Biology Reference
In-Depth Information
Direct Inter- and Intraspecific Transfer between Species and Lines
Direct transfer of
Wolbachia
into novel hosts has been a useful tool for the study of genetic
interactions in
Drosophila
(Boyle et al., 1993; Braig et al., 1994; Giordano et al., 1995; Poinsot
et al., 1998; McGraw et al., 2001, 2002). This approach transfers the microbe (and small amounts
of egg cytoplasm) into uninfected host eggs via direct cytoplasmic transfer by microinjection, and
infected lines are established (reviewed in Karr, 1994). Once infected lines are established, CI and
bacterial densities can be measured and compared to the parental line.
The Ýrst successful interspecies transinfection of
Wolbachia
(Boyle et al., 1993) from
D.
simulans
into
D. melanogaster
showed a clear effect of host genetic background on CI expression.
Further, the lack of CI expression was due to the presence of far fewer bacteria in the novel
D.
melanogaster
host. This suggested two important points: Ýrst, CI expression was not an exclusive
intrinsic property of
Wolbachia
, and, second, the host could regulate bacterial load. A linear
correlation between bacterial numbers in the egg and the level of CI expression in isofemale lines
selected for increased levels of CI further suggested a host genetic basis for this trait (Boyle et al.,
1993; Sinkins, 1995a). A number of subsequent studies in a variety of systems have shown the
efÝcacy and challenges of cross-taxa transfer of
Wolbachia
(Braig et al., 1994; Poinsot et al., 1998;
van Meer and Stouthamer, 1999).
Recently, horizontal transfer of a virulent strain of
Wolbachia
(ÑpopcornÒ [Min and Benzer,
1997]) has revealed the importance of hostÏsymbiont interactions in the expression, growth, and
maintenance of infection, particularly the role of host genetic background (McGraw et al., 2001).
In its original host, the popcorn-strain variant does not express CI (but see Reynolds and Hoffmann,
2002; Weeks et al., 2002) and does not reside in the testis (McGraw et al., 2001; Veneti et al., in
review). Upon transfer to an uninfected
D. simulans
strain known to harbor a CI-inducing strain
of
Wolbahcia
(the DSR strain), the popcorn strain infected the testis and expressed CI, clearly
showing a recurring theme of the importance of host genetic background.
Mutant Analyses
The three approaches described above reveal general patterns of host genotype on
Wolbachia
biology. However, they cannot provide information about individual genes and their potential
impact on
Wolbachia
growth, tissue distributions, or expression of CI. Genetic analyses would
greatly beneÝt from the application of classical genetic techniques (e.g., identiÝcation of mutants
that effect symbiosis parameters mentioned above and the concomitant identiÝcation of individual
genes involved). As mentioned above, the lack of
in vitro
cultures precludes direct genetic
analyses of
Wolbachia
, but fortunately insights into the genetics of this system can be obtained
by study of host genetics relying on the extensive genetic information and genomic data available
for
D. melanogaster
(Lindsley and Zimm, 1992). Although in its infancy, this approach has
proven successful in identifying situations where mutant phenotypes are affected by
Wolbachia
infection, as described below. In its broader application, these initial studies strongly suggest
that a dedicated screen for other genetic loci will identify host genes that directly or indirectly
interact with
Wolbachia
.
The sxl
4
mutation.
Wolbachia
has been found to restore fertility to
D. melanogaster
females
carrying the
sxl
4
mutation (Starr and Cline, 2002). This mutation results in sterile females because
of its effect on sex determination (Cline and Meyer, 1996). Prior phenotypic analyses had been
performed in this mutant line without knowledge of its infection status. Interestingly, restoration
of fertility by
Wolbachia
is not observed to the same extent in other germline-speciÝc
sxl
alleles;
therefore, fertility restoration most probably results from a speciÝc interaction with
Sxl
protein
rather than from a bypass of the normal germline requirement for this developmental pathway. As
such,
sxl
Ï
Wolbachia
interactions indicate the importance of
Wolbachia
insinuation of host repro-
duction and the degree to which it has occurred.
The chico mutation.
Mutations in the insulin-receptor pathway and the PTEN/MMAC signaling
pathway affect body, tissue, and organ size in
D. melanogaster
(Bohni et al., 1999; Goberdhan
