Biology Reference
In-Depth Information
When the
Wolbachia
infections have gone to Ýxation in populations, the infections do not
seem to have a negative impact on offspring production. In two
Trichogramma
species with Ýxed
populations,
T. cordubensis
and
T. oleae
, sexual lines could be established after curing (Silva,
1999; van Meer, 1999). Infected females and sexual females from cured lines had the same
offspring production in both species. In all other studies, the effects on host Ýtness of PI-
Wolbachia
have been studied only through comparison of untreated infected females with infected
females treated with antibiotics of heat. Infected wasps of
M. uniraptor
produced as many
offspring as antibiotic-fed wasps (Horjus and Stouthamer, 1995). Another study on the same
species conÝrmed the absence of a negative effect of
Wolbachia
on fecundity, longevity, and
offspring survival (Zchori-Fein et al., 2000).
In
Eretmocerus mundus,
fecundity was unaffected by
Wolbachia
,
but offspring of infected
females had much higher survival than offspring from antibiotic-treated females (De Barro and
Hart, 2001). This low survival may be caused by the very high antibiotic dose of 29.8 mg/ml
Rifampicin that was used. Two cases of
Wolbachia
-induced parthenogenesis outside the
Hymenoptera, with the population Ýxed for the infection, also do not show a negative effect of
Wolbachia
on host Ýtness. In the parthenogenetic mite species
B. praetiosa
and
B.
sp.
x
, antibiotic-
treated females (0.15% tetracycline) produced fewer offspring than infected females (Weeks and
Breeuwer, 2001). Arakaki et al. (2000) studied the predatory thrips species
F. vespiformis
and found
the same longevity and offspring production for infected and treated females. Pijls et al. (1996)
compared parthenogenetic
Apoanagyrus diversicornis
females with sexual uninfected conspeciÝc
females, which were allopatric, and studied the effect of antibiotic treatment in both forms. They
found a lower offspring survival for treated originally parthenogenetic females. For
A. diversicornis
the results are, however, slightly ambiguous because allopatric females were compared. At the time
of their study
Wolbachia
had not been detected yet in
A. diversicornis
, but this was later conÝrmed
by van Meer et al. (1999). In general, infection induces a Ýtness cost in mixed populations but not
in populations Ýxed for the infection. For some aspects of host Ýtness, infection might have evolved
to neutrality or even benevolence in Ýxed populations.
DYNAMICS OF PI-
WOLBACHIA
AND THE EVOLUTION
OF “VIRGINITY” MUTANTS
The dynamics of PI-
Wolbachia
infections are in general very simple. A PI-
Wolbachia
infects a
female in a population, and as long as the infected female produces more infected daughters than
an uninfected female produces uninfected daughters the infection will spread in the population,
barring stochastic events. The infection frequency among females that such an infection can reach
is a function of a number of variables: (1) the Ýtness cost of the infection, in terms of offspring
production, (2) the transmission Ýdelity of the PI-
Wolbachia
infection, and (3) the egg-fertilization
frequency (Stouthamer, 1997). Models show that the transmission Ýdelity of the infection is one
of the most important variables that determine the equilibrium infection frequency. Once the
infection has reached equilibrium it may stay at that level for a long time, unless suppressor alleles,
i.e., traits that either kill off the
Wolbachia
or suppress the
Wolbachia
-induced phenotype, evolve
and spread (Stouthamer et al., 2001). Alternatively,
Wolbachia
may also evolve to a higher level of
transmission and eventually reach Ýxation. However, another option is very likely to be the most
common. In this model, the infection reaches Ýxation in the population because a mutation that
causes females not to fertilize their eggs any longer has an enormous Ýtness advantage when a
substantial part of the population has become infected with the PI-
Wolbachia
.
Imagine a population where half of the females are infected and produce mainly infected
daughters as offspring, and assume 10% of the offspring of infected females is uninfected because
of inefÝcient transmission of the
Wolbachia
. Let us also assume that all infected females mate. The
uninfected females produce male and female offspring with a sex ratio that is optimal for the
