Early adaptation to an unusual host in the bean weevil <i>Zabrotes subfasciatus</i> is associated with changes in body size and reproductive physiology

Abstract Host shifts can present unique challenges and exert selective pressures that influence the evolutionary trajectory of phytophagous insect species. In Bruchinae beetles, which are highly specialized feeders on Fabaceae plants, host shifts are often linked to changes in life‐history traits and are typically accompanied by reduced fitness. Since fitness is closely tied to reproductive success, morphophysiological traits such as body size, gonadal development and the expression of vitellogenic genes may serve as indicators of how well a population adapts to a novel host. To test this hypothesis, we conducted a short‐term selection experiment (12 generations) involving the use of an atypical host, Cicer arietinum (chickpea), by the bean weevil Zabrotes subfasciatus (Boheman, 1833) (Chrysomelidae: Bruchinae: Amblycerini). We estimated eight body measures, assessed the development of ovaries and testes and quantified the expression of vitellogenin ( vg ) and vitellogenin receptor ( vgR ) genes in the fat body and ovary of newly emerged individuals (within 24 h) from both selected populations. Our results reveal that the adaptation involves an increase in body size (e.g., the length of the abdomen not covered by elytra in females), which correlates with higher oocyte storage in the oviducts. This morphological change was coupled with a reduction in the size of the germarium in selected females and a decrease in vgR transcription levels. These findings provide valuable insight into the initial phases of phytophagous insect adaptation to an unconventional host, illustrating how such adaptations are accompanied by significant morphophysiological changes that affect crucial life‐history traits and ultimately lead to a decline in fitness.