Biology Reference
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the paternal chromosomes in fertilized eggs are functionally destroyed and only the maternal
chromosomes and the PSR
factor itself remain. Such fertilized eggs develop into sons that carry a
haploid set of chromosomes from the mother and the PSR chromosome from their father. PSR
therefore causes uninfected and infected females to produce sons from their fertilized eggs. The
mating structure of the uninfected
T. kaykai
is such that a large fraction of the uninfected females
mate with their brothers (54 to 65%, Stouthamer and Kazmer, 1994). In contrast, the infected
females mate with males from the population, and 10% of these are carriers of PSR. As a conse-
quence, approximately 10% of the infected females mate with PSR males and only 5.5 to 6.4% of
the uninfected females. The consequence of this asymmetry is that the higher daughter production
of the infected population is suppressed more than that of the uninfected population, allowing for
a stable coexistence of both forms within the population (Stouthamer et al., 2001).
CONCLUSION
Since the beginning of the last decade, when the association between
Wolbachia
and parthenogenesis
was Ýrst identiÝed, our knowledge of this association has grown extensively. After the Ýrst evidence
of bacterial involvement in parthenogenesis through antibiotic treatment and the identiÝcation of
Wolbachia
, the recent success of Ýnding intraspeciÝc (Huigens et al., 2000) and interspeciÝc
(Grenier et al., 1998) horizontal transfer delivered the Ýnal proof that this symbiont was a causal
agent of parthenogenesis. Since the last review it has also become clear that PI-
Wolbachia
may
inÞuence cytogenetic events in different ways to cause parthenogenesis, even through meiotic
modiÝcations (Weeks and Breeuwer, 2001), and obviously more cytogenetic studies are needed to
determine if yet unknown mechanisms of PI exist. One of the main questions that remains to be
answered is: Which
Wolbachia
genes cause the traits expressed in their hosts? And similarly, how
important are host effects in expressing the
Wolbachia
phenotype?
The different genome projects that are under way should result in the sequence of several CI-
Wolbachia
, a PI-
Wolbachia
, and a feminizing
Wolbachia
. Cooperative studies on these sequences
may help in solving this problem. It is hoped that we will then also be able to explain why PI-
Wolbachia
are scattered throughout the phylogenetic trees based on several genes.
The study on the CFB bacterium that causes parthenogenesis in several
Encarsia
wasps showed
that the oviposition behavior of the infected females was modiÝed by the infection (Zchori-Fein
et al., 2001). We do not know if PI-
Wolbachia
also modify their hostÔs behavior.
On the population level, in at least one species we can now explain why infected and uninfected
forms coexist sympatrically. The presence of a suppressor in the form of the
psr
chromosome in
T. kaykai
is able to keep the infection in the population at a low level. A
psr
-like factor could occur
in
other species as well (Stouthamer et al., 2001).
More work remains to be done on the
WolbachiaÏ
host association in Ýxed populations. Studies
on genetic variation in these populations can tell us something about how
Wolbachia
went to
Ýxation. Furthermore, the genetics of the ÑvirginityÒ mutation remains to be studied.
Little has been done on applied aspects of the PI-
Wolbachia
. One greenhouse study by Silva
et al. (2000) showed that in inundative biocontrol the use of PI-infected
Trichogramma
wasps is
more economic than the use of sexual forms. However, more studies are needed to determine if
this is a general pattern or only a speciÝc example. In addition, similar studies should be done with
wasps that are used in classical biological control. The use of PI-infected wasps for augmenting
native sexual populations should be monitored closely because it may lead to the replacement of
native sexual forms with the released infected form.
Rendering wasps parthenogenetic through microinjection of PI-
Wolbachia
may only be effective
when the inoculum originates from mixed populations.
Wolbachia
from Ýxed populations are
expected to have coevolved to some extent with their hosts, which may have led to the loss of
genes needed to function in other host species
