Endosymbiont load dictates reproductive fate: Experimental validation for the bacterial dosage model in a parthenogenetic weevil (Coleoptera, Curculionidae)

In many arthropods, reproductive manipulations induced by maternally inherited symbionts appear to depend upon surpassing a bacterial density threshold. The Naupactini tribe harbours a diverse array of Neotropical weevils, many of which exhibit parthenogenetic reproduction linked to Wolbachia pipientis and Rickettsia sp., with parthenogenetic species typically displaying high Wolbachia and Rickettsia sp. densities, and sexually reproducing species characterized by either low density or absence of infections altogether. The main focus of this work was testing the bacterial dosage model, that is, that a threshold density of Wolbachia and/or Rickettsia sp. is required for parthenogenetic reproduction. Through tetracycline-curing experiments coupled with bacterial density quantification by real-time PCR, we analysed clutch viability in Pantomorus postfasciatus as a function of Wolbachia and Rickettsia sp. density. Clutch size was not affected by the antibiotic treatment, but clutch viability (proportion of eggs hatched) declined to zero, coinciding with a significant reduction in bacterial densities without complete clearance. Fitting a three-parameter log-logistic (Hill-type) model revealed a sigmoidal relationship between bacterial density and clutch viability, demonstrating a quantitative, dosage-dependent effect. In contrast, clutch viability of sexually reproducing females was unaffected by antibiotic treatment, confirming that the reduced egg hatching in parthenogenetic females results from symbiont depletion rather than direct drug effects. Additionally, Rickettsia sp. proved to be more susceptible to tetracycline than Wolbachia, while Wolbachia densities decreased more markedly in reproductive tissues-a pattern potentially linked to the observed collapse in clutch viability. This work highlights bacterial load as a key determinant of parthenogenetic reproduction within the Naupactini.