Parthenogenesis in weevils of the tribe Naupactini (Coleoptera, Curculionidae): a Wolbachia-density dependent trait?

The intracellular bacteria Wolbachia pipientis can manipulate host reproduction to enhance their vertical transmission. It has been reported an association between parthenogenesis and Wolbachia infection in weevils from the tribe Naupactini. A curing experiment suggested that a threshold density of Wolbachia is required for parthenogenesis to occur. The aim of this study was to analyze Wolbachia infection status in the bisexual species Naupactus xanthographus and Naupactus dissimulator. Wolbachia infection was detected in both species from some geographic locations, not being fixed. In all positive cases, faint PCR bands were observed. Quantification through real time PCR confirmed that Wolbachia loads in bisexual species were significantly lower than in parthenogenetic ones; this strengthens the hypothesis of a threshold level. Strain typing showed that both species carry wNau1, the most frequent in parthenogenetic Naupactini weevils. These infections seem to be recently acquired by horizontal transfer. Wolbachia was located throughout the whole body, which reinforce the idea of recent transmission. Moreover, we demonstrated that this strain carries the WO phage. Finally, the analysis of eubacterial 16S rRNA gene showed intense PCR bands for both bisexual species, suggesting -the presence of additional bacteria. Interspecific competition might explain why the parthenogenetic phenotype is not triggered.