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possible that secondary endosymbionts may succeed the one formerly established as a consequence
of a switch in the insect environment.
Symbiogenesis is a fundamental biological process that has accompanied cell evolution from
the beginning of life and continues today. Indeed, similar genome reduction and specialization are
found in bacteriocyte-associated symbionts of insects.
(the endosymbiont of aphids)
has reduced its genome seven- to tenfold during 250 million years of coevolution with aphids
(Charles and Ishikawa, 1999; Gil et al., 2002), and
B. aphidicola
spp. (the endosymbionts of the
tsetse Þy) have had almost as much genome reduction during their 80 million years of evolution
(Akman and Aksoy, 2001). In both, bacterial genome specialization seems to correspond with the
composition of the insect diet. This phenomenon is strongly suggested with
Wigglesworthia
, where
the genome has lost most genes encoding for nonessential amino acids and the Krebs cycle
(Shigenobu
B. aphidicola
has kept genes for the biosyntheses of amino
acids essential for the aphid hosts, indicating that hostÏsymbiont complementarity and synthrophy
have helped selection to remove redundant functions from the prokaryotic genome and may favor
the maintenance of the functions adaptive for the unit.
Hence, regardless of the scenario by which serial genomes have become established in the
association, symbiogenesis creates the possibility for life to diversify through the colonization of
new and empty biotopes. For instance, aerobic constraints selected for the acquisition of
et al., 2000). However,
B. aphidicola
-
Proteobacteria by early eukaryotic host cells around 2 billion years ago, permitting the diversiÝ-
cation of Eukarya. One billion years later, animals and plants diverged when cyanobacteria invaded
the ancestor protist (Doolittle, 1997).
In
b
weevils, the SOPE did not reduce its genome size as much, perhaps because of
the relative recent age of the symbiosis in this model. However, from a symbiogenesis point of
view, at least four genomes have integrated serially into the weevil cells in the course of evolution:
the nucleus, mitochondria, SOPE, and
Sitophilus
et al., 1999). Each of them through the
interaction with the others has separate functions with regard to the biology of the association. The
most striking conclusion is that insect evolution should be considered not only the consequence of
the classical Ñeukaryotic systemÒ balance between mutation and selection but also the result of
selective constraints that have been exerted on the whole association.
Wolbachia
(Heddi
ACKNOWLEDGMENTS
I thank M. McFall-Ngai, C. Vieira, and P. Nardon for critical comments on the manuscript;
H. Charles for the phylogenetic tree construction; and C. Khatchadourian for assistance.
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