Biology Reference
In-Depth Information
little or no effect on the host Ýtness. Once an infection has spread throughout the entire host
population the selective pressures that act on both host and symbionts are assumed to minimize
the costs to the host of carrying these symbionts. (B) In cases where the infection has not reached
Ýxation in populations, a nuclear cytoplasmic conÞict between the
Wolbachia
and the nuclear
genome of the host is expected.
Wolbachia
favors a 100% female bias, whereas the nuclear genes
favor a sex ratio involving at least some males. In these situations, an arms race between the nuclear
genes and those of the
Wolbachia
may result in nuclear genes trying to suppress the
Wolbachia
or
its effect. Consequently, a much higher physiological cost of being infected is expected in mixed
host populations where infected and uninfected individuals co-occur.
In addition to the contribution of the genomic conÞict to the potential Ýtness cost of the PI-
Wolbachia
in mixed populations, the opportunities for horizontal transmission in such populations
may also select for additional costs of being infected. In Ýxed populations, all individuals are
already infected, and we expect the horizontal transmission to be selected against.
P
HYSIOLOGICAL
C
OST
It is generally assumed that the physiological costs are minimized in those populations where the
infection has gone to Ýxation. But this does not imply there is no cost at all because energy-
consuming bacteria are still present inside the host.
The best way to determine the effect of
Wolbachia
infection on offspring production is by
ÑcuringÒ a line of its infection. Such cured lines should be maintained for several generations; then
the offspring production of two genetically identical lines that differ in their infection status can
be compared. Unfortunately, this has been impossible to do for those cases where sexual lines
cannot be established because males and females are no longer capable of sexual reproduction. In
this case it is only possible to compare the offspring production of infected females with cured
(antibiotic-treated) females. When such comparisons are made, infected females fed antibiotics are
compared with those fed honey. It is important in these comparisons to avoid interference of toxic
effects that the antibiotics may have on the females. Therefore, the lowest possible effective dose
should be applied. Stouthamer and Mak (2002) showed that at a concentration of 50 mg/ml honey,
the antibiotic tetracycline was toxic to
E. formosa
females, while at 5 mg/ml the treated females
produced signiÝcantly fewer offspring than controls. At a concentration of 1 mg/ml no difference
in offspring production was found between treated and control females. Both the 5 mg/ml and the
1 mg/ml concentrations cured the treated females of their infection.
Fitness cost for infected hosts from mixed populations Ð a rare situation for
Wolbachia
-induced
parthenogenesis Ð has been studied only in
Trichogramma
spp. In
T. pretiosum
,
T. deion
,
and
T.
kaykai
,
Wolbachia
has a negative inÞuence on lifetime offspring production under laboratory
conditions (Stouthamer and Luck, 1993; van Meer, 1999; Silva, 1999). Infected wasps produce
fewer offspring than genetically identical wasps cured from their infection when given an abundance
of host eggs every day during their entire lifespan (Stouthamer and Luck, 1993). Sometimes the
number of daughters produced was even lower for infected mothers (Stouthamer et al., 1990b;
Stouthamer and Luck, 1993). These results show that the host can suffer severe Ýtness costs from
being infected, but lifetime offspring production cannot be extrapolated very well from the lab to
the Ýeld because females are expected to be host-limited in the Ýeld (Stouthamer and Luck, 1993).
Some parameters that may be important for host Ýtness in the Ýeld were also studied. Van Meer
(1999) determined preadult survival and showed it was reduced for infected
T. kaykai
and
T. deion
females. We compared several infected lines with several naturally uninfected lines of
T. kaykai
and found that offspring of infected females had a lower preadult survival and survived even
dramatically less when both forms competed for the same food source (M.E. Huigens, unpublished
results). Under high parasitoid densities, sharing of a food source may occur frequently in the Ýeld.
The high mortality rates of unfertilized eggs in infected
T. kaykai
and
T. deion
lines found by
Tagami et al. (2001) also conÝrm the general pattern of high Ýtness costs in mixed populations.
