A proximate mechanism for evolutionary trade-offs in the bean beetle <i>Callosobruchus maculatus</i>

Abstract The mechanisms underlying resource allocation and covariances in life history traits are at the core of evolutionary biology. Whilst previous studies have detailed multiple mechanisms for the differential allocation of resources towards life history traits, few studies have tested the contributions of these mechanisms towards both resource allocation and life history trait covariance. To address this, we explored the role of Juvenile Hormone (JH) in modulating resource allocation and trait correlations in the bean beetle Callosobruchus maculatus. We hypothesized that JH plays a critical role in determining the differential resource allocation to dispersal traits and gonads. To test our hypotheses, we manipulated JH levels in developing larvae using methoprene (a synthetic JH analogue) and precocene (a JH synthesis inhibitor). We found support for our hypothesis as decreased JH generated individuals with larger wings and smaller gonads relative to the control group. We found consistent negative covariances between reproductive and dispersal traits across treatments, with JH shifting individuals along this trade-off axis without altering the relationship. Notably, these effects were sex-specific: in males, wing size was shaped by both testes size and JH treatment, whereas in females, wing size was related to ovariole size but unaffected by JH manipulation. Interestingly, we found no significant effect of increased JH on wing and gonad size. These findings provide direct evidence of JH’s involvement in resource allocation and trait covariances in bean beetles.