Biology Reference
In-Depth Information
PHYLOGENETIC CHARACTERIZATION OF SOPE
AND
WOLBACHIA
The molecular phylogenetic positioning of insect intracellular bacteria began in 1989 with the
sequence of the 16S rDNA of
Buchnera aphidicola
, the endosymbionts of aphids (Untermann
et al., 1989; Munson
et al., 1991). Currently, the bacterial symbionts of more than ten insect
models are described, including those of aphids, psyllids, cicadas, planthoppers, mealy bugs,
glossines, weevils, cockroaches, termites, ants, and bugs. Most of the insect-integrated endosym-
bionts belong to the
-proteobacteria group except those from cockroaches and mealy bugs,
which are included in, respectively, the Flavobacteria and
h
c
-proteobacteria groups
(Figure 5.2)
(Bandi
et al., 1996; Fukatsu and Nikoh, 2000). Using a phylogenetic
analysis based on a heterogeneous model of DNA evolution that takes into account the GC-
content heterogeneity among bacterial sequences, we have shown recently that the majority of
the
et al., 1994; Kantheti
-proteobacteria endosymbionts are closely related within the Enterobacteriaceae family,
located between the genera
h
et al., 2001). This result suggests that
the Enterobacteriaceae group may possess features allowing the establishment and maintenance
of stable symbiotic relationships with insects. The nature of such features and traits is unknown
currently, but endosymbiont-genome-sequencing projects, as well as host-symbiont molecular-
interaction studies, may provide insights into this aspect.
In the weevils of the genus
Proteus
and
Ye rsinia
(Charles
Sitophilus
, two types of symbionts are present, SOPE and
Wolbachia
(Figure 5.2), and both are transmitted to the offspring maternally. The 16S rDNA sequences of
Wolbachia
spp. that occur in
Sitophilus
spp. fall into the B-group of
Wolbachia
with which they
form a monophyletic group relative to the other
-proteobacteria. However, the SOPE 16S rDNA
sequence was placed within the Enterobacteriaceae (Heddi
b
et al., 1998). It shares 95% sequence
identity with
Escherichia coli
and 87% sequence identity with
B. aphidicola
. SOPE is also afÝliated
with the endosymbionts of the tsetse Þy (Aksoy
et al., 1997) and those of the carpenter ants,
Camponotus
spp. (Schrder
et al., 1996). Interestingly, the closest intracellular endosymbiont to
SOPE is
Sodalis glossinidius
, the secondary endosymbiont of tsetse. Recent data suggest that SOPE
and
may have evolved from the same ancestor or that horizontal gene transfer may
have occurred between tsetse and weevil over the course of evolution (unpublished data). These
hypotheses are under investigation in conjunction with phylogenetic studies of the Dryophthoridae
family endosymbionts.
S
.
glossinidius
INTRACELLULAR BACTERIAL GENOME EVOLUTION
It is now well documented that intracellular bacteria, as well as other intracellular pathogenic
organisms, undergo particular evolutionary constraints with respect to their DNA G+C content and
their genome sizes (Moran, 1996; Heddi
et al., 2001). In general, and regardless
of the bacterial physiology and behavior, genome sizes are reduced and genomes themselves are
biased toward A+T, as compared with their closest free-living bacterial relatives. Presently, Ýve
insect endosymbiont genomes have been analyzed for genome size and G+C content, four belonging
to the Enterobacteriaceae family and the Ýfth one from the
et al., 1998; Sun
group. As shown in
Table
5.1,
all intracellular bacterial genomes that have been studied so far have reduced genome sizes
and are A+T-biased with regard to the free-living bacterium
Wolbachia
. The reasons for these genetic
changes are not precisely understood, but they could be related to the vertical mode of bacterial
transmission. Indeed, most intracellular bacteria are transmitted to the progeny through the oocytes,
which generates a bottleneck effect and prevents endosymbiont recombination with free-living
organisms, thereby facilitating accumulation of deleterious mutations and compositional bias due
to directional mutation pressure (Moran, 1996; Heddi
E. coli
et al., 1998). Moreover, genome-size reduc-
tion could be interpreted as being the result of serial deletions of gene fragments, which thus are
no longer necessary for the new hostÏsymbiont unit. Likewise, gene transfer to the nucleus could
