Background: Horizontal gene transfer occurs frequently in prokaryotes and unicellular eukaryotes. Anciently acquired genes, if retained among descendants, might significantly affect the long-term evolution of the recipient lineage. However, no systematic studies on the scope of anciently acquired genes and their impact on macroevolution are currently available in eukaryotes.
Results: Analyses of the genome of the red alga Cyanidioschyzon identified 37 genes that were acquired from non-organellar sources prior to the split of red algae and green plants. Ten of these genes are rarely found in cyanobacteria or have additional plastid-derived homologs in plants. These genes most likely provided new functions, often essential for plant growth and development, to the ancestral plant. Many remaining genes may represent replacements of endogenous homologs with a similar function. Furthermore, over 78% of the anciently acquired genes are related to the biogenesis and functionality of plastids, the defining character of plants.
Conclusion: Our data suggest that, although ancient horizontal gene transfer events did occur in eukaryotic evolution, the number of acquired genes does not predict the role of horizontal gene transfer in the adaptation of the recipient organism. Our data also show that multiple independently acquired genes are able to generate and optimize key evolutionary novelties in major eukaryotic groups. In light of these findings, we propose and discuss a general mechanism of horizontal gene transfer in the macroevolution of eukaryotes.