Biology Reference
In-Depth Information
population, this will produce a strong selective advantage in being able to induce CI. Once
Wolbachia
has become Ýxed in the population, the CI-inducing capacity will lose its selective advantage. If this
Wolbachia
is not capable of horizontal transmission to another population (and thus has no chances
to exploit CI induction for its spreading), there will be no strain in which preservation of CI capacity
will be under strong selective pressure. It is, of course, possible to hypothesize that CI-inducing
capacity could be maintained, at least in the short term, in the presence of incomplete transovarial
transmission of
Wolbachia
. In any case, the lack of invasion of new compatibility types (determined
by the absence of horizontal transmission) and the clonal structure of the vertically transmitted
bacterial population could possibly provide conditions for virulence reduction through kin-selection
phenomena. We can thus expect a reduction of the arms race between
Wolbachia
and its host, with
optimization of
Wolbachia
replication and transmission. In the presence of perfect vertical transmis-
sion, in the long term we should also expect the development of some coadaptation to the reciprocal
presence and possibly the establishment of some forms of mutualistic interactions. If
Wolbachia
becomes in some way needed for embryonic development, pressure for preservation of the CI-inducing
capacity should disappear (but there could be a phase in which CI becomes a system by which to
avoid nonuseful partial development of embryos if
Wolbachia
is needed by the host in successive
phases of development). In the long term, if the presence of
Wolbachia
is in some way needed by the
host, we should expect development of host mechanisms to ensure its transmission and deÝnitive loss
of pressure for maintaining CI-inducing capacity.
The establishment of
Wolbachia
symbiosis in Ýlarial nematodes could have followed a similar
pathway, but it is also possible that the initial
Wolbachia
phenotype was different. A CI-inducing
Wolbachia
is, however, attractive as a hypothetical starting point for the development of obligatory
symbiosis: it promotes its own survival and diffusion while not being too costly to the host when
it nears Ýxation.
In other forms of reproductive parasitism, Ýtness costs to the host remain high even at Ýxation
(e.g., in male killing phenotypes), and the arms races is likely to be constant between
Wolbachia
and its hosts. Thus, where the behavior of a given kind of reproductive parasite is not incompatible
with obligatory symbiosis, we do not expect this kind of association to develop if the effect on
host Ýtness is above the threshold that will determine counter-responses by the host (i.e., the
establishment of an arms race).
Rapid Evolutionary Mechanisms for Transition
The above hypothesis concludes that evolution from a facultative to an obligatory symbiosis requires
long coevolution to obtain species-speciÝc coadaptation between partners and reduction of viru-
lence. However, the two examples in insects where
Wolbachia
infection is obligatory for reproduc-
tion suggest another transitive mechanism, which could occur more rapidly through the evolutionary
time. Indeed, in both the thelytokous wasps and
A. tabida
, host dependence has probably resulted
from the accumulation of deleterious mutations in the insect genome. In thelytokous wasps, because
sexual functions are no longer under selective pressure, random mutations in genes related to this
function can Ýxate and accumulate, making
Wolbachia
-induced parthenogenesis the only possible
mode of reproduction. In
A. tabida
, we have proposed two different hypotheses that could account
for the total
Wolbachia
dependence of females in producing eggs. The Ýrst one postulates a novel
reproductive
Wolbachia
manipulation (i.e., the
Medea
phenotype) and will not be discussed here.
The alternative hypothesis postulates that certain bacterial factors are directly involved in the
oogenesis of the insect. In this case, we have speculated on an evolutionary scenario that includes
a step in which both host and bacterial genomes would have a genetic determinant encoding for a
factor involved in and necessary for oogenesis. As in parthenogenetic wasps, we can therefore
expect that obligatory symbiosis would have been acquired rapidly by accumulation of deleterious
mutations in the genetic determinant of the insect, making those of
Wolbachia
the only functional
genetic determinants for insect reproduction.
