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 engul fed cells (known as a nucleomorph) is much reduced and encodes only a handf ul of the numerous essential plastid proteins normally encoded by the nucle us of chloroplast-containing organisms. In cryptomonads and chlorarachnioph ytes these proteins are thought to be encoded by genes in the secondary hos t nucleus, Genes for these proteins were potentially transferred from the n ucleomorph (symbiont nucleus) to the secondary host nucleus; nucleus to nuc leus intracellular gene transfers, We isolated complementary DNA clones (cD NAs) for chlorophyll-binding proteins from a cryptomonad and a chlorarachni ophyte. In each organism these genes reside in the secondary host nuclei, b ut 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.