Discovery and Description of a Subgenital Bacterial Organ within Female Ant Nest Beetles (Coleoptera: Carabidae: Paussini)

Abstract

In this study, I investigated the female reproductive system in paussine beetles and identified a specialized structure—the subgenital bacterial organ. This is a membranous invagination between abdominal segments VII and VIII, lined with cuticular spikes, and densely populated with bacteria. While previously thought to be part of the primary gonopore, I now recognize this structure as something anatomically separate from the reproductive system. I conclude that it has uniquely evolved to house and sustain bacteria. Where this organ is present, I also see that it is flanked by two conspicuous lateral glands. I hypothesize that these glands produce substances that support a species-specific bacterial community. This organ is not present in all species of the subfamily Paussinae. To understand both the geographic and phylogenetic distribution of this organ we compared female reproductive anatomy across multiple taxa. I found this organ to be unique to and consistently present throughout the tribe Paussini, with a few exceptions. I selected Paussus favieri as our test subject for further investigation of the bacteria within this organ. I used metagenomic sequencing, Scanning Electron Microscopy (SEM), and both traditional and Fluorescent in-Situ Hybridization (FISH) histology to confirm the presence of and visualize bacteria within the subgenital bacterial organ and other reproductive tissues. I corroborated our findings through tests on other species in the genus Paussus. Through this work, I have expanded our understanding of the female reproductive system in Paussinae, and described the previously misattributed subgenital bacterial organ. I have also documented a previously undescribed postabdominal membrane and contextualized its function within the beetle’s postabdominal reproductive system.