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bacteria in symbiotic insects. The two other proteins (h! and e) are from the bacteria; h! has not
yet been characterized, and e was shown to be a chaperonin (GroEL) (Charles
et al., 1995).
Ex
vivo
protein labeling showed that the GroEL protein was prominent among the SOPE-produced
proteins and could represent up to 40% of the total neosynthesized symbiont proteins. In contrast,
GroEL expression diminishes when SOPE is incubated
in vitro
, indicating that the
GroEL
gene
may play an important role in the association, and its regulation may be under the control of
bacteriocyte cytosolic parameters.
Currently, although many symbiotic teams have been studying the
GroEL
gene (Ishikawa, 1985;
Aksoy, 1995; Jeon, 1995), the gene function has not been clearly elucidated in relation to intra-
cellular symbiotic conditions. Nevertheless, considerating that most endosymbionts have reduced
their genome considerably in the course of their intracellular life history and consequently lack
several ORFs, one possible
GroEL
gene function could be that of protein folding and unfolding
during the shuttling of protein between bacteriocytes and bacteria. Regardless of the exact function
of these proteins in symbiotic systems, their differential expression proÝles suggest the existence
of hostÏsymbiont molecular interactions.
CONCLUSION
Symbiotic associations are widespread through the biotic world, and they occur at different levels
of organismal complexity, ranging from bacterial associations to animal and plant symbioses. The
newly formed unit involves at least two separate species and may include several organisms
belonging to distant phyla (Fukatsu and Ishikawa, 1996; Fukatsu, 2001).
In insects, symbionts can occur as fungi, protists, or bacteria that exhibit extraorganismal
relationships with ectosymbiosis or intraorganismal associations with endosymbiosis. The latter
could be extracellular, such as in the gutÏlumen endosymbioses, or intracellular, such as in insects
that feed permanently on a nutritionally poor and unbalanced diet. The early steps in symbiogenesis
are often believed to be harmful (or mildly harmful), during which time the partners exchange
virulence gene products, leading to population selection (Paillot, 1933; Jeon, 1983). When mutual
genome complementarity occurs establishing new adaptive traits on the interactive unit, and when
constraint pressures exerted on the association do not completely eliminate the populations, viru-
lence genes can be silenced (through expression or gene deletion) or at least controlled by the
association favoring hostÏsymbiont coadaptation. Insects may have selected the bacteriome organ
for the control of the symbiont population.
A successful coadaptation often leads to symbiontÏhost coevolution (Baumann
et al., 1995),
where the symbiont Ýnds a way to transmit to the offspring permanently. In insect bacteriocyte-
inducing symbioses, symbionts are always transmitted through the female germ cells, where they
persist, as in the weevils, or they infect the oocytes during the vitellogenesis process, as in
cockroaches, bugs, and aphids. This restricted way of symbiont transmission, along with particular
selection pressures exerted by the intracellular environment, appears to result in different selection
pressures on microorganism evolution. Symbionts are no longer able to recombine (or can recom-
bine only indirectly) with extracellular microorganisms; their DNA undergoes directional mutation
bias toward A+T, and their genomes reduce in size either through gene-fragment loss or gene
transfer to the host chromosomes. Early DNA rearrangements lead to bacterial domestication by
the host. From this symbiogenesis phase, intracellular bacteria start losing the ability to grow and
divide outside the association until their genomes reach a critical gene composition where the
bacteria become dependent on the host. The ultimate case of this progression is the organogenetic
symbioses of mitochondria, plastids, and hydrogenosomes (Gupta
et al., 1989; Palmer, 1997; Sogin,
1997; Cavalier-Smith, 1999; Gray, 1999; Doolittle, 2000), where not only are genome sizes dras-
tically reduced but also gene composition for a given organelle exhibits functional specialization.
However, it should be emphasized that not all symbioses end in organogenetic relationships.
Secondarily integrated symbioses occur very frequently in insects (Fukatsu
et al., 2000), and it is
