Biology Reference
In-Depth Information
10
5
0
three-quarter
sibling
cousin
non-kin
Fig. 11.6
Kin discrimination and odour in Belding's grounds squirells. Individuals spent
more time investigating cubes which had been rubbed over closer relatives. From Mateo
(2002). With permission on the Royal Society. Photograph of a group of pups at the
mouth of a burrow. Photo © George D. Lepp.
not possible, then individuals would be expected to show fixed strategies, where natural
selection fine-tunes behaviour in response to the average relatedness between interacting
individuals over evolutionary time. In this case, natural selection would favour less
selfish (or more cooperative) behaviour in populations or species where relatedness is, on
average, higher. Bill Hamilton (1964) pointed out that limited dispersal (population
viscosity) could lead to a high relatedness between interacting individuals, and that this
would favour indiscriminate altruism (without kin discrimination), because the altruism
would be directed towards individuals who are likely to be close relatives.
Limited dispersal
provides a
mechanism to
generate high
relatedness
between
interacting
individuals,
without kin
discrimination
Cooperative iron scavenging in bacteria
Ashleigh Griffin and colleagues tested how dispersal rates could influence relatedness,
and hence selection for cooperation, by examining the iron scavenging behaviour of the
bacteria
Pseudomonas aeruginosa
. Iron is a major limiting factor for bacterial growth
because most iron in the environment is in the insoluble Fe(III) form, and, in the context
of bacterial parasites, is actively withheld by hosts. Many species of bacteria address this
problem by producing iron scavenging siderophore molecules, which are released from
the cell and then bind with iron, allowing it be taken up into cells.
The production of factors such as siderophores leads to the problem of cooperation,
because cells who avoided the cost of producing them ('free riders') could still exploit the
benefits of those produced by others (Fig. 11.7). This was confirmed experimentally, by
showing that when a normal siderophore producing strain is grown in a mixture with
a free riding mutant that does not produce siderophores, the mutant increases in
frequency. These experiments were facilitated by the handy fact that cooperating and
free riding bacteria can be readily distinguished by eye, because siderophores are green,
and so colours the colonies of cooperative bacteria, whereas the free riders are white.
Kin selection is likely to be a key explanation for cooperation between bacterial cells,
The production of
iron scavenging
molecules by
bacteria is a
cooperative
behaviour
