Biology Reference
In-Depth Information
A simple mathematical model predicted that with sufficiently high expression of CcdB in
the predator and the QS module in the prey (i.e. LuxI and LasR), the system could exhibit
oscillatory dynamics. By experimentally modulating these two factors, and observing the
population dynamics using a custom-made microfluidic device, the authors confirmed this
prediction (
Fig. 13.2B
).
This consortium was further extended to examine the role of cellular motility and spatial
structure and their effects on biodiversity (i.e. coexistence of the predator and prey
strains).
26
While previous studies had shown that reducing motility or dispersal promoted
biodiversity, it was unclear how cellular motility could impact biodiversity if the
interactions occurred over a long distance. As the synthetic predator
prey interaction is
mediated by QS signals, which can diffuse over a long distance, the consortium was
suitable to address this question. Unlike the previous study, in which the cells were cultured
in liquid medium,
25
Song et al. inoculated the predator and prey populations at two
separate spots on an agar plate.
26
Cellular motility was modulated by changing the agar
density, while seeding the two strains at various segregation distances modulated the spatial
configuration. Through modeling and experimentation, the authors found that motility had
a negligible impact on biodiversity when the two strains were well mixed in liquid medium
or if they were inoculated at the same spot on an agar plate. In contrast, when the two
strains were seeded at a relatively long distance from one another, increasing cellular
motility was found to significantly decrease biodiversity. Using a mathematical model, the
authors determined that motility plays a critical role in biodiversity only when the length
scale of the predator
prey interaction matched the segregation distance.
While the predator
prey interaction represents a canonical example of antagonistic
relationships in ecology, such interactions in nature often involve more than two
populations. Along this line, Kerr et al. constructed a synthetic rock
scissors
system consisting of three
E. coli
populations.
27
The system revolves around the clever
use of colicin, a toxic protein that is secreted into the environment. Three strains, a
colicin-producing strain (P), a colicin-sensitive strain (S), and a colicin-resistant strain (R)
(
Fig. 13.2C
), realize the rock
paper
249
scissors interaction as follows: P kills S through colicin
secretion; S outgrows R due to the cost associated with colicin resistance; and R outgrows
P due to the metabolic cost associated with the plasmid housing the colicin gene, as well as
the genes encoding colicin immunity proteins, which protect P from colicin. Using this
consortium, the authors examined previous theoretical predictions that spatial scales of
dispersal, movement, and interaction are important factors in determining whether multiple
competitive species can coexist. When the three strains were cocultured in liquid medium,
where dispersal and movement are high and the interactions among the three populations
occurred globally, the community was unstable and only R persisted in the community.
In contrast, when the populations were grown on an agar plate in patches, where dispersal
and movement are limited and the interactions occur only locally, the three strains stably
coexisted for at least 70 generations. These observations thus provided a direct
demonstration of the theoretical prediction that local dispersal and interaction promote
biodiversity.
paper
A similar rock
scissors system was employed to examine the evolution of restrained
growth.
28
Restraint in the use of common resources (i.e. restrained growth) can be seen
as an altruistic trait. Prudent use of common resources likely prolongs the persistence of
a community, but such a trait has a competitive disadvantage against less-restrained
individuals who enjoy a higher fitness than restrained individuals.
29,30
As such, how
restrained growth can evolve stands as a fundamental question in ecology and evolutionary
biology. Previous computational studies had suggested that a rock
paper
scissors
interaction in a spatially structured community could lead to evolution of restrained
growth.
31,32
For example, if the rock population increased its growth rate by mutation,
paper
