Escherichia Coli
Figure 4: Two bacterial cells caught in the act of
plasmid-mediated conjugation. Many plasmids are able to transfer
horizontally from an infected donor (top) to an uninfected
recipient (bottom) via conjugation. Conjugation is initiated
by contact between donor and recipient cells via a plasmid-encoded
protein appendage called a sex pilus. Conjugation results
in the one-way transfer of a copy of the plasmid genome from
donor to recipient.
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Evolution of horizontal transfer in E. coli plasmids
Evolution of mutualisms
Evolution of horizontal transfer in E. coli plasmids
For many parasites, a fundamental conflict should exist between
modes of horizontal (infectious) and vertical (intergenerational)
transmission. Parasite activities that increase infectious transmission
are presumed to generally reduce host fitness (growth rate). In
turn, reduced host fitness impedes vertical transmission of the
parasite and thereby causes a tradeoff between transmission routes.
We use conjugative plasmids (Fig. 4) and their E. coli hosts as
a model to examine the evolution of parasite transmission. One experiment
allowed plasmids to evolve for hundreds of generations in environments
that contained different densities of bacterial hosts (Turner et
al. 1998, Evolution 52:315-329). The plasmid’s rate of conjugative
(infectious) transfer increased at the expense of host fitness,
indicating a tradeoff between horizontal and vertical modes of transmission.
Surprisingly, susceptible host density had no significant effect
on which mode of transfer was selectively favored. Continued research
focuses on other environmental factors mediating the evolution of
conjugation rates, epistatic interactions between genes for antibiotic-resistance
and conjugative-transfer, and phenotypic plasticity in traits governing
plasmid transmission.
Evolution of mutualisms
“I get by with a little help from my friends”
– John Lennon and Paul McCartney (A Little Help from my Friends,
1967)
Mutualisms (cooperative interactions) have traditionally received
less attention than parasitic interactions. This is surprising given
that cooperation among individuals is extremely common in nature.
For instance, the stability of bacterial communities may often rely
upon cross-feeding, whereby certain bacteria excrete metabolites
(nutrients) that are essential for the persistence of other strains
in the local environment.
We are currently using E. coli bacteria to examine the ecology
and evolution of mutualisms. In particular, an earlier study found
that stable genetic polymorphisms can evolve even when bacteria
are cultured in simple habitats containing only the single limiting
resource glucose (Turner et al. 1996, Ecology 77:2119-2129). Ongoing
projects concern the stability of the coexistence in different laboratory
environments, the metabolites that contribute to the observed cross-feeding,
and the vulnerability of mutualisms to invasion by exploitative
genotypes and parasites.
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