Biology Reference
In-Depth Information
PROCESS OF SYMBIONT-HOST HORIZONTAL GENE TRANSFER
At present,
neither the mechanism nor the process involved in the
Wolbachia
-insect horizontal gene
transfer is understood. Through generations of host insects,
Wolbachia
are maternally inherited via
infection of developing oocytes. In eggs,
Wolbachia
cells localize at the posterior pole, where germ
cells develop (HadÝeld and Axton, 1999), and are closely associated with astral microtubules of
the host cells during mitosis (Kose and Karr, 1995). Therefore, it is conceivable that these intimate
associations of
Wolbachia
with proliferating germ cells have provided a favorable condition for the
observed horizontal gene transfer.
EVOLUTIONARY ORIGIN
Absence of
Wolbachia
in other
Callosobruchus
species (
Figure 18.2)
suggests that wBruAus was
acquired by the ancestor of
C. chinensis
through horizontal transmission from an unrelated host.
In the DNA databases, a
wsp
sequence from the tephritid fruit Þy
Dacus destillatoria
(accession
No. AF295344) shows a very high similarity (98.9%) to the
wsp
gene of wBruAus, although a
biological connection between the fruit Þy and the bruchid beetle is obscure. The acquisition of
bacterial wBruAus probably preceded the transfer of its genome fragment to the host chromosome.
If so, a worldwide survey of
C. chinensis
populations might lead to the discovery of relic bacterial
wBruAus, which would provide further insights into the evolutionary origin and process of the
horizontal gene transfer.
MAINTENANCE MECHANISM IN HOST POPULATIONS
In all local populations of
C. chinensis
examined in Japan, the frequency of wBruAus was
consistently more than 90% (
Figure 18.4)
.
At present, the mechanism whereby the
Wolbachia
genome fragment on the host chromosome prevails in populations is a mystery. Fixation by
chance through drift cannot be ruled out but appears unlikely on account of the consistent
prevalence in many local populations. It appears conceivable that the common ancestor of
Japanese
C. chinensis
carried wBruAus on the X chromosome, which has been passed to present
local populations. The chromosomal
Wolbachia
might be able to increase its frequency by
hitchhiking with coexisting bacterial
Wolbachia
that cause CI. The chromosomal
Wolbachia
might be tightly linked to genes that confer a positive Ýtness effect to the insect or genes that
enhance its own transmission in a selÝsh manner like meiotic drive genes (Lyttle, 1993).
Alternatively, the chromosomal
Wolbachia
itself might behave as a selÝsh genetic element like
Medea
, a maternal-effect chromosomal factor known from Þour beetles (Beeman et al., 1992).
The last possibility is intriguing in that the
Medea
phenotype and
Wolbachia
-induced CI can
be explained by the same ÑpoisonÏantidoteÒ or ÑmodiÝcationÏrescueÒ mechanism (Bull et al.,
1992; Hurst and McVean, 1996).
OTHER SYMBIONT-HOST HORIZONTAL GENE TRANSFERS
TO BE FOUND
Earlier studies reported that genetic recombination could occur between coinfecting
Wolbachia
strains (Werren and Bartos, 2001; Jiggins et al., 2001). In this study, we Ýrst demonstrated that
genetic materials could be exchanged between
Wolbachia
and host insect. It is unknown whether
the
Wolbachia
Ïhost gene transfer is an orphan exception or whether other cases are to be found.
It should be noted that in previous extensive surveys of infection and diversity of
Wolbachia
(Werren
et al., 1995a; Werren and Windsor, 2000; Jeyaprakash and Hoy, 2000), only PCR detection was
conducted without examining the inheritance pattern of the genes. Given that insects are the most
