Evolution of Expending Extra Effort in Making a Dung Mass before Making a Brood Ball in the Nesting Behavior of the Female Dung Beetle ICopris acutidens/I

Nutrient limitations have often caused the evolution of mechanisms for efficient nutrient acquisition. The mouthparts of adult dung beetles efficiently acquire nutrients from a fiber-rich diet. Conversely, primitive mouthparts force larvae to survive on a low-quality diet despite experiencing the most demanding growth stages. In this study, we investigated the nutritional conditions and microbial community of the larval diet through the nesting behavior of the dung beetle Copris acutidens. We revealed that diet quality (C/N ratio) increased during the process of making the brood ball, irrespective of dung type. The sequencing of the bacterial community based on a partial 16S rRNA gene and the fungal community that targeted ITS2 region revealed that the fungal community in the female gut was the closest to the larval diet, whereas the bacterial community was not. The proportion of fungal Trichosporonaceae tended to increase with a decreasing C/N ratio irrespective of dung type and was alive in the larval gut. We suggest that Trichosporonaceae is a gut symbiont of both the adult female and larvae of C. acutidens, which is transmitted to the dung mass and then to larval gut through the brood ball, and that females have evolved the extra effort processes in their nesting behavior to compensate for larval diet quality, which is likely associated with symbiont fungi within the family Trichosporonaceae.