Researchers using DNA analysis have shown that there may be at least one hidden domain of life, a fourth branch of cellular organisms, says Jonathan Eisen at UC Davis, and colleagues.

The researchers analyzed metagenomic data and used them to search the Global Ocean Sampling (GOS) Expedition dataset for novel lineages in three gene families commonly used in phylogenetic studies: trees that use small subunit ribosomal RNA, recA, and rpoB genes — highly conserved genes that appear in nearly all organisms, including the domains of prokaryotes, the bacteria, and archaea.

They found some deep branches in the recA and rpoB phylogenetic trees that might represent viral genes. However, they acknowledge that the novel sequences possibly come from a new fourth major branch of cellular organisms on the tree of life.

Follow-up studies will be required to determine whether the source of the novel, deeply branching sequences are cellular organisms or viruses, and whether the novelty extends to all genes in the genome or is just seen for a few gene families, says Eisen.

Ref.: “Stalking the Fourth Domain in Metagenomic Data: Searching for, Discovering, and Interpreting Novel, Deep Branches in Marker Gene Phylogenetic Trees,” PLoS ONE (open access), March 18, 2011.