Forward genetics in Wolbachia: Regulation of Wolbachia proliferation by the amplification and deletion of an addictive genomic island

Abstract

Wolbachia is one of the smost prevalent bacterial endosymbionts, infecting approximately 40% of terrestrial arthropod species. Wolbachia is often a reproductive parasite but can also provide fitness benefits to its host, as, for example, protection against viral pathogens. This protective effect is currently being applied to fight arboviruses transmission by releasing Wolbachia-transinfected mosquitoes. Titre regulation is a crucial aspect of Wolbachia biology. Higher titres can lead to stronger phenotypes and fidelity of transmission but can have a higher cost to the host. Since Wolbachia is maternally transmitted, its fitness depends on host fitness, and, therefore, its cost to the host may be under selection. Understanding how Wolbachia titres are regulated and other aspects of Wolbachia biology has been hampered by the lack of genetic tools. Here we developed a forward genetic screen to identify new Wolbachia over-proliferative mutant variants. We characterized in detail two new mutants, wMelPop2 and wMelOctoless, and show that the amplification or loss of the Octomom genomic region lead to over-proliferation. These results confirm previous data and expand on the complex role of this genomic region in the control of Wolbachia proliferation. Both new mutants shorten the host lifespan and increase antiviral protection. Moreover, we show that Wolbachia proliferation rate in Drosophila melanogaster depends on the interaction between Octomom copy number, the host developmental stage, and temperature. Our analysis also suggests that the life shortening and antiviral protection phenotypes of Wolbachia are dependent on different, but related, properties of the endosymbiont; the rate of proliferation and the titres near the time of infection, respectively. We also demonstrate the feasibility of a novel and unbiased experimental approach to study Wolbachia biology, which could be further adapted to characterize other genetically intractable bacterial endosymbionts.

Wolbachia load variation in Drosophila is more likely caused by drift than by host genetic factors

“…In the D. melanogaster / wMelPop system, the maintenance of the virulence phenotype has thus frequently been associated with the fact that virulence is only expressed in conditions rarely observed in nature, so that the between-host selection against highly prolific variants (such as those with high Octomom copy numbers) is weak at 25°C. A recent study however shows that strains with 8-9 Octomom copies are pathogenic from 18°C to 29°C (Duarte et al, 2021). In addition, another selective force, the within-host selection, could explain the virulence of wMelPop in certain environmental conditions.…”

“…Bottlenecks during transmission can thus eventually lead to a gradual shift of the 'initial' density over time. Bottlenecks can also influence density levels through random differential transmission of bacterial variants from one generation to the next (Funk et al 2001;Kaltenpoth et al 2010), especially if these variants exhibit different reproductive rates (as it is the case with variants carrying different numbers of Octomom copies (Duarte et al 2021)). To summarize, if not counteracted by host or symbiont density control, drift is expected to induce instability over generations by a combination of quantitative (i.e., transmission of a non-equivalent number of bacteria to the eggs) and qualitative/genetic (i.e., random transmission of different variants) bottlenecks.…”

wMel Wolbachia genome remains stable after 7 years in Australian Aedes aegypti field populations

“…S4e). However, another published w Mel genome sequence from D. melanogaster reports only three Ts at this position, rather than the four present in the reference genome, supporting this indel is also likely an error [ 55 ]. Given these results, it is clear these indel events are again errors in the reference sequence, rather than mutations that have occurred since transinfection of w Mel into Ae.…”

Wolbachia -Conferred Antiviral Protection Is Determined by Developmental Temperature

“…However, it is known that temperature can affect the interaction between Wolbachia and Drosophila . For instance, higher temperatures lead to greater proliferation, and, in the case of the pathogenic variants w MelPop and w MelOctoless, higher cost, in D. melanogaster ( 8 , 41 – 43 ). Similarly, lower developmental temperatures lead to lower titers of w Yak in Drosophila yakuba , which in turn increases imperfect vertical transmission of this Wolbachia strain ( 44 ).…”