Extracellular symbiont colonizes insect during embryo development

Intracellularity, extracellularity, and squeezing in the symbiotic organ underpin nurturing and functioning of bacterial symbiont in leaf beetles

Cassidine leaf beetles are associated with genome-reduced symbiotic bacteria Stammera involved in pectin digestion. Stammera cells appear to be harbored in paired symbiotic organs located at the foregut-midgut junction either intracellularly or extracellularly, whereas the symbiont is extracellular in the ovary-accessory glands of adult females and during caplet transmission in eggs. However, using fluorescence and electron microscopy, an intracellular symbiotic configuration of Stammera was observed in Notosacantha species. Detailed inspection of other cassidine species revealed fragmented cell membrane and cytoplasm of the symbiotic organs, wherein Stammera cells are in an intermediate status between intracellularity and extracellularity. We also identified a mitochondria-rich region adjacent to the symbiont-filled region and well-developed muscle fibers surrounding the whole symbiotic organ. Based on these observations, we discuss why the Stammera genome has been reduced so drastically and how symbiont-derived pectinases are produced and supplied to the host's alimentary tract for plant cell wall digestion.

Paleocene origin of a streamlined digestive symbiosis in leaf beetles

Timing the acquisition of a beneficial microbe relative to the evolutionary history of its host can shed light on the adaptive impact of a partnership. Here, we investigated the onset and molecular evolution of an obligate symbiosis between Cassidinae leaf beetles and Candidatus Stammera capleta, a γ-proteobacterium. Residing extracellularly within foregut symbiotic organs, Stammera upgrades the digestive physiology of its host by supplementing plant cell wall-degrading enzymes. We observe that Stammera is a shared symbiont across tortoise and hispine beetles that collectively comprise the Cassidinae subfamily, despite differences in their folivorous habits. In contrast to its transcriptional profile during vertical transmission, Stammera elevates the expression of genes encoding digestive enzymes while in the foregut symbiotic organs, matching the nutritional requirements of its host. Despite the widespread distribution of Stammera across Cassidinae beetles, symbiont acquisition during the Paleocene (∼62 mya) did not coincide with the origin of the subfamily. Early diverging lineages lack the symbiont and the specialized organs that house it. Reconstructing the ancestral state of host-beneficial factors revealed that Stammera encoded three digestive enzymes at the onset of symbiosis, including polygalacturonase-a pectinase that is universally shared. Although non-symbiotic cassidines encode polygalacturonase endogenously, their repertoire of plant cell wall-degrading enzymes is more limited compared with symbiotic beetles supplemented with digestive enzymes from Stammera. Highlighting the potential impact of a symbiotic condition and an upgraded metabolic potential, Stammera-harboring beetles exploit a greater variety of plants and are more speciose compared with non-symbiotic members of the Cassidinae.