Biology Reference
In-Depth Information
hostÏ
Wolbachia
interactions. IdentiÝcation of these genes will be a major breakthrough in deci-
phering the biology of this unculturable bacterium, understanding
WolbachiaÏ
host symbiotic asso-
ciations, and uncovering the evolution of intracellular symbiosis.
G
ENETICS
AND
C
ELL
B
IOLOGY
OF
W
OLBACHIA
-I
NDUCED
CI
Genetic Analysis of Host/Symbiosis
Although Ýve
Wolbachia
types have been described in
D. simulans,
the exact number of distinct
Wolbachia
strains found in
D. simulans
is under question (e.g., see James et al., 2002, for a
discussion of strain nomenclature). The origins, evolution, and genetic basis of these different
incompatibility types are not known. However, their presence does suggest that each unique strain
of
Wolbachia
differentially modiÝes and rescues sperm, which is almost certainly the case for
bidirectional incompatibility (Braig et al., 1994; Rousset et al., 1999). In this section, the genetic
basis of this host/symbiosis system will be discussed. Unfortunately, our present knowledge of
Wolbachia
genetics is extremely limited. This is due to a lack of molecular genetic analyses of the
kind that has proven so extremely powerful for microorganisms such as
E. coli
. Unlike free-living
facultative anaerobes such as
E. coli
,
Wolbachia
is an obligate intracellular microbe for which cell-
free culture is not yet available. Therefore, essentially nothing is known of its genetic structure,
although this will certainly change as whole genomic sequencing and subsequent analyses of
Wolbachia
genomes are realized.
A number of factors have been identiÝed that effect the expression of CI in
Drosophila
.
These include (1)
Wolbachia
type, (2) host strain and host age, (3) heat treatment, and (4) mating
history (Boyle et al., 1993; Turelli and Hoffmann, 1995; Sinkins et al., 1995b; Hoffmann et al.,
1996; Bourtzis et al., 1998; Clancy and Hoffmann, 1998; Karr et al., 1998; Poinsot et al., 1998;
James and Ballard, 2000; Snook et al., 2000). Each of these identiÝed factors are potential targets
for genetic analyses of CI. Below, we provide a brief general background of previous work
relevant to genetic analyses of the system and further focus on one particular area of contemporary
research, the inÞuence of host genetic structure on
Wolbachia
growth and tissue distribution in
the
Drosophila
testis.
Historically, inferences about the genetics of
Wolbachia
symbiosis have been indicated by
studies involving (1) comparative work on different host-strain genetic backgrounds, (2) replace-
ment of the host genome by repeated backcrosses, (3) direct transfer of
Wolbachia
into novel or
nave hosts, or (4) genetic analyses of the effect of host mutations on
Wolbachia
biology. Each of
these approaches provides a wealth of information about the overall impact of host genetics in each
system studied and has generally suggested extensive genetic interactions between host and microbe.
Each of these areas of study is summarized below.
Comparative and Functional Studies
A variety of
Drosophila
species have been analyzed for
Wolbachia
infection (Bourtzis et al.,
1994, 1996; Giordano et al., 1995; Werren and Jaenike, 1995; Hurst et al., 2000; Lachaise et al.,
2000). Taken together, only 12 out of 52 tested species have been identiÝed as infected. From
this limited data it appears that
Wolbachia
may be present in about 23% of
Drosophila
species
in agreement with current estimates that these infect anywhere from 16 to 25% of insects (Werren
and Windsor, 2000).
Studies on different
D. melanogaster
strains exhibited an interesting variation in the expression
of CI associated with
Wolbachia
infection (Bourtzis et al., 1996; Clark et al., 2002). Presumably,
the considerable variation in CI expression observed in infected
D. melanogaster
mutant and wild-
type lines reÞects at some level the effect of host genetic backgrounds on CI expression. For
example, egg mortality rates among various
D. melanogaster
lines range from as low as 10% to
as high as 50% (Bourtzis et al., 1996). This would indicate, assuming the same
Wolbachia
infects
