Biology Reference
In-Depth Information
heat treatment disrupted SOPE only. From these experiments we have determined that
Wolbachia
are not highly involved in weevil physiology but rather in the reproduction of the insect by
causing unidirectional cytoplasmic incompatibility (Heddi
et al., 1999). The molecular mecha-
nism that triggers the host-genome deregulation was not studied. Nevertheless, the intimate
contact between
Wolbachia
and the germ-cell nucleus
(Figure 5.1)
argues in favor of proteinÏpro-
tein or DNAÏprotein interaction processes.
W
EEVIL
-SOPE P
HYSIOLOGICAL
I
NTERACTIONS
In the last decade, studies of the weevil symbiosis were mainly focused on how integrated bacteria
inÞuence the behavior and physiology of the insect. Nardon provided the Ýrst data when he
succeeded in obtaining aposymbiotic insects from a wild-type symbiotic strain named SFr (Nardon,
1973). This system is unique in insect symbioses as a heat treatment (35 C) and 90% relative
humidity for 1 month resulted in bacterial elimination from both somatic and germ cells. However,
the resulting aposymbiotic strain is less fertile than the wild type; it develops slowly during the
larval stages and is unable to Þy as an adult. These perturbations have led to the conclusion that
SOPE inÞuences the physiology and behavior of the insect (Nardon and Grenier, 1988, 1989), and
these impact host
Ýtness
. These data provided evidence that intracellular symbiosis has an impact
on the evolution of insect characters.
Several biochemical aspects were tested to understand how SOPE enhanced the host
Ýtness
.
First, SOPE provides the insect with many components that are poorly represented in wheat
grains or completely absent from the albumen part of the grain on which
Sitophilus
spp. larvae
feed. These include pantothenic acid, biotin, riboÞavin (Wicker, 1983), and amino acids, partic-
ularly the aromatic amino acids phenylalanine and tyrosine (Wicker and Nardon, 1982). Second,
SOPE interferes with insect metabolism either directly through the modiÝcation of some products
or indirectly by increasing some insect enzymatic pathways (Gasnier-Fauchet et al., 1986; Heddi
et al., 1993).
Investigating the metabolic pathways of amino acids, Gasnier-Fauchet and Nardon (1987) have
noticed that methionine, an amino acid in excess in wheat grains, is not metabolized the same way
in the aposymbiotic insect as in the symbiotic. Methionine sulfoxide is always at high levels in
symbiotic insects, while sarcosine levels increase regularly during the last instar larvae of aposym-
biotic insects (Gasnier-Fauchet and Nardon, 1986), reaching the highest value at the end of this
instar (43.5 nmol/insect). In symbiotic insects, sarcosine levels never exceed 3.8 nmol/insect. Hence,
it was demonstrated that SOPE helped the insect to catabolize methionine into methionine sulfoxide
by a nonenergy-consuming and reversible reaction. In contrast, aposymbiotic insects fail to carry
out this pathway, and methionine is preferably demethylated via a glycine
N
-methyltransferase-
like activity leading to the accumulation of sarcosine, which is neither incorporated into proteins
nor excreted in the feces (Gasnier-Fauchet
et al., 1986).
An additional indirect metabolic effect was shown on mitochondrial energy metabolism.
Mitochondria isolated from symbiotic insects exhibited high levels of mitochondrial respiratory
control (RCR) when compared with the aposymbiotic insect, indicating that ATP production is
more efÝcient in insects harboring intracellular bacteria (Heddi and Lefebvre, 1990a; Heddi
et al.,
1991). This metabolic aspect was conÝrmed later by measuring six mitochondrial enzymatic
activities, three belonging to the oxidative phosphorylation chain (cytochrome
c
oxidase, succi-
nate cytochrome
c
reductase, and glycerol-3-phosphate cytochrome
c
reductase) and three other
enzymes from the Krebs cycle (isocitrate dehydrogenase, pyruvate dehydrogenase, and b-keto-
glutarate dehydrogenase) (Heddi
et al., 1993). As a result, symbiotic insects always show higher
speciÝc enzymatic activities regardless of the insect stage. However, the differences between
symbiotic and aposymbiotic insects are attenuated in the adult stage, where symbiont density
decreases with age until the third week of adult development, at which time the mesenteric caeca
are completely devoid of symbionts.
