Another genome from a single-celled organism has been sequenced. This time it is the green algae, Chlorella. Chlorella are tiny algae that can reproduce quite rapidly. Yet despite the stream-lined nature of the organism, it retains most of the phytohormone biosynthesis pathways necessary to the development and growth of land plants.
Another interesting feature of the NC64A genome was the presence of homologs of receptors and biosynthetic enzymes of land plant hormones, such as abscisic acid, auxin, and cytokinin. The presence of these homologs does not necessarily imply the existence of plant hormones and their related functions in Chlorella but supports the hypothesis that genes involved in phytohormone biosynthesis and perception were established in ancestral organisms prior to the appearance of land plants.
Not only does this genome add more evidence to the growing plausibility of front-loading, but it also seems to offer a clue that the horizontal transfer of genetic information played a key role in the evolution of one of its key features – it’s unique chitin cell wall.
Green algae, like plants, have cells walls composed of cellulose. Chitin, on the other hand, is used in fungal cell walls and the exoskeleton of arthropods. Consider (from the same link above):
Finally, the analysis revealed surprising observations with respect to genes involved in cell wall metabolism. The Chlorella cell wall contains glucosamine polymers, such as chitin and chitosan, instead of cellulose and hemicelluloses that make up the cell walls of land plants. The genomic analysis indeed revealed no homologs of Arabidopsis genes involved in synthesis of cellulose or hemicelluloses. However, homologs were found to chitin metabolism genes present in Chlorella DNA viruses, leading to the remarkable hypothesis that components of Chlorella chitin metabolism could have been acquired via lateral gene transfer from viruses. If this is indeed the case, large DNA viruses predominantly associated with microalgae and marine protists might have played a larger role in the evolution of their hosts than previously recognized.
So chitin has been independently employed among algae, fungi, and arthropods. And a global network of viruses may have facilitated the emergence of these evolutionary “innovations.”