Gut microbiome insights from 16S rRNA analysis of 17-year periodical cicadas (Hemiptera: Magicicada spp.) Broods II, VI, and X.

Periodical cicadas (Hemiptera: Magicicada) have coevolved with obligate bacteriome-inhabiting microbial symbionts, yet little is known about gut microbial symbiont composition or differences in composition among allochronic Magicicada broods (year classes) which emerge parapatrically or allopatrically in the eastern United States. Here, 16S rRNA amplicon sequencing was performed to determine gut bacterial community profiles of three periodical broods, including II (Connecticut and Virginia, 2013), VI (North Carolina, 2017), and X (Maryland, 2021, and an early emerging nymph collected in Ohio, 2017). Results showed similarities among all nymphal gut microbiomes and between morphologically distinct 17-year Magicicada, namely Magicicada septendecim (Broods II and VI) and 17-year Magicicada cassini (Brood X) providing evidence of a core microbiome, distinct from the microbiome of burrow soil inhabited by the nymphs. Generally, phyla Bacteroidetes [Bacteroidota] (> 50% relative abundance), Actinobacteria [Actinomycetota], or Proteobacteria [Pseudomonadota] represented the core. Acidobacteria and genera Cupriavidus, Mesorhizobium, and Delftia were prevalent in nymphs but less frequent in adults. The primary obligate endosymbiont, Sulcia (Bacteroidetes), was dominant amongst core genera detected. Chryseobacterium were common in Broods VI and X. Chitinophaga, Arthrobacter, and Renibacterium were common in Brood X, and Pedobacter were common to nymphs of Broods II and VI. Further taxonomic assignment of unclassified Alphaproteobacteria sequencing reads allowed for detection of multiple copies of the Hodgkinia 16S rRNA gene, distinguishable as separate operational taxonomic units present simultaneously. As major emergences of the broods examined here occur at 17-year intervals, this study will provide a valuable comparative baseline in this era of a changing climate.

Genome expansion via lineage splitting and genome reduction in the cicada endosymbiont Hodgkinia.

Comparative genomics from mitochondria, plastids, and mutualistic endosymbiotic bacteria has shown that the stable establishment of a bacterium in a host cell results in genome reduction. Although many highly reduced genomes from endosymbiotic bacteria are stable in gene content and genome structure, organelle genomes are sometimes characterized by dramatic structural diversity. Previous results from Candidatus Hodgkinia cicadicola, an endosymbiont of cicadas, revealed that some lineages of this bacterium had split into two new cytologically distinct yet genetically interdependent species. It was hypothesized that the long life cycle of cicadas in part enabled this unusual lineage-splitting event. Here we test this hypothesis by investigating the structure of the Ca. Hodgkinia genome in one of the longest-lived cicadas, Magicicada tredecim. We show that the Ca. Hodgkinia genome from M. tredecim has fragmented into multiple new chromosomes or genomes, with at least some remaining partitioned into discrete cells. We also show that this lineage-splitting process has resulted in a complex of Ca. Hodgkinia genomes that are 1.1-Mb pairs in length when considered together, an almost 10-fold increase in size from the hypothetical single-genome ancestor. These results parallel some examples of genome fragmentation and expansion in organelles, although the mechanisms that give rise to these extreme genome instabilities are likely different.

Sympatric speciation in a bacterial endosymbiont results in two genomes with the functionality of one.

Mutualisms that become evolutionarily stable give rise to organismal interdependencies. Some insects have developed intracellular associations with communities of bacteria, where the interdependencies are manifest in patterns of complementary gene loss and retention among members of the symbiosis. Here, using comparative genomics and microscopy, we show that a three-member symbiotic community has become a four-way assemblage through a novel bacterial lineage-splitting event. In some but not all cicada species of the genus Tettigades, the endosymbiont Candidatus Hodgkinia cicadicola has split into two new cytologically distinct but metabolically interdependent species. Although these new bacterial genomes are partitioned into discrete cell types, the intergenome patterns of gene loss and retention are almost perfectly complementary. These results defy easy classification: they show genomic patterns consistent with those observed after both speciation and whole-genome duplication. We suggest that our results highlight the potential power of nonadaptive forces in shaping organismal complexity.