Diverse wMel variants of Wolbachia pipientis differentially rescue fertility and cytological defects of the bag of marbles partial loss of function mutation in Drosophila melanogaster

Abstract

In Drosophila melanogaster, the maternally inherited endosymbiont Wolbachia pipientis interacts with germline stem cell genes during oogenesis. One such gene, bag of marbles (bam) is the key switch for differentiation and also shows signals of adaptive evolution for protein diversification. These observations have led us to hypothesize that W. pipientis could be driving the adaptive evolution of bam for control of oogenesis. To test this hypothesis, we must understand the specificity of the genetic interaction between bam and W. pipientis. Previously, we documented that the W. pipientis variant, wMel, rescued the fertility of the bamBW hypomorphic mutant as a transheterozygote over a bam null. However, bamBW was generated more than 20 years ago in an uncontrolled genetic background and maintained over a balancer chromosome. Consequently, the chromosome carrying bamBW accumulated mutations that have prevented controlled experiments to further assess the interaction. Here, we used CRISPR/Cas9 to engineer the same single amino acid bam hypomorphic mutation (bamL255F ) and a new bam null disruption mutation into the w1118 isogenic background. We assess the fertility of wildtype bam, bamL255F/bamnull hypomorphic, and bamL255F/bamL255F mutant females, each infected individually with ten W. pipientis wMel variants representing three phylogenetic clades. Overall, we find that all of the W. pipientis variants tested here rescue bam hypomorphic fertility defects with wMelCS-like variants exhibiting the strongest rescue effects. Additionally, these variants did not increase wildtype bam female fertility. Therefore, both bam and W. pipientis interact in genotype-specific ways to modulate female fertility, a critical fitness phenotype.

A model of functionally buffered deleterious mutations can lead to signatures of positive selection

“…We have studied the population genetics of two Drosophila genes that are critical for gametogenesis in D. melanogaster ( bag of marbles, bam , and Sex-lethal, Sxl ) and they both show evidence, using the MK test, of episodic bursts of positive selection (Bauer DuMont et al 2007; Flores et al 2015a; Bauer DuMont et al 2021). These genes genetically interact with the endosymbiont bacteria Wolbachia pipientis in that Wolbachia rescues the reduced fertility of a partial loss-of-function bam mutant and the reduced egg production of multiple partial loss-of-function Sxl mutants (Starr and Cline 2002; Flores et al 2015b; Bubnell et al 2021). The bursts of positive selection in bam in only certain lineages of Drosophila is consistent with the episodic nature of bacterial infections and the heterogeneous presence of Wolbachia reported across the genus (Richardson, et al 2012; Turelli, et al 2018; Meany, et al 2019).…”

“…Secondly, we propose and evaluate a new model of interaction that is based on the nature of the observed genetic interactions of Wolbachia with bam and Sxl . Wolbachia partially rescues the fertility defects of a single amino acid replacement hypomorphic mutant of bam (Flores et al 2015b, Bubnell et al 2021) as well as for several hypomorphic alleles of Sxl (Starr and Cline 2002) in D. melanogaster. We posit that in the presence of Wolbachia , slightly deleterious mutations may accumulate in the bam gene without significantly reducing bam’s function.…”

“…We have studied the population genetics of two Drosophila germline stem cell genes, bag-of-marbles (bam) and Sex-lethal (Sxl), that show strong evidence of episodic positive selection (Bauer DuMont, et al 2007;Flores, DuMont, et al 2015;Bauer DuMont, et al 2021;Bubnell et al 2021). However, the typical selective pressures that we expect on reproductive genes, including sexual selection and conflict, do not obviously apply to bam and Sxl because these genes function so early in gametogenesis.…”

“…However, the typical selective pressures that we expect on reproductive genes, including sexual selection and conflict, do not obviously apply to bam and Sxl because these genes function so early in gametogenesis. Instead, the positive selection has been proposed to be due to a change in gene function and/or a genetic conflict with the endosymbiont bacteria Wolbachia that genetically interacts with both genes and resides in the germarium where they function (Bauer DuMont, et al 2007; Flores, Bubnell, et al 2015; Flores, DuMont, et al 2015; Bubnell et al 2021, Bauer DuMont et al 2021). The bursts of positive selection at bam in only certain lineages of Drosophila is consistent with the episodic nature of bacterial infections and the heterogeneous presence of Wolbachia reported across the genus (Richardson, et al 2012; Turelli, et al 2018; Meany, et al 2019).…”

Functional divergence of the bag of marbles gene in the Drosophila melanogaster species group

“…One possibility is that the maternally inherited, endosymbiotic bacterium Wolbachia pipientis is driving the evolution of bam. We previously identified a genetic interaction between bam and W. pipientis (Werren et al 2008) where infection with the wMel strain of W. pipientis partially rescued the fertility and cellular GSC daughter differentiation defects of a bam partial loss-of-function mutant (hypomorph) in D. melanogaster females (Flores, Bubnell, et al 2015;Bubnell et al 2021). That W. pipientis rescues a bam hypomorph in females, and that functional consequences of bam divergence between D. melanogaster and D. simulans is female specific, are consistent with the possibility that interactions between the Drosophila germline and W. pipientis could drive positive selection at bam.…”