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upstream region of
ftsZ
, where a non-LTR retrotransposon-like sequence was identiÝed in wBruAus,
comM
and an unidentiÝed ORF were found in wMel. These results indicated that the genome
fragments of wBruAus certainly retain the
Wolbachia
genome structure.
THE NATURE OF
W
B
RU
A
US
AS “CHROMOSOMAL
WOLBACHIA
”
Taken together, these results lead to the conclusion that wBruAus, a strain of
Wolbachia
identiÝed
from
C. chinensis
, has no bacterial entity but is a genome fragment of
Wolbachia
on the genome
of the host insect. The
Wolbachia
genome fragment is at least 11 kb in size (excluding the non-
LTR retrotransposon region), and probably much larger, contains a number of bacterial ORFs, and
is located on the X chromosome of the host insect. Based on the
wsp
sequence analyses (cf.
Figure
18.3)
,
we Ýrst thought that
C. chinensis
was infected with three distinct strains of endosymbiotic
bacteria of the genus
Wolbachia
. Actually, however, it turned out that the beetle is harboring two
ÑbacterialÒ and one ÑchromosomalÒ
Wolbachia
strains.
UNIVERSAL OCCURRENCE OF HORIZONTAL GENE TRANSFERS
BETWEEN PROKARYOTES
For a long time, horizontal gene transfers between genomes of different organisms were believed
to be rather exceptional phenomena. However, recent accumulation of microbial genome data has
revealed an exciting view of evolution according to which horizontal gene transfers have commonly
taken place between unrelated prokaryotic lineages in a dynamic manner (Ochman et al., 2000;
Koonin et al., 2001; Bushman, 2002). Horizontal gene transfer is now widely accepted as an
important and universal pathway for bacteria to reorganize their genome and to quickly acquire
novel features such as drug resistance, pathogenicity, metabolic properties, and others (Mazel and
Davies, 1999; Hacker and Kaper, 2000; Bushman, 2002).
HORIZONTAL GENE TRANSFERS BETWEEN PROKARYOTE
AND EUKARYOTE
On the other hand, horizontal gene transfers between prokaryote and eukaryote are still regarded
as unusual, except for those derived from mitochondria and chloroplasts (Martin et al., 1998; Gray
et al., 1999). A number of reports have described putative prokaryoteÏeukaryote gene transfers
based on identiÝcation of prokaryote-like genes on eukaryote genomes (Bushman, 2002). However,
many, if not all, of them are considered to be dubious or inconclusive for the following reasons:
(1) since the prokaryote-like gene on the eukaryote genome is fairly divergent from related prokary-
otic genes, the transfer is likely to be a quite ancient event; (2) at present, complete genome-
sequence data from a wide variety of eukaryotes are not available; (3) therefore, it is difÝcult to
exclude the possibility that the prokaryote-like gene was not horizontally acquired but has been
maintained in the lineage from the common ancestor of prokaryotes and eukaryotes, whereas most
eukaryotic lineages have lost the gene over evolutionary time.
The most impressive case of Ñdubious prokaryoteÏeukaryote horizontal gene transfersÒ recently
came from the human genome project. In the monumental article of the draft human genome sequence
(International Human Genome Sequencing Consortium, 2001), it was reported that the human genome
contained more than a hundred genes putatively transferred from bacteria. Many biologists were
brieÞy very surprised at the abundance of alien genes in our genome, but the excitement soon vanished.
By careful and intensive reanalyses of the genome data from human and other eukaryotes, Salzberg
et al. (2001) and Stanhope et al. (2001) independently came to the conclusion that descent through
common ancestry and gene loss provided a biologically more plausible explanation than horizontal
gene transfer for the origin of Ñthe human genes putatively from bacteria.Ò
