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Résumé :
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Insects that feed on plant sap depend on symbiotic bacteria for nutrients that are not present in the diet. These bacteria live within host cells and are transmitted from mother to offspring. The symbionts of aphids, in the genus Buchnera, are the best characterized of insect endosymbionts. They result from an ancient infection of a common ancestor of modem aphids,;Ind they possess genes underlying pathways for production of essential amino acids that are rare in the phloem sap diet of aphids. Thus these bacteria appear to be highly coadapted with hosts and to have evolved as mutualists for millions of years. Nonetheless, Buchnera and other endosymbionts possess some genetic traits that appear to be deleterious to both hosts and, ultimately, the bacterial symbionts themselves. The most likely basis for these traits is the fixation of slightly deleterious mutations in the context of genetic drift in these bacterial populations, which possess small genetic population sizes relative to many free-living bacteria. Investigations during the next few years will reveal the: extent of convergence among independently derived symbionts of different insect families and the extent to which symbionts are adapted to species-specific aspects of their host's ecology and nutrition.
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