Biology Reference
In-Depth Information
TABLE 5.1
Genome Sizes and A+T Content of Insect Intracellular Bacteria
A+T
Content
Insect Host
Genome Size
Ref.
Buchnera
spp.
Aphid PE
From 450 kb
to 670 kb
74%
Charles and Ishikawa,
(1999); Shigenobu et al.
(2000); Gil et al. (2002)
Wigglesworthia
spp.
Tsetse PE
From 705 kb
to 770 kb
60%
a
Akman and Aksoy (2001)
Sodalis glossinidius
Tsetse SE
2.2 Mb
45%
a
Akman et al. (2001)
SOPE
Weevil PE
3 Mb
46%
Charles et al. (1997);
Heddi et al. (1998)
Wolbachia
Invertebrate
endosymbiont
From 0.95 Mb
to 1.66 Mb
60%
a
Sun et al. (2001)
Escherichia coli
Free-living bacteria
4.6 Mb
47%
Smith et al. (1987)
a
Values determined from gene sequences only. PE = principal endosymbiont; SE = secondary endosymbiont.
living Enterobacteriaceae. This may also partly explain why the weevil association could be
artiÝcially dissociated (Nardon, 1973) or why
Sodalis
can grow outside the host (Beard
et al., 1993),
whereas the other endosymbionts (
B. aphidicola
and
Wigglesworthia
spp.)
are
strictly dependent
on their hosts.
INSECT-BACTERIA INTERACTIONS IN THE WEEVIL
SYMBIOTIC ASSOCIATION
Living in association implies interaction, which starts from the Ýrst contact between partners
and continues during the process of permanent mutual establishment. Today, most biologists
admit that the evolution of eukaryotes resulted from symbiotic interaction between several
independent ancestors such as ancestors of mitochondria and plastids (Sogin, 1997), although
the origin of other cellular organelles, such as undulipodia, microtubules, and nucleus, is con-
troversial (Maynard Smith, 1989; Margulis, 1993b). However, what remains unresolved is an
understanding of the behavior of bacteria at the early phases of host infection as well as the
mechanism leading to stable unit formation. This section describes how SOPE interacts with the
weevil at three levels: genetic, showing how the insect controls the SOPE population; physio-
logical, by which the association improves its
Ýtness
; and molecular interactions that deÝne the
molecular ÑconversationÒ between partners.
G
ENETIC
I
NTERACTIONS
BETWEEN
E
NDOSYMBIONTS
AND
W
EEVILS
As reported above, SOPE is located speciÝcally in the bacteriocytes that form the bacteriome; no
bacterial proliferation has been observed elsewhere. Moreover, symbiont density is different among
Sitophilus
strains, but it is consistent within a given population. Finally, symbiont numbers from
the egg (20,000 cells/embryo) to the last larval stage (3 10
6
cells/larvae) indicate that SOPE may
divide seven times over 21 d (Nardon
et al., 1998). Taken together, these results support the existence
of a host genetic system controlling both the location and the population size of bacteria within
the insect. To shed light on this aspect, genetic experiments were conducted on strains selected on
the basis of larval development times (
Figure 5.3)
.
One Low (LL) and one Rapid (RR) strain were
obtained from the SFr wild-type strain, and reciprocal crosses and backcrosses were conducted
between them (Nardon
et al., 1998). Interestingly, the number of symbionts is identical in both
