Behavioral Ecology Blog

What is a bacterial species ecotype?

Cohan, 2002. Ann. Rev. Microbio.

OLD-SCHOOL OPERATIONAL DEFINITIONS

Using DNA-DNA hybridization-

> 70% sequence homology over the entire genome

Using 16S rRNA

> 3% sequence divergence

§ But this was calibrated using DNA-DNA hybridization methods- i.e. 3% is the number which correlated with 70% homology. There is no “new science” here

MORE CONCEPTUAL

Cohesion Species Concept- Maglitsch and Templeton

> A bacterial species is comprised of a group of organisms whose divergence is constrained by intermittent bouts of natural selection.

§ When 2 groups reach the point when which they can no longer withstand the others selection, they form 2 species.

> Cohan argues that bacterial ecotypes are the unit which possesses the properties we call species in other groups.

Recombination is the cohesive force in sexual species

> The rate of recombination in most bacteria is akin to the rate of mutation- i.e. pretty low.

§ Importantly, recombination is not limited to exchange between similar species (as it is in plants and animals)

§ Recombination is not limited to the exchange of homologus loci.

§ MAIN POINT- Recombination cannot serve as a cohesive force in bacteria (and other asexual species)

Bacterial speciation is very frequent:

> Bacterial speciation required only ecological divergence (not reproductive divergence, as in sexual species)

> Allopatry is not necessary for bacterial species (because recombination is so rare)

> Population sizes are very large, making new mutation accessible.

> Heterologous gene transfer (5%-15% of genes in bacterial species have come form other species!!!) Inter-specific adaptation transfer.

How to identify an ecotype

> By Phylogenetic clustering methods.

§ But this comes with the problem of which cluster/subcluster represents the ecologically important ecotypes.

> Cohan proposes using his unpublished “star clade’ approach.

> MLST is also an important way of identifying ecotypes.