From CI phenotype shift to underlying genomic basis

Slab and Ist isofemale lines are maintained at the lab since 2005 [10] and were crossed several times since: Slab females were compatible with Ist males at their establishment in 2005 (HRs above 70%, not precisely computed at that time) [10]. Crosses performed in 2017 (published in [41]) showed compatibility between Slab females and Ist males, but with highly variable hatching rates (HR ranging from 8% to 84% but all egg rafts produced larvae [41], S1 Fig). In the present study (crosses performed in 2021), we witnessed for the first time the emergence of fully incompatible crosses between some Slab females and Ist males. Twenty five rafts out of 94 had a 0% hatch rate (full CI), and the remaining 69 clutches had HRs between 3.2% and 96.4% (S1 Fig).

At least 3 cidA repertoires coexist in the Slab isofemale line

To decipher the genomic changes that led to this recent CI shift, we sequenced the cidA and cidB repertoires present in Wolbachia infecting 4 individuals from the Ist isofemale line and 8 females from the Slab isofemale line (4 compatible and 4 incompatible) using Nanopore Technologies sequencing of cidA and cidB polymerase chain reaction (PCR) products [42]. While Wolbachia present in all Ist individuals had the same cid repertoire (presented in S1 Table), we found polymorphism in cid repertoires of Slab individuals: they all had the same cidB (i.e., toxins) diversity, composed of cidB-III-ae3 and cidB-III-ag1 variants, whereas cidA (i.e., antidotes) repertoires varied among individuals. Wolbachia present in compatible and incompatible Slab females all exhibited the variants cidA-III-α(5)-25 and cidA-III-γ(3)-12, although additional variants were found in compatible females: cidA-III-β(2)-25, either alone or accompanied by cidA-III-β(2)-16. The observed differences in cidA repertoires can be summed up by the presence/absence of 2 specific cidA regions: the upstream region cidA-III-β(2) and the downstream region cidA-III-16 (Figs 1 and 2A for nomenclature).

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Fig 1. Intra-line polymorphism in cidA repertoires matches the compatibility patterns.

Three distinct cidA repertoires coexist in the Slab isofemale line. Repertoires perfectly match with compatibility patterns, with females having repertoires (β+,16+) and (β+,16-) compatible with Ist males, while females (β-,16-) are incompatible with Ist males. The presence or absence of cidA-III-β(2) and cidA-III-16 regions was determined by specific qPCRs. The presence of cidA-III-β(2) significantly influences HRs (χ = 13.094, p < 0.0001). Data supporting this figure are found in S1 Data. HR, hatching rate; qPCR, quantitative polymerase chain reaction.

https://doi.org/10.1371/journal.pbio.3002493.g001

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Fig 2. Matching recombination in the CidA-CidB interaction interface I is key for rescue.

(A) cid gene polymorphism is located in 2 distinct regions for both cidA and cidB, named upstream and downstream regions. The upstream region of cidA and the downstream region of cidB show few variability, while the other 2 regions are highly variable. Variants names are composed of: (1) the name of the gene cidA or cidB, (2) the wPip phylogenetic group concerned (I to V), (3) the name of the upstream region (in Greek or Latin letters for cidA and cidB, respectively), and (4) the name of the downstream region (in numbers for both genes). The name of a given upstream or downstream region corresponds to the same sequence across all wPip groups. (B) CidA and CidB interact head-to-tail through 3 distinct interfaces I, II, and III, highlighted here in green, yellow, and red, respectively. The cidA-III-β and cidB-IV-2 regions, which are key to the (in)compatibility between Slab and Ist isofemale lines, are both exposed in the interaction interface I. (C) Network phylogenies of the low-variability regions using the Neighbor-Net method on variants from this study and from [21,41,43]. Each edge (or set of parallel edges) corresponds to a split in the dataset with a length equal to the weight of the split. Similar regions are grouped within clusters. Within these clusters: (i) cidB regions are identical and are thus named the same way; (ii) in each given cidA cluster, sequences differ by at most 3 SNPs (located outside of any interface region). All variants within a cluster are named by the same Greek letter (alpha, beta…), with a number between brackets to reflect the existence of distinct variants (e.g., cidA-I-α(1) or cidA-I-α(2)). Recombinant variants and regions of interest in the study are highlighted in green. (D) Recombinations in key variants lead to original sequences at CidA-CidB interaction interfaces. Alignments of the protein sequences of the interaction interface I of CidA (up) and CidB (down) show that focal variants (in green) exhibit an original amino acid sequence. The sequence represented is the sequence between the first and last amino acid belonging to interface I (Wang and Hochstrasser, pers. com.). Accession numbers for sequences used in Fig 2D are found in S3 (previously sequenced variants) and S4 (newly sequenced variants) Tables. SNP, single-nucleotide polymorphism; wPip, Wolbachia infecting Culex pipiens.

https://doi.org/10.1371/journal.pbio.3002493.g002

cidA repertoire variation fully explains compatibility polymorphism

To determine if compatibility variations were linked to these specific cidA repertoire variations, we performed 3 replicates of the cross between Slab females and Ist males. We isolated egg rafts from a total of 102 Slab females from the same isofemale line (28, 34, and 40 females for replicates 1, 2, and 3, respectively) and studied their HRs (i.e., number of hatched larvae over the number of eggs laid). Eight females produced unfertilized rafts and were thus excluded from further analyses. For the remaining 94 females, HRs ranged from 0% (fully incompatible raft) to 96.4%, thus highlighting the strong differences in their rescuing abilities. Of these 94 females, 25 (i.e., 27%) were fully incompatible with Ist males representing the new emerged CI phenotype.

Since repertoires differed in the presence/absence of 2 specific regions of cidA repertoires, we designed primers and specifically screened the 94 mated females, of which 89 were successfully analyzed (S1 Data). The PCR assay confirmed that 3 distinct Wolbachia cidA repertoires coexisted in the Slab isofemale line: (i) individuals hosting Wolbachia with both the upstream β(2) and downstream 16 regions, named (β+,16+); (ii) individuals hosting Wolbachia with only the upstream region β(2) but not the downstream 16 region, named (β+,16-); and (iii) individuals hosting Wolbachia genomes with neither of these regions, named (β-,16-) (S1 Data). Hereinafter, we refer to a mosquito by its Wolbachia infections’ cidA repertoires ((β+,16+), (β+,16-), or (β-,16-)). Of the 89 females analyzed, 58 were (β+,16+), 8 (β+,16-), and 23 (β-,16-) (S1 Data).

Egg rafts produced by (β-,16-) females had a null HR, with a single exception, in which only 1 larva hatched, resulting in an HR of 1.8%. Fully incompatible crosses were only observed with (β-,16-) mothers, as all (β+,16+) and (β+,16-) mothers were compatible with Ist males (but with variability in HR) (Fig 1).

cidA repertoire variations do not explain HR variability in compatible crosses

Examining the compatible crosses involving (β+,16+) or (β+,16-) Slab females and Ist males, we found strong variations in HRs ranging from 3.2% to 96.4% (Fig 1 and S1 Data) similar to those recorded in 2017 for which the HR ranged from 8% to 84% [41] (S1 Data). While infection levels (quantified by quantitative polymerase chain reaction (qPCR)) in full female DNA extracts were variable, with 0.11 to 10.1 Wolbachia genomes per host genome and an average of 1.0 ± 0.2 Wolbachia genome per host genome (mean ± standard error), this variability did not correlate with HR variations (S2A Fig, Spearman correlation test, ρ = −0.09, p = 0.49, S1 Data). Similarly, HR variations were not correlated with the cidA copy numbers (total cidA, cidA-III-β, or cidA-III-16 copy numbers) per wPip-Slab genome investigated in Slab females (Spearman correlation test, ρ = −0.13, p = 0.28; ρ = 0.01, p = 0.94; ρ = −0.17, p = 0.16 for the total cidA, cidA-III-β, and cidA-III-16, respectively; S2B–S2D Fig and S1 Data).

For each of the 3 cidA repertoires found in the original Slab isofemale line, we established 2 isofemale sublines. HR variations could also be observed in intra-line crosses within these newly established isofemale lines, with HRs ranging from 29% to 100% (S3 Fig and S2 Data). While HR variance was overall similar in all intra-line crosses (Levene test, p = 0.05), HRs were significantly more variable in “compatible” crosses between Slab females and Ist males than in intra-line crosses (Levene test, p < 0.0001).

Recombination in cid variants leads to original CidA-CidB interaction zones

Recent works revealed that there were 3 interaction interfaces between CidA and CidB proteins, named interfaces I to III (Fig 2B, [23,24]). The upstream region cidA-III-β(2), present in both (β+,16+) and (β+,16-), seems key to enable Slab females to rescue Ist males. This region corresponds to the interface I of the CidA protein (Fig 2). Looking closer at the protein sequences at this interface, we found that CidA-III-β(2) has an original combination of amino acids resulting from a recombination of the CidA-α and CidA-γ regions (Fig 2D). Building a phylogenetic network of all the described cidA upstream regions [21,22,41,43] confirmed that β-like regions were rare (only found in the wPip group IV to which the Ist line belongs and Slab to date) and that they came from a recombination of the cidA-γ-like and cidA-α-like regions, with these last 2 regions being frequent and found in all wPip groups surveyed to date (Fig 2C). We also showed that the cidA-δ region was recombinant.

The CidB-IV-2 downstream region, present in wPip-Ist, had previously been associated with incompatibility when crossed with a wide range of females [22]. This region interacts with cidA-β at the interface I (Fig 2B). When looking at wider phylogenetic scales based on all the previously published sequences, we found a clear recombinant pattern with 2 downstream regions shared among wPip groups (CidB-1 and CidB-3) and a unique intermediate recombinant region, only found in wPip group IV so far, CidB-IV-2 (Fig 2C).

Evolution of cid repertoires

Rapid evolution of cid repertoires through gene losses

In 2021, there were approximately one quarter of females which had 3 cidA-cidB tandems. These three-copy females were absent in the original line in 2005, showing that 2 cid tandems had been lost since the line’s establishment. cid copy number distributions are bimodal, with 2 peaks at 3 and 5 copies and very rare with intermediate repertoires of 4 copies. These rare intermediates are likely due to the qPCR variability. This could mean either that (i) four-copy repertoires were a transient state that disappeared quickly due to lower fitness or drift or that (ii) four-copy repertoires did not exist, meaning that 2 tandems were lost in a simultaneous genomic structural variation.

While the loss of 2 tandems affects cidA and cidB numbers similarly, we observed a decrease in the number of distinct cidA variants without a change in the cidB repertoires: all (β+,16+), (β+,16-), and (β-,16-) repertoires described here share the same cidB variants (cidB-III-ag1 and cidB-III-ae3), while they have 4, 3, and 2 distinct cidA variants, respectively (Fig 1). Literature on CI evolution suggests that mod factors or toxins (cifB variants) could degenerate first, as their evolution is neutral for maternally transmitted Wolbachia [48,49]. Only once cifB variants have lost their function is it possible for the loss of cifA (resc factors or antidotes) [50]. In such a conceptual framework, losing cifA without losing cifB would lead to self-incompatibility, which is an immediate evolutionary dead-end. Independent from CI, the presence of cidB without an associated cidA might be toxic for the host, as shown recently in insect cells [4]. These predictions have been corroborated by a large-scale phylogenetic study of cif genes, indicating that cifB genes were often disrupted, while cifA were not [26]. This apparent discrepancy with the predictions may be explained by the fact that wPip genomes contain several cid genes. Unlike conceptual framework where the loss of cifA was thought to lead to a loss of the self-rescuing ability, wPip has several cidA variants, which makes it possible to lose 1 variant while still maintaining compatibility with the surrounding mosquitoes.

Laboratory conditions could facilitate this loss of antidotes. Indeed, in natural populations, individuals with multiple cid repertoires coexist in the same population (as shown by sequencing and revealed by numerous crosses [21,22,43,51]), meaning that crosses can involve a wide range of toxins. A greater diversity in the cidA repertoire enables the rescue of a wider range of toxins, which is thus under positive selection. While cidA diversity is advantageous in natural populations, in an isofemale line cage, crosses are involving a restricted and fixed set of toxins. Loss of cidA variants, which are not strictly required for the rescue of the cidB in a given cage, is not counter-selected. Furthermore, as numerous gene copies can be costly (e.g., [52]), the loss of cidA-cidB tandems could confer a higher fitness to Wolbachia with lower numbers of copies and thus contribute to the fixation of new Wolbachia genomes in mosquitoes, which could explain the rapid Wolbachia turnover. It is conceivable that the three-copy repertoire of (β-,16-) Wolbachia, which contains 2 distinct variants of cidA and 2 distinct variants of cidB, is the minimum viable repertoire with only pairs of matching toxins and antidotes.

No lasting coinfection involved in changes in CI patterns in wPip

One plausible explanation for the observed shift in CI could be that Slab is coinfected by 2 strains of Wolbachia with different cid repertoires, which are usually co-transmitted, and that some individuals recently lost one of these strains, as observed in Aedes albopictus [53,54]. Using common MLST markers, Baldo and colleagues initially found no variability in the wPip strains [55], showing that if coinfection occurs, it should be between highly similar strains. The development of a wPip-specific MLST led to the uncovering of wPip polymorphism and the description of 5 monophyletic groups within wPip [56]. However, while these markers highlighted wPip polymorphism, no sign of coinfection was found, as every mosquito clearly hosted one of the 5 groups [56]. In this study focusing on cid genes, which are highly variable even within a wPip group, we likewise found no sign of coinfection. We quantified several cid markers and analyzed the distribution of their values. In the case of coinfections with 2 Wolbachia strains with 3 and 5 cid copies, respectively, quantification outcomes would have varied depending on the proportion of each Wolbachia in each mosquito, leading to a unimodal distribution centered on an intermediate value. For the 3 distinct cid markers analyzed in 89 females, the quantifications displayed a bimodal distribution (S6D–S6F Fig), with a few intermediate values probably due to technical variations in the qPCR measures (S10 Fig). While coinfection must occur at some point (all Wolbachia within one individual cannot be wiped out and instantly replaced by another Wolbachia), it had to be brief with no lasting coinfection in a single host individual. This could be explained by 2 alternative hypotheses: (i) an exclusion between the 2 distinct genotypes (3 and 5 copies) for any non-CI-related reason; or (ii) a small effective population of Wolbachia (number of Wolbachia transmitted from mother to offspring) in wPip.

Variability in hatching rates in compatible crosses not explained by cid repertoires of females

Studying the cross between Slab females (wPip-III) and Ist males (wPip-IV), we found intermediate HRs in “compatible” crosses. While on/off compatibility (hatching versus no hatching at all) was clearly explained by the presence/absence of specific cidA variants, these intermediate HRs did not correlate with any of the cidA markers nor with the Wolbachia infection rate. Several factors could contribute to such variable HRs in “compatible” crosses: (i) some Wolbachia-independent effects, with loss of fertility in these strains due to inbreeding following a prolonged lab rearing (as previously noted by, e.g., [57]); and (ii) some Wolbachia-linked effect, with partial incompatibilities, as we recently described, for the first time in C. pipiens [41]. The first hypothesis is supported by previous findings, showing variable HRs even in crosses between Wolbachia-devoid mosquitoes [41]. This decrease in fertility could also be combined with a Wolbachia-linked effect. Indeed, we previously showed that male mosquitoes infected with a Wolbachia from wPip group IV (such as Ist males) induced more partial incompatibilities [41]. Furthermore, it could be envisaged that there are cidB (toxin) copy number variations in males, which we could not detect here since our protocol isolated the females to lay their eggs and did not track the males. We also found that intra-line crosses exhibited variable HRs, but that this variability was significantly higher in Slab x Ist crosses, strongly suggesting Wolbachia-linked effects.

Recombination as the key to the emergence of new CI patterns

The presence/absence of a specific cidB region, named cidB-IV-2, was previously correlated with the ability or not of males to sterilize a wide range of females [21,22]. This specific cidB-IV-2 region is present in the genome of the Wolbachia infecting the Ist males studied here. We showed that this specific region is issued from a recombination between the 2 main groups of cidB downstream regions (1 and 3), which are shared by all sequenced wPip groups (Fig 2).

Furthermore, it was previously shown that the only females able to rescue the modifications induced by Ist males were those infected with wPip-IV (as Ist) as well as Slab females (wPip-III, studied here) [21,22]. We explained the rescuing ability of Slab females by the presence of a specific cidA upstream region known as cidA-β: all females with this region can rescue Ist males, while females that have lost it cannot. This variant was also generated by a recombination between 2 main groups of upstream regions shared by all wPip groups. Here, there were 2 types of recombinants, the cidA-β and cidA-δ regions (Fig 2). All these recombinant regions were previously found in field populations, confirming they are not lab oddities [21,22].

When looking at the repertoires found in females infected with wPip-IV, which can all rescue Ist males, we found that they all had 1 recombinant region in their repertoire (either cidA-β or cidA-IV-δ) [21,22]. In other words, all females from field populations or laboratory lines able to rescue a toxin containing the recombinant region cidB-IV-2 have a recombinant cidA-β or cidA-δ antidote in their repertoire. Since the cidA upstream region (e.g., β or δ) interacts with the cidB downstream region (e.g., cidB-IV-2) in the interaction interface I [24], this points to recombinant-for-recombinant binding (Fig 2). New CI phenotypes could thus emerge through the recombination of existing variants. As it demonstrates a key role for binding in compatibility, this finding strongly supports the toxin–antidote hypothesis, for the first time in a natural system.

In conclusion, we found that CI evolved in less than 4 years in a Culex isofemale line. This evolution is linked to the loss of some cidA (antidote) variants in their cid repertoire. Our results lead to propose how new incompatibilities could emerge as a two-step process in the wild: first, local changes take place in the CI repertoires while maintaining compatibility with the surrounding mosquitoes, and then migration and secondary contact occur with the incompatible lines. Here, we showed that all repertoires present in a cage were mutually compatible: changes in the repertoires modified the CI pattern only when crossed with an external line. We demonstrated that new compatibility types can emerge rapidly, as a result of the rapid evolution of Wolbachia genomes. A common point in the few studies addressing Wolbachia microevolution is that both Octomom in wMel [37–39] and cid genes in wPip are amplified genomic regions, suggesting the strong role played by gene amplification in the speed of Wolbachia genome evolution. This rapid evolution should therefore be considered when releasing Wolbachia-infected mosquitoes, inducing CI, for vector control purposes.

Isofemale lines

All the mosquito lines used were isofemale lines, i.e., lines created by rearing the offspring resulting from a single egg raft and thus from a single female. We used the following lines: Slab, infected with Wolbachia from the phylogenetic group wPip-III, and Istanbul (Ist), infected with Wolbachia from the phylogenetic group wPip-IV. Both lines were established in our laboratory in 2005.

We also used 6 isofemale (sub)lines, which we established for this study in 2021: 2 (β+,16+) lines bearing both the cidA-III-β and cidA-III-16 regions; 2 (β+,16-) lines bearing only the cidA-III-β marker; and 2 (β-,16-) lines with neither of these markers. In short, to establish the isofemale line, a cage containing the Slab isofemale line was blood fed. After 5 days, females were isolated in glasses for egg laying. After egg laying, DNA was extracted, and females were genotyped for cidA-III-β and cidA-III-16 (S2 Table). For each genotype, the 2 most numerous progenies were kept in order to establish 2 separate isofemale lines.

All isofemale lines were reared in 65 dm³ screened cages in a single room maintained at 26°C under a 12-h light/12-h dark cycle. Larvae were fed with a mixture of shrimp powder and rabbit pellets, and adults were fed on a honey solution. Females were fed with turkey blood using a Hemotek membrane feeding system (Discovery Workshops, United Kingdom) to enable them to lay eggs.

Crosses

For each cross performed, around 100 virgin females were put in a cage with around 50 virgin males. After 5 days in cages, females were fed a blood meal, and 5 days later, individual females were isolated for egg laying. This allows us to precisely link a female with its egg raft. Egg rafts were deposited into 24-well plates. The eggs were photographed using a Leica camera and counted using ImageJ. Two days after egg laying, larvae issued from each egg raft were counted manually. HR was calculated as the proportion of eggs that hatched into larvae. Egg rafts with a null HR were mounted on a slide and examined with a binocular magnifier to ensure that the eggs were fertilized and thus confirm that null HR resulted from CI rather than non-fertilization.

Repertoire acquisition using Nanopore sequencing of PCR products

We acquired the cidA and cidB repertoires of 8 individuals from the Slab isofemale line, 4 individuals from the Ist isofemale line, and 2 individuals from each of the newly established isofemale lines following the pipeline described in [42]. We also acquired cidAB combinations for all newly established isofemale lines. DNA was extracted from the individuals. In short, the target sequence was amplified using the adequate primer set (S2 Table and S8 Fig). PCR products were then sequenced by Nanopore sequencing with the MGX Platform (Montpellier) using a MinION (Oxford Nanopore Technologies). This gave an average coverage of 30,000 to 50,000 reads for each gene. Repertoires were then acquired following [42]. Briefly, reads were mapped on a reference base containing all the variants previously identified, along with variants created in silico. Single-nucleotide polymorphism (SNP) calling was then performed, along with haplotype phasing if necessary. For cidAB combinations, the pipeline was adapted: cidA and cidB repertoires were first obtained, and then the combinations between all cidA and cidB were artificially created and used as a reference base to run the pipeline.

Wolbachia genotyping

Variant copy number analysis

Variant copy numbers were obtained using both the cidA qPCR data and the relative coverage of Nanopore sequencing. Knowing the total number of cidA copies, we deduced each variant copy number. For example, in the case of 3 cidA copies, if out of the 2 variants present, the coverage of the first was twice the coverage of the second, it could be concluded that there were 2 copies of the first variant and 1 copy of the second.

Grouping analysis of upstream/downstream regions of variants

Phylogenetic analysis of variants was performed using all the variants already published [21,22,41,43] and present in this study. Network phylogenies of the 2 regions of interest (cidA upstream variable region and cidB downstream region, which respectively corresponded to nucleotides 1–800 and 1201-end) were made using Splitstree [61].

Nomenclature of cid variants

The nomenclature established by [21] was updated, since it could result in identical sequences being given different names in different wPip groups. The fundamentals of the previous nomenclature were retained: names of variants were composed of the name of their upstream region (in Greek letters for cidA and in Latin letters for cidB) and that of their downstream region (digits). A fixed limit was established between the upstream and downstream regions for both genes (downstream regions starting at nucleotides 801 and 1201 for cidA and cidB, respectively). A phylogenetic network revealed 4 clusters of cidA upstream regions (Fig 2C), regions within a cluster differing by at most 3 SNPs. We renamed regions so that all those belonging to the same cluster were named using the same Greek letter (e.g., all named α). When SNPs were present, to distinguish the sequences, we introduced a number between brackets such as α(1). The same was done for cidB downstream regions. Similarly, cidA downstream regions and cidB upstream regions were renamed so that distinct regions had different names, and identical regions had identical names, regardless of the wPip group in which they were found.

For all variant names, the group in which the variant occurs is mentioned. For example, cidA-III-α(5)-25) is found in a wPip-III.

The correspondence between the previously used names and the names based on this new nomenclature is provided in S3 Table.

Statistical analyses and graphical representation

All statistical analyses were run using R version 4.2.0 [62]. Correlation analyses were performed using a Pearson correlation test. Homoscedasticity was tested using Levene tests. All boxplots and histograms were done using ggplot2 [63], and sequence representation was conducted using the function msaPrettyPrint from the msa package [64]. HRs analysis were done using a generalized linear model (GLM), with a binomial distribution. The significance of the different terms was testing using likelihood ratio tests (LRT).