Blind killing of both male and female Drosophila embryos by a natural variant of the endosymbiotic bacterium Spiroplasma poulsonii

Abstract

Spiroplasma poulsonii is a vertically transmitted endosymbiont of Drosophila melanogaster that causes male‐killing, that is the death of infected male embryos during embryogenesis. Here, we report a natural variant of S. poulsonii that is efficiently vertically transmitted yet does not selectively kill males, but kills rather a subset of all embryos regardless of their sex, a phenotype we call ‘blind‐killing’. We show that the natural plasmid of S. poulsonii has an altered structure: Spaid, the gene coding for the male‐killing toxin, is deleted in the blind‐killing strain, confirming its function as a male‐killing factor. Then we further investigate several hypotheses that could explain the sex‐independent toxicity of this new strain on host embryos. As the second non‐male‐killing variant isolated from a male‐killing original population, this new strain raises questions on how male‐killing is maintained or lost in fly populations. As a natural knock‐out of Spaid, which is unachievable yet by genetic engineering approaches, this variant also represents a valuable tool for further investigations on the male‐killing mechanism.

Rapid molecular evolution of Spiroplasma symbionts of Drosophila

“…From a practical perspective, S. poulsonii has many features of a desirable model for symbiont–host interactions: fast rates of evolution make it more likely that adaptation and spontaneous changes in phenotypes can be determined over short time scales, as has been observed previously [32, 35, 36]. However, fast evolutionary changes make experiments less predictable, and because stochastic effects become more pronounced, links of genomic changes with phenotypes may be obscured.…”

“… Spiroplasma -induced phenotypes have been commonly observed to be dynamic compared with other symbiont traits, with repeated observation of phenotype change within laboratory culture over relatively short time frames. For example, spontaneous loss of male killing was found in Spiroplasma symbionts of Drosophila nebulosa [33] and Drosophila willistoni [34], and spontaneous emergence of non-male-killing Spiroplasma in D. melanogaster has occurred at least twice in a single culture [32, 35]. In addition, Spiroplasma symbionts artificially transferred from their native host Drosophila hydei into D. melanogaster initially caused pathogenesis, but evolved to become benign over just a few host generations [36].…”

Rapid molecular evolution ofSpiroplasmasymbionts ofDrosophila

“…From a practical perspective, Spiroplasma poulsonii has many features of a desirable model for symbiont-host interactions: fast rates of evolution make it more likely that adaptation and spontaneous changes in phenotypes can be determined over short time scales, as has been observed previously (Harumoto and Lemaitre, 2018;Masson et al, 2020;Nakayama et al, 2015). On the other hand, fast evolutionary changes make experiments less predictable, and because stochastic effects become more pronounced, links of genomic changes with phenotypes may be obscured.…”

“…Spiroplasma induced phenotypes were commonly observed to be dynamic compared with other symbiont traits, with repeated observation of phenotype change within laboratory culture over relatively short time frames. For example, spontaneous loss of male killing was found in Spiroplasma symbionts of Drosophila nebulosa (Yamada et al, 1982), and Drosophila willistoni (Ebbert, 1991), and spontaneous emergence of non male-killing Spiroplasma in Drosophila melanogaster has occurred at least twice in a single culture (Harumoto and Lemaitre, 2018;Masson et al, 2020). In addition, Spiroplasma symbionts artificially transferred from their native host Drosophila hydei into D. melanogaster initially caused pathogenesis, but evolved to become benign over just a few host generations (Nakayama et al, 2015).…”

Dual proteomics ofDrosophila melanogasterhemolymph infected with the heritable endosymbiontSpiroplasma poulsonii

“…To this end, we extracted total hemolymph from uninfected and infected 10 days old females. At this age, Spiroplasma is already present at high titers in the hemolymph but does not cause major deleterious phenotypes to the fly (Herren and Lemaitre, 2011; Masson et al, 2020a). Extraction was achieved by puncturing the thorax and drawing out with a microinjector.…”