The defensive Spiroplasma

Abstract

Defensive microbes are of great interest for their roles in arthropod health, disease transmission, and biocontrol efforts. Obligate bacterial passengers of arthropods, such as Spiroplasma, confer protection against the natural enemies of their hosts to improve their own fitness. Although known for less than a decade, Spiroplasma's defensive reach extends to diverse parasites, both microbial and multicellular. We provide an overview of known defensive phenotypes against nematodes, parasitoid wasps, and fungi, and highlight recent studies supporting the role of Spiroplasma-encoded ribosome-inactivating proteins in protection. With cellular features well-suited for life in the hemolymph, broad distribution among invertebrate hosts, and the capacity to repeatedly evolve vertical transmission, Spiroplasma may be uniquely equipped to form intimate, defensive associations to combat extracellular parasites. Along with insights into defensive mechanisms, recent significant advances have been made in male-killing -a phenotype with interesting evolutionary ties to defense. Finally, we look forward to an exciting decade using the genetic tools of Drosophila, and the rapidly-advancing tractability of Spiroplasma itself, to better understand mechanisms and evolution in defensive symbiosis.

Whitefly genomes contain ribotoxin coding genes acquired from plants

“…Moreover, we have shown that, in Aedes aegypti , these RIP genes are transcribed and their expression levels are modulated across the developmental stages 15 . Recently, evidence has been accumulated on the role of Spiroplasma spp RIPs in the protective mechanisms generated by these endosymbiotic species against arthropod infection by natural enemies (see 16 for a review).…”

Evolutionary costs and benefits of infection with diverse strains of Spiroplasma in pea aphids*

“…). Similar to other heritable endosymbionts, some Spiroplasma have evolved the ability to defend their hosts against other infections (Ballinger and Perlman ). For example, the male killing strain MSRO of S. poulsonii protects Drosophila melanogaster against parasitoid wasps (Xie et al.…”

“…It is estimated that between 5% and 10% of insects carry heritable infections with Spiroplasma (Duron et al 2008). Similar to other heritable endosymbionts, some Spiroplasma have evolved the ability to defend their hosts against other infections (Ballinger and Perlman 2018). For example, the male killing strain MSRO of S. poulsonii protects Drosophila melanogaster against parasitoid wasps (Xie et al 2014;Paredes et al 2016), illustrating that reproductive manipulation and protection are not mutually exclusive strategies of symbionts to spread in host populations.…”

Recurrent acquisition of alien ribotoxin encoding genes by insect genomes

“…Our research also evidenced that these genes are derived from a single HGT event from a bacterial donor species [14]. Recently, evidence has been accumulated on the role of Spiroplasma spp RIPs in the protective mechanisms generated by these endosymbiotic species against arthropod infection by natural enemies (see [15] for a review).…”