Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Automated summary

Insect host-parasitoid interactions are common in nature and characterized by host specialization. Many host species rely on heritable bacterial endosymbionts for defense against parasitoids. These symbionts provide strong and specific resistance, potentially due to variation in symbiont-produced toxins. Defensive symbionts are thus important for diversifying selection and may alter host-parasitoid food web structures. Additionally, other defensive mutualisms may also exert diversifying selection on insect parasitoids.