Sizing up the genomic footprint of endosymbiosis.

A flurry of recent publications have challenged consensus views on the tempo and mode of plastid (chloroplast) evolution in eukaryotes and, more generally, the impact of endosymbiosis in the evolution of the nuclear genome. Endosymbiont-to-nucleus gene transfer is an essential component of the transition from endosymbiont to organelle, but the sheer diversity of algal-derived genes in photosynthetic organisms such as diatoms, as well as the existence of genes of putative plastid ancestry in the nuclear genomes of plastid-lacking eukaryotes such as ciliates and choanoflagellates, defy simple explanation. Collectively, these papers underscore the power of comparative genomics and, at the same time, reveal how little we know with certainty about the earliest stages of the evolution of photosynthetic eukaryotes.