Organ structure and bacterial microbiogeography in a reproductive organ of the Hawaiian bobtail squid reveal dimensions of a defensive symbiosis

Many plants and animals house symbiotic microorganisms in specialized tissues or organs. Here, we used multidimensionalin-situimaging techniques to illuminate how host organ structure and bacterial microbiogeography contribute to the symbiotic function of an organ in the Hawaiian bobtail squid,Euprymna scolopes. Along with the well-studied light organ, femaleE. scolopesharbor a community of bacteria in the accessory nidamental gland (ANG). The ANG is a dense network of epithelium-lined tubules, some of which are dominated by a single bacterial taxon. These bacteria are deposited into squid eggs, where they defend the developing embryos from harmful biofouling. This study used a combination of imaging techniques to visualize different dimensions of the ANG and its bacterial communities. Imaging entire organs with light sheet microscopy revealed that the ANG is a composite tissue of individual, non-intersecting tubules that each harbor their own bacterial population. The organ is bisected, with tubules converging towards two points in the organ. At these points, tubules empty in a space where bacteria can mix with squid jelly to be deposited onto eggs. Observations of bacterial populations correlated bacterial taxa with cell morphology and show that tubule populations varied: some contained populations of mixed phyla while some tubules contained only one genus of bacteria. Together, these data shed light on how bacterial populations interact within the ANG and how the host uses physical structure to maintain and employ a symbiotic bacterial population in a defensive context.