Evidence for phylogenetic congruence among sulfur-oxidizing chemoautotrophic bacterial endosymbionts and their bivalve hosts

Bacterial colonization of the developing light organ of the squid Euprymna scolopes is shown to be highly specific, with the establishment of a successful association resulting only when the juvenile host is exposed to seawater containing one of a subset of Vibrio fischeri strains. Before a symbiotic infection the organ has elaborate epithelial structures covered with cilia and microvilli that are involved in the transfer of bacteria to the incipient symbiotic tissue. These structures regressed within days following infection; however, they were retained in uninfected animals, suggesting that the initiation of symbiosis influences, and is perhaps a prerequisite for, the normal developmental program of the juvenile host.

Aphids (superfamily Aphidoidea) contain eubacterial endosymbionts localized within specialized cells (mycetocytes). The endosymbionts are essential for the survival of the aphid hosts. Sequence analyses of the 16S rRNAs from endosymbionts of 11 aphid species from seven tribes and four families have indicated that the endosymbionts are monophyletic. Furthermore, phylogenetic relationships within the symbiont clade parallel the relationships of the corresponding aphid hosts. Our findings suggest that this endocytobiotic association was established in a common ancestor of the four aphid families with subsequent diversification into the present species of aphids and their endosymbionts.