Biology Reference
In-Depth Information
Rhynchophoridae) comprises as many as 145 genera that feed on plant stems, roots, and seeds.
Among them,
spp. are the most characterized in terms of symbiosis because of the
damage they cause on cereals. Three cereal-feeding species (
Sitophilus
S. oryzae
,
S. zeamais
, and
S.
granarius
) destroy up to 40% of stored cereals, particularly in tropical regions. All are naturally
symbiotic and transmit maternally to the offspring an intracellular
-proteobacterium. Females
lay eggs inside the seeds, and larvae develop within the grain and emerge from the grain as
adults. During this period, larvae feed on seed albumen only, which is starch rich but with low
amino acid, lipid, and vitamin content. A strong database has demonstrated that intracellular
bacteria help the host to balance these nutritional deÝciencies (Wicker, 1983; Wicker
h
et al., 1985;
Nardon and Grenier, 1988).
This chapter describes intracellular symbioses in the weevils of the genus
, reviews
the recent knowledge on genetic, physiological, and molecular interactions between the host and
the symbiont, and explores the role of intracellular symbiosis as a driving force in evolution.
Sitophilus
THE BIOLOGY OF
SITOPHILUS
SSP. SYMBIOSIS
Symbiosis in members of the genus
was Ýrst described by Pierantoni (1927), then by
Mansour (1930, 1934, 1935). The best-understood symbiont is the intracellular bacterium,
recently called SOPE (
Sitophilus
(1998), which
is a Gram-negative bacterium located in the ovaries and in the larval bacteriome formed by
polyploid cells called bacteriocytes
(Figure 5.1).
It occurs naturally in all populations of the
three
Sitophilus oryzae
principal endosymbiont) by Heddi
et al.
) and always lives free
in the cytosol of oocytes and bacteriocytes without any surrounding membrane. This feature
suggests that either SOPE does not behave (or behaves only slightly) as a pathogenic cell or that
host cells regulate their immune system to control and tolerate the bacterium. Nevertheless, the
tissue speciÝcity of SOPE, which is restricted to two kinds of tissues (i.e., bacteriocytes and
oocytes), argues in favor of molecular signals or ÑdialogueÒ between partners leading to the
control of the bacterial population.
In the young nymph, the larval bacteriome dissociates and bacteriocytes migrate to reach the
apex of mesenteric caeca, where they form mesenteric bacteriomes in the young adults (Figure
5.1). The density of SOPE changes with the developmental stage and with the age of the insect.
Their number increases from the Ýrst instar larvae and reaches the highest level in the fourth and
last instar larvae, with as many as 3 million bacteria per larval bacteriome (Nardon and Wicker,
1981). In adults, SOPE density decreases with age, and 3-week-old adults are completely devoid
of symbiotic bacteria (Mansour, 1930; Schneider, 1956). However, adult females retain them in the
apical bacteriome of the ovaries and in the oocytes from which the bacteria are transmitted to the
progeny. Thus, from a physiological point of view, symbiosis in the weevil seems to be necessary
during the larval and the young-adult developmental stages only.
In addition to SOPE,
Sitophilus
species analyzed (
S. oryzae
,
S. zeamais,
and
S. granarius
Sitophilus
weevils harbor a second type of bacterium, which belongs
to the genus
et al., 1999). This rickettsia-like organism
is widespread in insects and is estimated to occur in at least 20% of the insect species (Werren,
1997; Jeyaprakash and Hoy, 2000). It has also been detected in crustaceans, nematodes, and
arachnids. Unlike integrated endosymbionts,
Wolbachia
(Figures 5.1 and
5.2)
(Heddi
spp. show different behavior in that, in
parallel with vertical inheritance throughout the egg cytoplasm, they have undergone an exten-
sive intertaxon transmission. Moreover,
Wolbachia
transmission by microinjection has been accom-
plished between insect taxa, particularly within the
in vitro
Trichogramma
genus (Grenier et al., 1998).
In the weevils, several populations are infected with
Wolbachia
. Among 23
Sitophilus
stains
belonging to three species (
S. oryzae, S. zeamais, S. granarius
) collected worldwide, 13 are
infected with
Wolbachia
, constituting 9 strains totally infected and 4 strains partially infected
(Heddi
et al., 1999). Cytologically, the distribution of
Wolbachia
exhibits different features from
that of SOPE. Fluorescence
in situ
hybridization (FISH) using speciÝc oligonucleotide probes
