A class of RNA molecule, called a microRNA, has been found in a unicellular green alga. The discovery, made independently by two labs, dismantles the popular theory that the regulatory role of microRNAs in gene expression is tied to the evolution of multicellularity.

MicroRNAs found in green algae hint at the organism's complexity. Credit: M.I. WALKER/SPL

The researchers confirmed the existence of dozens of microRNAs in the genome of the green alga Chlamydomonas reinhardtii, and think that hundreds are likely. The finding is as startling as the discovery ten years ago that the nematode Caenorhabditis elegans has 19,000 genes, just 1,000 short of the human count, says Gregory Hannon from Cold Spring Harbor Laboratory in New York and a co-author of one of the studies (T. Zhao et al. Genes Dev. 21, 1190–1203; 2007). “People were shocked that the complexity of the genomes in these simpler creatures was similar to our own,” he says. Now it seems that the RNA in simple unicellular organisms could be as complex as that in higher creatures.

The second study, which independently came to the same conclusion, appears online in Nature this week (A. Molnár et al. doi:10.1038/nature05903; 2007).

Ever since the discovery of RNA interference — the selective blocking of gene expression by small RNAs — biologists have identified a growing family of these tiny molecules in eukaryotic organisms. But hunts in yeast and in the protist Tetrahymena have yielded fewer types of RNA molecule than found in plants and animals. MicroRNAs were never found.

This, combined with the fact that RNA sequences differ between plants and animals, helped give rise to the idea that microRNAs evolved independently in plant and animal lineages as parts of complex regulatory mechanisms associated with multicellularity. Now it seems that these molecules may predate that evolutionary development.

“It shows how basing conclusions on studies of just one or two model organisms can really lead you astray in terms of how you think about evolutionary processes,” says Jim Umen from the Salk Institute in La Jolla, California.

A combination of factors led to the recent findings: high-throughput sequencing can now sieve through thousands of RNA molecules in search of microRNA. And having a nearly completed map of the genome for the alga means that its sequences can be interpreted relatively easily.

Nobody knows why such a simple organism needs microRNAs, nor how or when they first appeared. But researchers say they may help C. reinhardtii adapt to extremely diverse environments.

Whatever their role, their presence indicates that microRNAs could be much more ancient than previously thought; they might have persisted for more than a billion years.