Genus of bacteria
Arsenophonus is a genus of Morganellaceae, of the Gammaproteobacteria. Members of the "Arsenophonus" genus are increasingly discovered bacterial symbionts of arthropods that are estimated to infect over 5% of arthropod species globally and form a variety of relationships with hosts across the mutualism parasitism continuum. "Arsenophonus" bacteria have been identified in a diversity of insect taxa, including economically important species such as the Western honey bee and the rice pest "Nilaparvata lugens".
The majority of work on "Arsenophonus" has been done on the type species "Arsenophonus nasoniae" for which genetic manipulation has been successful in achieving "in vivo" tracking of the bacterium. "Arsenophonus nasoniae" infects "Nasonia" parasitic wasps, is vertically transmitted, passed from a female wasp to the fly host during parasitisation, and then acquired by her hatching larvae feeding on the microbe . It has a male-killing phenotype. Infection with "Arsenophonus nasoniae" triggers the death of approximately 80% of the wasps male offspring. Killing male offspring is thought to facilitate the spread of "Arsenophonus" through the host population as it releases more resources to female offspring, and it is the female line that "Arsenophonus" is transmitted through.. However, horizontal transmission during superparasitism of a single fly pupae by multiple wasp females is required for symbiont spread .
Within the genus a number of "Arsenophonus" strains have known roles as mutualistic endosymbionts. In both "Pediculus humanus" and "Lipoptena cervi" "Arsenophonus" symbionts are essential to host functioning and are involved in vitamin synthesis, and are vertically transmitted across host generations. In other hosts "Arsenophonus" is suspected to be parasitic. In the Western honey bee "Arsenophonus" can be horizontally transmitted via social behaviour, and the presence of "Arsenophonus" in a colony has been linked to poor bee health. This species has been formally described as "Arsenophonus apicola". The majority of associations between "Arsenophonus" and host taxa remain uncharacterized.
The diversity of interactions between "Arsenophonus" and insects has led to the genus being adopted, alongside "Sodalis", as one where it is possible to investigate the genetic and evolutionary changes associated with different types of symbiosis. Siozios et al demonstrated early stages of transition to vertical (parent-offspring) transmission were associated with increases in "Arsenophonus" genome size, associated with proliferation of prophage. This was considered a consequence of loss of CRISPR-Cas defences, hypothesized to be driven by reduce phage predation rates in strongly host associated endosymbionts. Indeed, "Arsenophonus nasoniae" has one of the more complex microbial genomes, carrying over 50 prophage elements and 17 plasmids.
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