Biology Reference
In-Depth Information
lines, that the variation observed was due to the differing genetic backgrounds
of the host. However, the precise genetic basis for these differences is not known, and these
observations suggest that further genetic tests that target speciÝc cellular processes and traits will
identify key host genetic loci involved in the symbiosis.
Because
all
D. melanogaster
Wolbachia
affect sperm function during fertilization, research has focused on
Wolbachia
distribution and behavior during spermatogenesis (Bressac and Rousset, 1993; Ndiaye and Mattei,
1993; Karr et al., 1998; Clark and Karr, in press; Clark et al., in press).
presumably impair
the male pronucleus but not the extranuclear component of the sperm (Presgraves, 2000). Recent
studies have demonstrated the presence of two major groups of
Wolbachia
DrosophilaÏWolbachia
associations
within infected testes: group I is characterized by the presence of high numbers of
Wolbachia
-infected
sperm cysts, while in group II
are absent from cyst cells and found mainly within somatic
cells (Clark et al., 2002, in press; Veneti et al., in review). In all cases studied,
Wolbachia
infection
of sperm cysts was tightly linked to CI expression. For example, group I infection consisted of two
subgroups,
Wolbachia
D. simulans
Coffs (
w
Cof) and
D. mauritiana
(
w
Ma), that exhibit high numbers of heavily
Wolbachia
-infected sperm cysts, but they did not express CI, while the second contained variable
numbers of
-infected sperm cysts and express variable levels of CI. Based on these studies,
a ÑWISÒ hypothesis (for
Wolbachia
-Infected Spermatocyte) was proposed and a model built on this
hypothesis suggested. In this model, cyst infection is a necessary, but not sufÝcient, condition for
the expression of CI. Using this deÝnition as a starting point, genetic studies can focus on host
mutations that change or otherwise alter
Wolbachia
in the developing spermatocyst.
While no concrete conclusions can be reached as to the genetic bases underlying the growth
and maintenance of
Wolbachia
in the expression of a WIS phenotype, the consistency of the results
strongly imply such a basis. The relative contributions of host genetic backgrounds to the expression
of CI have been studied by introgression of host genetic backgrounds into a
Wolbachia
-infected
maternal cytoplasm (see Genome Replacement via Introgression, below). This is a more direct
approach to the study of genetic interactions, as it compares differing host genetic backgrounds in
an otherwise identical
Wolbachia
Wolbachia
strain infecting the same maternal cytoplasm.
Genome Replacement via Introgression
The contributions of host genetic backgrounds to the expression of CI in the parasitic hymenopteran
Nasonia
have been well studied (Breeuwer and Werren, 1993; Bordenstein and Werren, 1998). In
this system, two species,
Nasonia vitripennis
and
N. giraulti,
harbor double
Wolbachia
infections
(A and B strains). Introgressions of the
N. giraulti
genome into a doubly-infected
N. vitripennis
line did not affect the expression of bidirectional CI or unidirectional CI when compared to the
parental
N. vitripennis
males, i.e., the
N.
giraulti
genome did not affect this modiÝcationÏrescue
system. However, introgression of
N. giraulti
into a singly-infected
Wolbachia
A strain of
N.
vitripennis
did affect CI expression levels compared to this same A strain in
N. vitripennis
,
suggesting the differences noted were due to host genetic backgrounds.
Parallel studies in
Drosophila
conÝrmed and extended the basic conclusions obtained in the
Nasonia
system. Reciprocally introgressed genomes between host lines harboring different
Wolba-
chia
strains were measured for their ability to express CI (Clark and Karr, in press). These
introgression experiments demonstrated that the
type determined the ability or inability
to modify sperm independent of host genetic background and conÝrmed the comparative studies
mentioned above. Although an intrinsic
Wolbachia
Wolbachia
factor appears to determine CI type, host genetic
backgrounds were shown to clearly affect
Wolbachia
growth in the testis and, as a result, to affect
the level of CI in those lines with mod+
.
Introgression experiments are limited because the original host genotype cannot be completely
replaced, and epistatic interactions in the host may inÞuence the results and obscure direct genetic
interactions of host and symbiont (although epistasis in this context might reveal interesting
interactions themselves). Another useful avenue of research that avoids these limitations is direct
transfer of
Wolbachia
Wolbachia
via microinjection, as described below.
