The draft genome of strain Cpun from biting midges confirms insect are not a monophyletic group and reveals a novel gene family expansion in a symbiont.

BACKGROUND

It is estimated that 13% of arthropod species carry the heritable symbiont . 16S rRNA and gyrB sequence divides this species into at least four groups (A-D), with the A group infecting a range of arthropods, the B group infecting nematode worms, the C group infecting biting midges, and the D group associated with the marine copepod . To date, genome sequence has only been available for strains from groups A and B, impeding general understanding of the evolutionary history of the radiation. We present a draft genome sequence for a C group , motivated both by the paucity of genomic information outside of the A and B group, and the importance of biting midge hosts as arbovirus vectors.

METHODS

We reconstructed the genome of Cpun, a strain from group C that naturally infects , through Illumina sequencing of infected host specimens.

RESULTS

The draft genome presented has high completeness, with BUSCO scores comparable to closed group A genomes. Phylogenomic analysis based on concatenated single copy core proteins do not support from arthropod hosts as a monophyletic group, with nematode strains nested within the two groups infecting arthropod hosts. Analysis of the genome of Cpun revealed expansion of a variety of gene families classically considered important in symbiosis (e.g., ankyrin domain containing genes), and one set-characterized by DUF1703 domains-not previously associated with symbiotic lifestyle. This protein group encodes putative secreted nucleases, and the Cpun genome carried at least 25 widely divergent paralogs, 24 of which shared a common ancestor in the C group. The genome revealed no evidence in support of B vitamin provisioning to its haematophagous host, and indeed suggests may be a net importer of biotin.

DISCUSSION

These data indicate strains of within nematodes cluster within strains found in insects. The draft genome of Cpun further produces new hypotheses as to the interaction of the symbiont with the midge host, in particular the biological role of DUF1703 nuclease proteins that are predicted as being secreted by Cpun. In contrast, the coding content of this genome provides no support for a role for the symbiont in provisioning the host with B vitamins.