Evolution of an alternative genetic code in theProvidenciasymbiont of the haematophagous leechHaementeria acuecueyetzin

Abstract

ABSTRACTStrict blood-feeding animals are confronted with a strong B vitamin deficiency. Blood-feeding leeches from the Glossiphoniidae family, similarly to haematophagous insects, have evolved specialised organs called bacteriomes to harbour symbiotic bacteria. Leeches of theHaementeriagenus have two pairs of globular bacteriomes attached to the oesophagus which house intracellular ’CandidatusProvidencia siddallii’ bacteria. Previous work analysing a draft genome of theProvidenciasymbiont of the Mexican leechHaementeria officinalisshowed that, in this species, the bacteria hold a reduced genome capable of synthesising B vitamins. In this work, we aimed to expand our knowledge on the diversity and evolution ofProvidenciasymbionts ofHaementeria. For this purpose, we sequenced the symbiont genomes of three selected leech species. We found that all genomes are highly syntenic and have kept a stable genetic repertoire, mirroring ancient insect endosymbionts. Additionally, we found B vitamin pathways to be conserved among these symbionts, pointing to a conserved symbiotic role. Lastly and most notably, we found that the symbiont ofHaementeria acuecueyetzinhas evolved an alternative genetic code, affecting a portion of its proteome and showing evidence of a lineage-specific and likely intermediate stage of genetic code reassignment.