Ancient gene duplication and differential gene flow in plastid lineages: The GroEL/Cpn60 example

Cryptomonads, small biflagellate algae, contain four different genomes. In addition to the nucleus, mitochondrion, and chloroplast is a fourth DNA-con taining organelle the nucleomorph. Nucleomorphs result from the successive reduction of the nucleus of an engulfed phototrophic eukaryotic endosymbion t by a secondary eukaryotic host cell. By sequencing the chloroplast genome and the nucleomorph chromosomes, we identified a groEL homologue in the ge nome of the chloroplast and a related cpn60 in one of the nucleomorph chrom osomes. The nucleomorph-encoded Cpn60 and the chloroplast-encoded GroEL cor respond in each case to one of the two divergent GroEL homologues in the cy anobacterium Synechocystis sp. PCC6803. The coexistence of divergent groEL/ cpn60 genes in different genomes in one cell offers insights into gene tran sfer from evolving chloroplasts to cell nuclei and convergent gene evolutio n in chlorophyll a/b versus chlorophyll a/c/phycobilin eukaryotic lineages.