Host-associated microbial diversity in New Zealand cicadas uncovers elevational structure and replacement of obligate bacterial endosymbionts by Ophiocordyceps fungal pathogens

Host-microbe interactions influence eukaryotic evolution, particularly in the sap-sucking insects that often rely on obligate microbial symbionts to provision deficient nutrients in their diets. Cicadas (Hemiptera: Auchenorrhyncha: Cicadidae) specialize on xylem fluid and derive many essential amino acids and vitamins from intracellular bacteria or fungi (Hodgkinia, Sulcia, and Ophiocordyceps) that are propagated via transmission from mothers to offspring. Despite the beneficial role of these symbionts in nutrient provisioning, they are generally not considered to function within the gut where microbiota may play an important dietary role during insect diversification. Here, we investigate the relative impact of host phylogeny and ecology on gut microbial diversity in cicadas by sequencing 16S ribosomal RNA gene amplicons from 197 wild-collected cicadas and new mitochondrial genomes across 38 New Zealand cicada species, including natural hybrids between one species pair. We find a lack of phylogenetic structure and hybrid effects but a significant role of elevation in explaining variation in gut microbiota. Additionally, we provide evidence of Hodgkinia loss with gains of Ophiocordyceps fungal pathogens in all New Zealand cicadas examined that suggests convergent domestications of fungal pathogens. This highlights the macroevolutionary instability of obligate symbiosis and the relative importance of ecology rather than phylogeny for structuring gut microbial diversity in cicadas. ImportanceAn unresolved question in evolutionary biology is how beneficial associations between eukaryotes and microbes impact macroevolutionary patterns. We report substantial data from natural populations that suggest the absence of macroevolutionary impacts from gut microbiota in cicadas. Instead, gut microbial diversity is better explained by elevational variation across an island landscape. Cicadas, like many insects, have obligate nutritional associations with bacteria housed in organs outside of the gut, but we show that these associations seem also to be unstable at macroevolutionary scales. We report evidence for unexpected and widespread replacement of obligate bacteria by a domesticated and formerly pathogenic Ophiocordyceps fungus representing an evolutionarily convergent pattern across the cicada phylogeny.