Rooted tree key to understanding bacterial evolution

 

Integrating vertical descent and horizontal gene transfer can be used to infer the root of the bacterial tree and the nature of the last bacterial common ancestor.

 

Bacteria comprise a very diverse domain of single-celled organisms that can be found almost everywhere on Earth. All Bacteria are related and derive from a common ancestral Bacterial cell. Until now, the shape of the bacterial tree of life and the placement of its root has been contested, but is necessary to shed light on the early evolution of life on our planet.

 

Just as in plants and animals, the genomes of Bacteria are home to many different genes. However, Bacterial genes are not only inherited vertically from mother to daughter, but are also frequently exchanged horizontally between potentially distant family members. Amongst its many functions, horizontal gene sharing drives the rapid spread of antibiotic resistance amongst pathogenic Bacteria.

 

The combination of vertical and horizontal ancestry complicates how we think about evolutionary relationships among Bacteria, with the former best represented as a tree and the latter as a network. The team used phylogenetic methods that simultaneously consider the vertical and horizontal transmission of genes and found that, on average, genes travel vertically two-thirds of the time, suggesting that a tree provides a meaningful framework for interpreting bacterial evolution.

 

The analysis predicts a root between two major clades, the Terrabacteria (mostly Bacteria with single-membranes such as Bacillus) and the Gracilicutes (mostly Bacteria with a double-membrane such as E. coli). This contrasts with recent proposals placing the Candidate Phyla Radiation (CPR) at the base of the tree. CPR are unusual ultrasmall Bacteria reliant on other host cells for survival.

 

Bacteria can have one or two cell membranes, and one of the big questions in early evolution is how these two kinds of cells are related to each other. The analysis suggests that the common ancestor of all living bacteria already had two membranes, so the question now is how apparently simpler single-membrane cells evolved from double-membraned cells, and how many times this has occurred during evolution. The approach to integrating vertical and horizontal gene transmission holds promise for answering this and many other open questions in evolutionary biology.

 

See:

 

Gareth A. Coleman, Adrián A. Davín, Tara A. Mahendrarajah, Lénárd L. Szánthó, Anja Spang, Philip Hugenholtz, Gergely J. Szöllősi, Tom A. Williams. A rooted phylogeny resolves early bacterial evolution. Science, 2021; 372 (6542): eabe0511 DOI: 10.1126/science.abe0511

 

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)