No Transcriptional Compensation for Extreme Gene Dosage Imbalance in Fragmented Bacterial Endosymbionts of Cicadas

Bacteria that form long-term intracellular associations with host cells lose many genes, a process that often results in tiny, gene-dense, and stable genomes. Paradoxically, the same evolutionary processes that drive genome reduction and simplification may also sometimes cause genome expansion and complexification. A bacterial endosymbiont of cicadas, Hodgkinia cicadicola , exemplifies this paradox. In many cicada species, a single Hodgkinia lineage with a tiny, gene-dense genome has split into several interdependent cell and genome lineages. Each new Hodgkinia lineage encodes a unique subset of the ancestral unsplit genome in a complementary way, such that the collective gene contents of all lineages sum to the total found in the ancestral single genome. This splitting process creates genetically distinct Hodgkinia cells that must function together to carry out basic cellular processes, but also creates a gene dosage problem where some genes are encoded by only a small fraction of cells while others are much more abundant. Here, by sequencing DNA and RNA of Hodgkinia from different cicada species with different amounts of splitting – along with those of its structurally stable, unsplit partner endosymbiont Sulcia muelleri – we show that Hodgkinia does not transcriptionally compensate to rescue the wildly unbalanced gene and genome ratios that result from lineage splitting. We also find evidence that Hodgkinia has a reduced capacity for basic transcriptional control independent of the splitting process. Collectively, these findings reveal another layer of degeneration that further pushes the limits of canonical molecular and cell biology in Hodgkinia , and may in part explain its propensity to go extinct through symbiont replacement. Significance Many cicadas host two bacterial endosymbionts, Hodgkinia and Sulcia , which produce essential amino acids missing from the insect’s xylem sap diet. Following 100+ million years of strict host association, both bacteria have lost many genes and posses extremely tiny genomes. In some cicadas, Hodgkinia has split into multiple cell lineages, distributing its genes, with little respect to their function, among separate lineages present at (sometimes wildly) different abundances. We find no transcriptional response to this in Hodgkinia , resulting in similarly imbalanced mRNA abundances. We also find less control of transcription in Hodgkinia compared to Sulcia . Hodgkinia ’s transcriptome embodies an extreme, even relative to other highly-reduced endosymbionts, and raises questions about how cell biology in multi-lineage Hodgkinia can function at all.