Bacterial Endosymbionts and Insect Hosts Practice “Supply-Side” Metabolism

Endosymbiotic bacteria within some insects supply their hosts with essential nutrients that enable those hosts to live in specialized environmental niches. In particular, blood-feeders such as bedbugs and tsetse flies, and plant-sap feeders such as pea aphids, citrus mealybugs, and cicadas rely heavily on their microbial symbiotic partners for B vitamins and amino acids that are otherwise lacking from their food sources. The symbionts encode enzymes and steps inmetabolic pathways that produce the missing nutrients that their hosts cannot make themselves. “Endosymbionts provide novel biochemistry and metabolic traits to host insects that allows insects to exploit otherwise inaccessible niches,” says Alex Wilson of the University of Miami, Fla. Such symbioses are so critical that insect hosts have developed specialized cells and organs to house their symbionts (Fig. 1). Located within the host abdomen, bacteriomes are a specialized insect tissue consisting of cells called bacteriocytes that houses their microbial occupants, which, in turn, support the metabolic needs of the insect hosts. Bacteriocytes help their occupants by supplying them with metabolic precursors, transport functions, and by meeting other needs. Some consider this relationship to be very similar to that between other cells and organelles. These intimately associated insect-microbial partnerships co-evolved over tens of thousands of years or more, with each participant shaping the other’s genome, metabolic capabilities, and evolutionary trajectories. “Obligate symbiotic relationships are more accurately composed of interdependent parts of an intimately coevolving whole, the holobiont,” says Wilson (see p. 27). She and others discussed the intricacies of these symbiotic partnerships at the 2015 ASM General Meeting, held in New Orleans last May.