Understanding the cost of symbiosis in a Beetle–Fungal symbiosis model: the cigarette beetle Lasioderma serricorne and its symbiont Symbiotaphrina kochii

Research on insect symbiosis is dominated by insect–bacteria models, however, the role of fungal symbionts in developing insect hosts is poorly understood. Fungi are de novo synthesizers of essential nutrients, such as B vitamins, sterols, and amino acids, therefore, reciprocal pairing between a host and its microbial symbionts can permit it to thrive on diets devoid of essential nutrients. Here, we address the progression of fungal symbiosis using a system of beetle–fungal symbiosis: the cigarette beetle (Lasioderma serricorne) with its symbiont Symbiotaphrina kochii. The beetle fungal–symbiosis model provides a powerful means to investigate insect–fungal relationships for the following reasons: the symbiont is both intracellular and extracellular, beetles can be reared without their symbionts, and the beetles can be reinfected to create controlled symbiotic pairings. Here, we addressed the following questions: 1) Is the conferred fitness benefit of S. kochii to the cigarette beetle only present in low-quality diets, 2) what is the relationship between the beetle’s developmental stage and the number of viable symbionts, and 3) when reared on a low-quality diet, does the beetle increase the number of viable symbionts to compensate for a heightened nutritional demand? Preliminary findings reveal no cost to hosting a symbiont when the diet quality is high. Next, we find differences in the number of viable symbionts in larvae, pupae, and adults; however, symbiont growth is not different between beetles reared on low-quality versus high-quality diets.