First published: 23 February 2026.
A new paper, published in RES journal Insect Molecular Biology, explores how Wolbachia impacts parthenogenetic weevils: “Endosymbiont load dictates reproductive fate: Experimental validation for the bacterial dosage model in a parthenogenetic weevil (Coleoptera, Curculionidae)”
Reproductive manipulation by endosymbiotic bacteria such as Wolbachia is usually described in binary terms: hosts are either infected or not. However, for years many researchers have suspected that this view is too simplistic, and that bacterial density, rather than mere presence, plays a key role in determining host reproductive outcomes. This idea, known as the dosage model, was widely accepted but had not been tested explicitly.
Image: Marcela Silvina Rodriguero and Lucía Fernandez Goya viewing a microscope.
Photo Credit: Diego Caraballo
The authors’ interest in this question emerged from unexpected experimental results. In a classic curing experiment using parthenogenetic weevils, they found that antibiotic treatment caused sterility without fully eliminating Wolbachia.
Image: Weevil on a leaf
Photo Credit: Lucía Fernandez Goya
This suggested that reducing bacterial load below a critical threshold might disrupt reproductive manipulation. Further work revealed an additional surprise: a sexually reproducing weevil species, previously considered uninfected, carried Wolbachia at extremely low densities—undetectable by conventional PCR, but evident using quantitative approaches.
Image: Eggs
Photo Credit: Lucía Fernandez Goya
These observations led the authors to directly test whether a threshold of endosymbiont density is required to induce parthenogenesis. By quantifying bacterial loads, they demonstrated that both Wolbachia and Rickettsia must exceed specific density thresholds to trigger parthenogenetic reproduction in Pantomorus postfasciatus.
Their results show that reproductive manipulation is not a simple on–off process, but a quantitative phenomenon.
Image: Marcela Silvina Rodriguero and Lucía Fernandez Goya
Photo Credit: Diego Caraballo
In short, they tested the bacterial dosage model for two endosymbionts, Wolbachia and Rickettsia, in the induction of parthenogenetic reproduction. Their work highlights bacterial load—not mere presence or absence—as a key determinant of parthenogenesis in Naupactini weevils. They demonstrated what many had long suspected: it is not about having Wolbachia or not—it is about how much Wolbachia you have. Quantity matters.
“Endosymbionts do not act as switches. They act as dimmers – and in evolution, quantity often matters more than presence.”
– Marcela Silvina Rodriguero
Image: Marcela Silvina Rodriguero and Lucía Fernandez Goya
Photo Credit: Diego Caraballo
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