Biology Reference
In-Depth Information
species. The male offspring of these uninfected females have a high Ýtness compared to both
uninfected and infected females because they can mate with several females. Assume that a mutation
takes place in an uninfected female and that this mutation stops her either from mating or from
fertilizing her eggs so that now she produces exclusively male offspring. This ÑvirginityÒ mutation
has a large Ýtness advantage because of the female-biased sex ratio in the population. When the
mutant male mates with an infected female, part of her offspring will be heterozygous for the
mutant, and some of the offspring (50% of the unfertilized eggs) of these infected females will
become homozygote for the ÑvirginityÒ mutation and no longer mate with males. These infected
mutant females will produce some uninfected eggs because of the inefÝcient transmission. These
eggs will be unfertilized and thus become males that are carriers of the mutation. These males also
experience the large Ýtness beneÝt from being male in a population consisting largely of females.
If these males mate with uninfected females, they will increase the frequency of the mutant allele
in the uninfected population, thereby reducing the number of uninfected females produced. In the
infected population, an increasingly larger number of females will carry the mutation and become
unavailable for mating with males, while the infected females not yet homozygous for the virginity
mutation will still be willing to mate.
Simulations (R. Stouthamer and F. Vavre, unpublished results) have shown that even with a
large negative impact on the Ýtness of their carriers in terms of offspring production this ÑvirginityÒ
mutation will spread through the population and result in a rapid Ýxation of the
Wolbachia
infection.
In addition, the resulting all-female population will now consist entirely of mutant females that are
no longer able or willing to mate. However, if males are induced by feeding the infected females
antibiotics, these males are still capable of mating successfully with nonmutant females. This is
indeed the pattern that is observed in a number of populations where both completely infected and
uninfected populations exist. Pijls et al. (1996) and Arakaki et al. (2000) studied species where
both completely sexual and completely infected populations existed allopatrically. When males
were produced by antibiotic treatment from the infected population, they did not mate successfully
with females of the infected line, while successful matings took place with females from the sexual
line. The females from the infected line did not mate successfully with either males from the
infected or from the uninfected line. The lack of successful matings between cured males and
infected females is a common phenomenon in populations where the infection has gone to Ýxation.
For example, in parthenogenetic parasitoid species such as
E. formosa
(Zchori-Fein et al., 1992),
M. uniraptor
(Stouthamer et al., 1993, 1994; Zchori-Fein et al. 2000),
A. diversicornis
(Pijls et al.,
1996),
Aphytis
spp. (Zchori-Fein et al., 1995),
Telenomus
nawaii
(Arakaki et al., 2000), and
E.
mundus
(de Barro and Hart, 2001), females do not produce offspring from fertilized eggs, when
allowed to mate with conspeciÝc males derived by antibiotic treatment. In addition, in the parthe-
nogenetic mite species
B. praetiosa
, copulation took place between cured males and females, but
sperm was not used by the females (Weeks and Breeuwer, 2001). In cured females of the thrips
F.
vespiformis
, sperm was found after they mated with ÑparthenogeneticÒ males (Arakaki et al., 2001a)
but also not used.
In mixed populations, mating behavior of infected females does not seem to be different from
uninfected females. Within several species Ð
T. kaykai
,
T. pretiosum
, and
T. deion Ð
infected
females successfully mate with males from cured or uninfected forms (Stouthamer et al., 1990b;
Stouthamer and Luck, 1993; van Meer, 1999; Stouthamer et al., 2001). In all mixed populations,
the virginity mutation could evolve and should lead to Ýxation of the infection in the population.
The cases where mixed populations persist are an indication of the presence of some suppressing
factor. Suppressor genes as such have not yet been found for PI-
Wolbachia
. In
T. kaykai
, detailed
work on the prolonged persistence of infected and uninfected individuals within a population did
not Ýnd any evidence for the presence of suppressors but brought to light the presence of a second
sex-ratio distorter (paternal sex ratio, PSR) in the population. About 10% of the males carry this
chromosome (Stouthamer et al., 2001). Such a PSR chromosome was until then found only in the
parasitoid wasp
N. vitripennis
(Werren, 1991). When a female mates with a PSR
-
carrying male,
