Origins of Bacterial Diversity—Biology 337
February 20, 2003
The evolutionary consequences of rare sex.
Cohan, F. M. 2003. Selective sweeps and ecological diversity in bacteria. In: Selective
Sweeps, (D. Nurminsky, editor), Landes Bioscience. (accepted).
I assigned this paper originally for February 13, but realize now that we will have more
time to discuss it on February 20. This paper argues that periodic selection is a powerful
force that purges diversity within an ecotype. The sections of interest are pp. 1-5, 7-8,
and 19-26, and associated figures. When reading this, please keep in mind the arguments
by Lawrence that periodic selection has minimal effect as a diversity-purging force.
Bacterial sex is promiscuous, and implications for adaptation
Majewski, J. 2001. Sexual isolation in bacteria. FEMS Microbiology Letters 199:161169. OR
Cohan, F. M. 2002. Sexual isolation and speciation in bacteria. Genetica 116:359-370.
You can take your choice between these two overviews of the barriers to genetic
exchange between bacterial species. Both are quite similar, and as you’ll see, the later
Cohan article copiously cites the earlier Majewski piece. Nevertheless, the article by
Jacek Majewski (my former graduate student) emphasizes molecular mechanisms more
than mine, and mine emphasizes the consequences on adaptation and speciation more
than Jacek’s. Either way, you’ll be hearing my take on all of this in class next week.
Gogarten, P., W. F. Doolittle, and J. G. Lawrence. 2002. Prokaryotic evolution in light of
gene transfer. Molecular Biology and Evolution 19:2226-2238.
We have previously read and discussed this article. For the present discussion, please
focus on the section on “Organismal phylogeny,” on pages 2229-2230. Here Gogarten et
al. entertain the radical possibility that the rate of gene exchange might be determined
more by the ecological similarity of the potential donor and recipient species (and their
co-inhabiting the same microenvironment) than by their relatedness.
Majewski, J. and F. M. Cohan. 1999. DNA sequence similarity requirements for
interspecific recombination in Bacillus. Genetics 153:1525-1533.
This works out the mechanisms of sexual isolation for transformation between Bacillus
species. In contrast to the case for the enteric bacteria, mismatch repair has little impact,
and the main difficulty in cross-species recombination is the difficulty in finding an exact
match between donor and recipient at the ends of the recombining fragment; divergence
in the center of the fragment has no effect on the difficulty of recombination. Note that
this makes introduction of heterologous DNA much more likely than when mismatch
repair is the principal barrier: if a heterologous gene in the donor is sandwiched between
two genes that are homologous with the recipient, the heterologous gene can slip in as
part of a homologous recombination event.
Maynard Smith, J., C. G. Dowson, and B. G. Spratt. 1991. Localized sex in bacteria.
Nature 349:29-31.
This work demonstrates that adaptations have been carried across species boundaries on
very small segments of DNA (i.e., “localized” sex). Note that the localized nature of
genetic exchange in bacteria allows an adaptation to be passed between populations or
species with different niches, without the co-transfer of genes that might not be
compatible with the recipient’s genetic background or its ecological niche.