes, a group of unicellular amoeboid algae whose secondarily acquired nucleus, termed the nucleomorph, is located between the inner and outer pair of membranes surrounding its chloroplast (Fig. 1). The study by Van de Peer et al. (8) effectively settles a phylogenetic conundrum concerning the origin of the chlorarachniophyte's algal endosymbiont, while that by Gilson and McFadden (9) reveals a startling array of compaction mechanisms employed by the highly miniaturized nucleomorph genome of the chlorarachniophyte.