Post-Embryonic Gonad Development in the Capital-Breeding Seed Beetle Zabrotes subfasciatus (Bruchinae): A...

Post-embryonic gonad development determines when insects acquire reproductive competence, yet this process remains poorly characterized in Bruchinae seed beetles. Here, we examined ovarian and testicular development in the capital-breeding bean weevil Zabrotes subfasciatus, from larval stages to four-day-old adults, using stereomicroscopy, light microscopy, and scanning electron microscopy. Female gonads were already recognizable in larvae as ovaries composed of three ovarioles, each dominated by a proliferative germarial region and posterior duct-like domains. During pharate-pupal and pupal development, these structures remained largely previtellogenic but showed ductal integration associated with future calyx formation. The most extensive ovarian reorganization occurred in pharate adults, when terminal filaments, individualized oocytes, follicular cells, trophic cords, and the germarium-vitellarium transition became evident; acidophilic inclusions consistent with early yolk deposition further indicate that vitellogenesis begins before emergence. Male gonads also differentiated early, with larval testes containing germ stem cells, spermatogonia, spermatocytes, spermatids, cyst cells, and the internal portion of the efferent ducts. From the pharate-pupal stage onward, spermiogenesis intensified, and four-day-old adults displayed a fully organized reproductive tract. Together, these findings show that gonad development begins during larval life but attains functional organization mainly during late metamorphosis, supporting rapid reproductive readiness in this capital-breeding pest.