Intraspecific microbiome dynamics across the life cycle of the milkweed bug Oncopeltus fasciatus

The microbiome is an important part of the complete nutritional and genomic profile of insects. The species-rich insect order Hemiptera (aphids, cicadas and true bugs) is highly diverse for mode of microbiome acquisition, with the conundrum that species in the seed-feeding subfamily Lygaeinae have lost obvious anatomy for housing bacteria, either in bacteriocytes or midgut crypts. Here, we characterize the microbiome of the milkweed bug <i>Oncopeltus fasciatus</i> as a tractable lygaeinid, using <i>16S rRNA</i> gene sequencing. We assess how bacterial taxa vary between the sexes and across life history stages in a controlled environment, focusing on maternal-to-embryo transmission and distinguishing egg-stage constituents that are superficial or internal (transovarially transmitted). Among a core microbiome of 28 genera, the egg stage shows the greatest diversity, with a particular expansion of the family <i>Comamonadaceae</i>. We also analyse inter-individual variability in nymphs and adults and validate structured, stage-specific detection of seed material. Comparative analysis identifies <i>Rhizobium</i> as a notable microbiome constituent in seed-feeding Hemiptera, which we had previously shown to lack nitrogen metabolism components in the genome. Overall, we provide a nuanced assessment of bacterial abundance dynamics between individuals and across the life cycle and discuss the implications for acquisition and potential relevance as nutritional endosymbionts. This will underpin comparative investigations in seed-feeding bugs and future work in <i>O. fasciatus</i> on tissue-specific and diet-specific microbiome profiles, including in natural populations.