Biology Reference
In-Depth Information
diverse eukaryotic group in the terrestrial ecosystem (Wilson, 1989) and that infection frequency
of
Wolbachia
in natural insect populations reaches around 20 to 70% worldwide (Werren and
Windsor, 2000; Jeyaprakash and Hoy, 2000), it will not be surprising if future careful studies reveal
other cases of
Wolbachia
-host gene transfer. Other endosymbiotic systems, such as
Buchnera
in
aphids, in which the obligate symbiont genome exhibits remarkable degeneration and reduction
(Shigenobu et al., 2000), might conceal similar horizontal gene transfer events. Genome sequencing
of
Arabidopsis thaliana
revealed a continuous stretch of nearly 75% of mitochondrial genome
located on chromosome 2 of the plant (Lin et al., 1999). Similarly, other cases of symbiontÏhost
gene transfer might come from genome sequencing projects of various eukaryotic organisms now
in progress.
EVOLUTIONARY IMPLICATIONS
It has been pointed out that
Wolbachia
-induced CI can promote reproductive isolation of host insects
(Hurst and Schilthuizen, 1998; Bordenstein et al., 2001). If genes of
Wolbachia
responsible for CI
are transferred to the host genome in a functional form, this would also reinforce the reproductive
isolation and ultimately lead to speciation. It has been suggested that endosymbiotic associations
with microorganisms act as a source of evolutionary innovations for their hosts (Margulis and
Fester, 1991). Implications of the currently ongoing genome transfer between symbiont and host
can be far-reaching in this context.
PERSPECTIVE
Our studies on the
Wolbachia
triple infection system in
C. chinensis
have led to the exciting
discovery of the symbiontÏhost horizontal genome transfer. We now have a number of subjects to
be investigated further.
1.
Structure of the transferred genome fragment: The size and structure of the fragment,
the location on the host X-chromosome, and the structure of the junction between the
symbiont and host genomes must be determined for understanding the evolutionary
process and molecular mechanism of the horizontal genome transfer. To clone the full
length of the
Wolbachia
genome fragment, we are constructing a cosmid genomic library
from the total DNA of the
C. chinensis
strain jC
Aus
.
2.
Gene expression on the transferred genome fragment: Once the full sequence of the
transferred genome fragment is determined, all structurally intact ORFs on the fragment
will be subjected to an expression assay using RT-PCR. If
Wolbachia
genes expressed
on the host genome are identiÝed, they should provide an important clue to understanding
the biological function of the chromosomal
Wolbachia
.
3.
Biological effects of the transferred genome fragment: Fitness parameters of the host
insect, population and localization of the bacterial
Wolbachia
, and other aspects will
be quantitatively examined in the presence and absence of the chromosomal
Wolbachia
,
which would provide us with insights into the biological effects of the transferred
genome fragment.
4.
Diversity of chromosomal
Wolbachia
in natural populations: In Japanese populations of
C. chinensis
, diagnostic PCR analysis showed that wBruAus was prevalent at an average
frequency of 97%. However, the result was based only on PCR detection of
wsp
gene.
The possibility that part of the 3% negative insects are not free of the chromosomal
Wolbachia
but simply
wsp
-negative cannot be ruled out. It is conceivable that there should
be variants of chromosomal
Wolbachia
in the populations with different degrees of
deletions and rearrangements. To access the diversity of the chromosomal
Wolbachia
,
