Speaker: Piotr Lukasik (University of Montana)
Cicadas, like other sap-feeding hemipteran insects, depend for the production of essential amino acids and vitamins on specialized heritable microorganisms that inhabit their tissues. Two such endosymbiotic bacteria have co-diversified with cicadas for some 200 million years. I will explain how one of them, Hodgkinia, has repeatedly experienced a dramatic change in the genomic organization and function, with no known parallel outside of cicadas. In many cicada clades, this symbiont has fragmented into complexes comprising multiple genetically and cytologically distinct lineages. Individual Hodgkinia lineages may encode fewer than 20 total genes, but across all lineages present in a complex, nearly the complete set of the ancestral genes is present. This implies that the lineages rely on each other for the basic cellular functions, but also complement each other in producing the same nutrients as the single ancestral symbiont. I argue that the symbiont fragmentation is detrimental to the cicadas, decreasing biosynthetic efficiency and forcing adaptations to the maintenance and transmission of the symbiosis that are likely costly. This becomes more of a problem as the complexes fragment further over time, and these degenerative processes may ultimately lead to the extinction of the host lines. At least, unless the inefficient, degraded symbiont gets replaced by another microorganism and the degenerative cycle restarts.