Evidence for nucleomorph to host nucleus gene transfer: light-harvesting complex proteins from cryptomonads and chlorarachniophytes

Cryptomonads and chlorarachniophytes acquired photosynthesis independently by engulfing and retaining eukaryotic algal cells. The nucleus of the engulfed cells (known as a nucleomorph) is much reduced and encodes only a handful of the numerous essential plastid proteins normally encoded by the nucleus of chloroplast-containing organisms. In cryptomonads and chlorarachniophytes these proteins are thought to be encoded by genes in the secondary host nucleus. Genes for these proteins were potentially transferred from the nucleomorph (symbiont nucleus) to the secondary host nucleus; nucleus to nucleus intracellular gene transfers. We isolated complementary DNA clones (cDNAs) for chlorophyll-binding proteins from a cryptomonad and a chlorarachniophyte. In each organism these genes reside in the secondary host nuclei, but phylogenetic evidence, and analysis of the targeting mechanisms, suggest the genes were initially in the respective nucleomorphs (symbiont nuclei). Implications for origins of secondary endosymbiotic algae are discussed.