Maternally inherited microorganisms can influence the mtDNA pattern of variation in hosts. This influence is driven by selection among symbionts and can cause the frequency of mitochondrial variants in the population to eventually increase or decrease. Wolbachia infection is common and widespread in Drosophila melanogaster populations. We compared genetic variability of D. melanogaster mitotypes with Wolbachia genotypes among isofemale lines associated with different geographic locations and time intervals to study coevolution of the mtDNA and Wolbachia. Phylogenetic analysis of D. melanogaster mtDNA revealed two clades diverged in Africa, each associated with one of the two Wolbachia genotype groups. No evidence of horizontal transmission of Wolbachia between maternal lineages has been found. All the mtDNA variants that occur in infected isofemale lines are found in uninfected isofemale lines and vice versa, which is indicative of a recent loss of infection from some maternal fly lineages and confirms a significant role of Wolbachia in the D. melanogaster mtDNA pattern of variation. Finally, we present a comparative analysis of biogeographic distribution of D. melanogaster mitotypes all over the world.
Citation: Ilinsky Y (2013) Coevolution of Drosophila melanogaster mtDNA and Wolbachia Genotypes. PLoS ONE8(1): e54373. https://doi.org/10.1371/journal.pone.0054373
Editor: Luciano A. Moreira, Centro de Pesquisas René Rachou, Brazil
Received: August 10, 2012; Accepted: December 11, 2012; Published: January 17, 2013
Copyright: © 2013 Yury Ilinsky. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by grant number 12-04-01319-a from the Russian Foundation for Basic Research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The author has declared that no competing interests exist.
Wolbachia is a genus of maternally transmitted endosymbiotic bacteria that is found in a wide range of arthropods and nematodes –. The effects of Wolbachia on their hosts are quite diverse, including mutualism and reproductive parasitisms such as cytoplasmic incompatibility, parthenogenesis, male-killing, feminization, which can provide a reproductive advantage to infected females .
Wolbachia infection is common and widespread in Drosophila melanogaster –. The frequency of infected individuals in populations normally ranges from 10% to 90% and is on average about 50% per population. However, some populations have an extremely low frequency of infected individuals, e.g. a population from West Africa sampled in 2010, that had only one infected individual in a sample of over hundred flies .
In D. melanogaster, Wolbachia occurs as a single strain named wMel. This conclusion was made after different gene sequences were found monomorphic –; however, this strain was further subdivided into several genotypes (wMelCS, wMelCS2, wMel, wMel2, wMel3) by using genetic markers such as inversion, variable number tandem repeats (VNTRs) and transposon insertion sites . The wMel and wMelCS genotypes are found all over the world; however, the wMel genotype is most prevalent –. The other Wolbachia genotypes are rare and local.
Selection on maternally inherited symbionts can lead to changes in the mtDNA haplotype frequency in host populations (indirect selection on mtDNA variation). Hurst and Jiggins  classify the influence of microoganisms on mitochondrial diversity into four types: 1) symbiont-driven reduction in mtDNA diversity, 2) symbiont-driven increase in mtDNA diversity, 3) symbiont-driven change in mtDNA variation over space and 4) symbiont-associated mtDNA paraphyly. Facts about Wolbachia’s influence on mtDNA diversity are numerous, with many of them reviewed by Hurst and Jiggins , where they also suggested further studies in terms of evolutionary history of Wolbachia hosts , , –.
Several attempts have been made to find a link between mitochondrial DNA diversity in D. melanogaster and Wolbachia. Solignac et al.  compared the infection status and restriction-site polymorphism of D. melanogaster mtDNA and came to the conclusion that “cytoplasm infection is irrespective of mtDNA haplotypes”. Nunes et al.  performed a more detailed comparison of partial cox1 sequences for D. melanogaster with Wolbachia genotypes and concluded that “the Wolbachia infection was not randomly distributed among flies with different mtDNA haplotypes”. Previously, we used a similar approach and came to the conclusion that there were three haplotypes of the D. melanogaster mitochondrial cox1 gene, each associated with one of the three Wolbachia genotypes , . Here we present a more extensive phylogenetic analysis of 2757-bp and 1280-bp of mtDNA from D. melanogaster isofemale lines harboring as many Wolbachia genotypes as have been found in wild populations. We demonstrate a perfect consistency between major mitochondrial lineages and Wolbachia genotypes, which suggests an absence of Wolbachia horizontal transmission among D. melanogaster lineages or if such events exist then there is no conspicuous effect on the cytotype patterns. Similar results have been obtained recently by Richardson et al. , with a different sample of strains used. We extend our results and those of  to provide a comparative analysis of both datasets on the diversity and biogeography mitotypes and Wolbachia genotypes in D. melanogaster.
Methods and Materials
A total of 413 samples for mitochondrion polymorphism are used. There are such datasets: a) 62 stocks were sequenced by me in range 502–2757 bp mtDNA fragment (Table 1), b) 8 mitochondrial genomes present in GenBank sequenced by other authors (Table 1), c) 25 stocks tested for 37C/T polymorphism (see below), d) 28 sequences of 1515 bp fragment taken from Rand et al. ; e) 290 sequences of 2757 bp fragment derived from Richardson et al. . Most of lines in dataset “a” are from the Laboratory of Populations Genetics of the Institute of Cytology and Genetics of the Siberian Branch of the Russian Academy of Sciences. Lines 10030 and 10032 infected with wMel2 were courtesy of Masayoshi Watada (Ehime University, National Bio-Resource Project in Japan), w1118 infected with wMelPop  was courtesy of Elena Kiseleva (Institute of Cytology and Genetics, Russia). Dataset “b” also includes the 12508 bp sequence of w1118 stock produced by Clancy  that is indicated in footnotes of Table 1 because of the identity of Clancy’s and my results in the compatible region. The “d” dataset represents lines derived from populations of Africa, Europe, Asia, North and South America. The “e” dataset contains information on samples from a single population of Northern America , Europe, populations of Africa  and a chimerical sequence – NC001709 that is composed from Canton-S and Oregon-R stocks , –.
DNA Extraction, Wolbachia Genotyping and mtDNA Analysis
There was one female sampled from each line and incubated in 200 µl of extraction buffer (10 mM TRIS-HCl (pH 8.0), 25 mM EDTA, 0.5% SDS, 0.1 M NaCl, 0.1 mg/ml Proteinase К) for 2 h at 56°C. The DNA was precipitated and diluted in 50 µl of deionized water. 1 µl of this solution was used for all amplifications. PCR cycling conditions were 30 cycles in 20 µl of the total volume as follows: denaturing for 5 min at 95°C; 29 cycles each for 20 s at 94°C; annealing for 1 min at 55°C (57°C for the wsp gene); elongation for 1 min/kbp at 72°C. The Mg2+ was 2.5 mM and that of each primer was 0.3 mM. The Wolbachia infection status was determined by amplification with the 81F/691R primer set for the wsp gene , and the 99F/994R primer set for 16SrRNA gene . The Wolbachia genotypes were determined by using VNTRs, IS5 and inversion markers according to the protocol .
We developed a system called snpPCR for detecting the 37C/T polymorphism (position 2187 in GenBank accession number NC001709), which is a diagnostic substitution for discrimination between the M- and S-clades. A search for 37C/T SNPs was carried out in two independent PCRs, one of them with COIR1 5′-CCAGTAAATAATGGGTATCAGTG-3′ and 2187-MEL 5′-GCGTTTGATTTTTTGGTGAT-3′ as primers and the other with COIR1 5′-CCAGTAAATAATGGGTATCAGTG-3′ and 2187-CS 5′-GCGTTTGATTTTTTGGTGAC-3′ as primers; 25 cycles, annealing at 55°C, Mg2+ at 1.5 mM. The inference about the mitotype (37C vs. 37T) depended on which of the two PCR tubes contained an amplicon. The snpPCR system was validated and verified in two ways: 1) wild-type isofemale lines infected with different genotypes and 2) 300 mutant stocks from the Laboratory of Populations Genetics, Novosibirsk, Russia (Ilinsky Yu, unpublished data). So far as the lines with S-clade mitotypes (Canton-S, Oregon-R, w1118, and those derived from them) often used in Drosophila labs are concerned, the 37C/T snpPCR method is a reliable technique for monitoring stock contamination as well as in Drosophila crossing studies.
The 2757-bp region of mtDNA contains following genes: ATPase subunits 6 and 8, three tRNAs (tRNA-Leu, tRNA-Lys, and tRNA-Asp), cytochrome c oxidase subunit II and parts of cytochrome c oxidase subunits I and III, which were sequenced using COIF 5′-CCAGCTGGAGGAGGAGATCC-3′ , 2672r 5′-CCAGTAAATAATGGGTATCAGTG-3′ , At6F 5′-GCACCTATTAGATGATTATT-3′, At6R 5′-TCGTGATACATCTCGTCATC-3′, 01 5′-TTACAAGATAGAGCTTCTCC-3′, 02 5′-ATATCATTGATGGCCGATTC-3′, 03 5′-GACGGAATTATTAAAAGTCC-3′, 04 5′-TTAGCTGTTCGATTAACTGC-3′, and COIR1 5′-GAGTTCCATGTAAAGTAGC-3′ as primers. In some lines, only 2604-bp, 1280-bp or 502-bp fragments containing relevant information were sequenced (Table 1). The 502-bp region contains two diagnostic substitutions that account for three haplogroups: CT (GenBank accession number JF730694) associated with wMel, wMel2, wMel4; CC (JF730696) – wMelCS2; and TC (JF730695) – wMelCS genotypes. The mtDNA amplicons were purified by ExoSAP-IT reagent (USB Corporation) and sequenced by BigDye® Terminator v3.1 and v1.1 cycle sequencing kits (Applied Biosystems).
The maximum likelihood method, the Kimura 2-parameter model of nucleotide substitution  and 1000 bootstrap replications were used for reconstruction of the three phylogenetic trees generated by MEGA5 : 1) 33 samples, alignment 2757 bp (Figure 1; “a, b” datasets); 2) 43, 1280 (Figure S1; “a, b” datasets); and 3) 327, 1515 (Figure S2; “a, b, d, e” datasets). There are no principal differences in using other methods and models. A Bayesian approach (MrBayes 3.2.1), with a general-time-reversible (GTR) model of nucleotide substitution, 1.5×106 iterations of Markov chain Monte Carlo (MCMC) was used for reconstruction of phylogenetic tree with 323 samples and 2757 bp (Supporting Information S2, Supporting Information S3; “a, b, e” datasets). Posterior Bayesian probabilities were calculated on the basis of the last half of MCMC iterations.
The tree reveals two major clades, each associated with one of the two Wolbachia genotype groups. Names, origin, infection status of stocks and bootstrap (1000 replicates) values higher than 75 are provided. The samples infected with identical Wolbachia genotypes are indicated with the same colour.
We tried to find if there is a certain evolutionary relationship between Wolbachia genotypes and mtDNA diversity of D. melanogaster. The design of this study was to select lines with different infections status, from a broad set of locations. Coevolution changes must be observed in case of strict co-inheritance of both maternal factors: Wolbachia and mitochondrion. Discordance of inheritance would indicate the fact of a Wolbachia horizontal transmission.
The long-term association of Wolbachia and mitochondrion variants was investigated, “a, b, e” datasets used. Biogeography distribution of mitochondrion variants over the world was based on the analysis of “a, b, d, e” datasets. The dataset “b” was used for the analysis of D. melanogaster population structure and non-random distribution of the mitochondrion variants among uninfected lineages.
Each of the infected isofemale lines for which the data are provided here was unambiguously characterized by a particular Wolbachia genotype. In the course of these experiments, we have identified a new genotype, wMel4, in a population from the Sinai Peninsula (Egypt). It differs from the most abundant wMel genotype in wMel having seven variable number tandem repeats, VNTR-141, while wMel4 has five (Table 2). It is important to note, however, that the infection status and genotype of some stocks sequenced by other authors were unknown to us, since we used only mtDNA information available from GenBank (Table 1; “b” dataset). However, we assume the stock w1118, which harbors the wMelPop pathogenic strain , , , is infected with wMelCS. Based on genetic similarity, these Wolbachia genotypes fall into two groups: MEL (wMel, wMel2, wMel3, wMel4) and CS (wMelCS, wMelCS2) (Table 2).
An Association between D. melanogaster mtDNA Diversity and Wolbachia Genotypes
The reconstructed phylogenetic tree from 33 D. melanogaster stocks of different origins of 2757-bp fragment of mtDNA (“a, b” datasets) reveals two main clades: M and S (the label “M” inspired by – D.melanogaster and “S” – from Canton-S) (Figure 1, Table S1), each being strictly associated with one of the two major Wolbachia groups, MEL and CS respectively (Table 2). The major clades of the tree were the same when we used a shorter, 1280-bp mtDNA fragment for 43 D. melanogaster stocks (Figure S1; “a, b” dataset).
The mtDNA diversity in isofemale lines infected with CS genotypes is low, only 6 sites are variable. All the lines harboring wMelCS2 (derived from field collections of Eastern Europe, the Caucasus, Central Asia, and the Altai) have identical 2757-bp and identical 1280-bp sequences with the exception of laboratory stock 2–23 (origin unknown; maintained in the Laboratory of Populations Genetics since 1970), which differs from the others in only one nucleotide substitution (T → C at position 2589). The mtDNA sequences in the lines infected with the wMelCS genotype contain four variable sites. Our results show that the mitotypes associated with this genotype have no geographical pattern.
One of mtDNA variants found in MEL-infected flies is obviously widespread. The lines harboring wMel are s400 (Sochi, the Caucasus, Russia, 2004), Harwich (Massachusetts, USA, 1967), 11-Sinai (Sinai Peninsula, Egypt, 2010), and Z53 (Zimbabwe, 1990) have identical 2757-bp mtDNA sequences. We note that Z53 is infected, however the Wolbachia genotype of this strain has not been examined . 12-Sin (Sinai Peninsula, Egypt, 2010), infected with the newly discovered genotype wMel4, is quite close to the widespread mitochondrial variant of s400, Harwich, 11-Sinai, and Z53 lines but it differs in one substitution (A → T at position 1106). Another widespread variant occurs in Eurasia: Central Asia (Kyrgyzstan, 2004), Bi90; Northern Asia (Altai, Russia, 2003), 335; and Eastern Europe (Zvenigorodka, 2003, and Uman, 2004, the Ukraine), U4, 3310. Two wMel2-infected lines (both reported from Amamioshima, Japan, 1992) differ in four substitutions; however, they are most closely related to each other.
What are relative frequencies of the M and S mitotypes in D. melanogaster in the wild? As far as infected flies are concerned, the answer is easy to give: the M-clade is the most prevalent because it is associated with the most prevalent wMel genotype. To answer this question on uninfected flies it is possible to compare 1) the ratio of M/S uninfected wild flies (Figure 2; “a, c” datasets) with that of MEL/CS genotypes and 2) the mitotype diversity of infected and uninfected flies.
This distribution was inferred using the 37C/T diagnostic substitution (37C, the S-clade; 37T, the M-clade). Boxed geographical names were confirmed by sequencing; numbers after names stand for the number of lines corresponding to a particular location.
On the one hand, mitotype frequencies among infected flies can be expected to differ from uninfected ones. Indeed, mitotypes of infected flies undergo an indirect selection, i.e. selection of Wolbachia, whereas uninfected flies are under a direct selection of mitotypes. The genetic drift leads to shift of mitotype frequencies among infected and uninfected flies. As to uninfected flies the genetic drift must be stronger then direct selection, since most mitotypes are neutral or near-neutral. On the other hand, infected lineages lose bacteria (imperfect maternal transmission) and as a result the mitotype frequencies of uninfected flies are equalized with the infected ones. Continent-island model for gene flow can be a good illustration of the case. If the mitotype ratio of uninfected flies differs from that of infected ones this means that the value of bacteria loss in flies lineages is lower than the value of selection or genetic drift for mitotypes. If ratio of uninfected flies does not differ from infected ones – there is a high rate of bacteria loss in maternal lineages. Besides, it is important to compare mitotype diversity of uninfected flies with that of infected flies. If these diversity are identical or very close that means the mutation rate is lower than that of Wolbachia loss.
To distinguish M and S mitotypes there have been developed the snpPCR of the 37C/T diagnostic substitution. In the uninfected isofemale lines derived from flies in the collections from North Eurasian populations in 1978–1992, 29 lines were identified as being in the M-clade (37C) and 20 lines as being in the S-clade (37T) (Figure 2). In the infected lines developed from flies of the same collections 31 lines were identified as being infected with CS genotypes and 74 lines – with a MEL group, in particular wMel genotype . We performed a statistical comparison of the M/S mitotype ratio in uninfected flies and the M/S cytotype ratio (a cytotype results from a mitotype and the genotype of the infection) in infected lines. These differences are not significant (Fisher’s exact test, p = 0.199), which accounts for non-random sampling of uninfected lineages in populations and are likely to imply that uninfected flies had infected ancestors in the near past. In some of the uninfected lines, the 2757-bp, 1280-bp or 502-bp regions were also sequenced. The 502-bp region contains two diagnostic substitutions that account for three haplogroups: CT (GenBank accession number JF730694) associated with MEL, CC (JF730696) – wMelCS2, and TC (JF730695) – wMelCS. The result is that the infected and uninfected isofemale lines are observed to have identical sets of mtDNA variants (Figure 1, S1), which also means that Wolbachia infection has been recently lost from some maternal lineages of flies. The confirmation of this conclusion we find in the results of Richardson et al. , where the uninfected lines have the similar or identical mitotype diversity as the infected ones.
Thus the diversity and frequency of mitotypes among uninfected flies primarily depend on the gene pool of infected flies, uninfected flies replenished at the expense of bacteria loss in infected lineages.
The analysis of D. melanogaster mtDNA variation and Wolbachia genotypes suggests a significant role for Wolbachia shaping in the haplotype diversity in this species. The fly cytotype is derived from the mitotype and infection status. Each of the two mitochondrial clades is associated with one of the two Wolbachia genotype groups: M, with MEL, and S, with CS. There are four different cytotypes in the wild: M-MEL, M-w−, S-CS and S-w−; however, their relative frequencies are not equal. M-MEL and M-w− collectively make up about 90% or more, while S-CS and S-w− –10% or less.
A high frequency of the wMel genotype was reported previously –, . A few wMelCS cases are known; however, they were reported from different regions of the world . The wMelCS2 genotype is likely to be limited to D. melanogaster populations in Eastern Europe, the Caucasus, Central Asia, and the Altai –. The wMel2 genotype has been found in Japan, China, India and Southeast Asia , , while wMel3, in just one D. melanogaster stock kept under laboratory conditions . Additionally, we have found a new genotype, wMel4, in a population from the Sinai Peninsula (Egypt). Nunes et al.  also reported a new genotype from Uganda, the latter genotypically belongs to the MEL group; however, it is not named and there are no data on the status of its inversion marker.
There are data , ,  that most uninfected flies have the same mitotypes as flies infected with wMel genotype. Furthermore, it is possible to make inferences about M- and S-clade frequencies from the data on mitochondrial sequences in lines that were studied without reference to Wolbachia whatsoever. Previous studies , , ,  (“b, d, e” datasets) have focused on M-clade genomes. Seven complete or nearly complete mtDNA genomes presented in GenBank belong to the M-clade (AJ400907, AF200829, FJ190106–10) and only two sequences belong to the S-clade (AF200828 and FJ190105) (Table 1) , , . The genetic distance ranges from 0.0002 to 0.0019 within the M-clade and it is 0.0001 within the S-clade; while between these two clades it ranges from 0.0037 to 0.0042.
Findings of the recently performed study  in which complete Wolbachia and mtDNA genome sequences of 290 D. melanogaster lines were presented confirm our results: in particular they show a strict concordance between Wolbachia and mtDNA lineages. The most lines in this study also belong to the M-clade, 285 versus 5 of the S-clade. The detailed analysis  made it possible to subdivide M-clade into 5 clades (I–V) and to refer our S-clade to their VI-clade. A combined analysis of mtDNA from ,  and our data allow us to get a more comprehensive picture of the biogeography of mitotypes all over the world. The phylogenetic tree for 323 lines (290 of “e” +33 from “a, b” datasets) for 2757 bp alignment distinguishes all 6 clades (Figure 3, Supporting Information S2). In addition to clades I–VI there is a new clade VII that is associated with wMelCS2 genotype. Clade V in the original study  was not associated with Wolbachia and it was found in a very small number of stocks. Our results show that clade V is associated with the wMel genotype and its mitotypes are widespread in Eurasia. The samples that belong to clade IV in particular are associated with wMel2 (Figure 1, 3). Based on distribution of wMel2 genotype ,  these mitotypes spread not only in Africa  but over South, East and Southeast Asia. The samples of clade II are absent in our collection and according to  they are limited within Africa. Clades I and III are evidently the most spread over the world. In addition to mitotype distribution of “a, b, e” datasets we compared 1515 bp mtDNA of 27 isofemale lines (“d” dataset) from Africa, Eurasia and the Western hemisphere from  with the relevant genome information. The CAF line from Congo is the most close to S-clade whereas other lines (USA, Argentina, France, India, Japan, Zimbabwe) cluster with the I-II-III-IV clade-branch, which is not clearly resolved for 1515 bp analyzed region (Supporting Information S1).
Posterior Bayesian probabilities and SE indicated above and below nodes, respectively. Shown are 1) the position of our lines and those of , ,  in the tree and their associations with Wolbachia genotypes; 2) the correspondence between clades identified in Richardson et al.  and our datasets; clade VII*, identified here by us, is associated with wMelCS2 genotype; 3) the biogeography data of mitotypes; Southeast Asia** according to  where genotype wMel2 was found; 4) 28 samples from Rand et al.  (“d” dataset) belonging to the tree that is based on the analysis of relevant region of 1515 bp alignment (Supporting Information S1). Clade-branch I-II-III-IV is not clearly resolved in this case (Figure S2); 5) the correspondence of the tree to M and S clades.
S/M Clade Divergence
Tropical Africa is the ancestral range of D. melanogaster . This conclusion was made on the basis of allozymic variability analysis , nuclear gene sequences – and mtDNA restriction-site polymorphism . The world regions where D. melanogaster populations reside were subdivided into three categories: ancestral regions (tropical Africa), ancient regions (Eurasia) and new regions (Australia and the Western Hemisphere) , .
In light of the current D. melanogaster biogeography, it is interesting to ask whether mtDNA diversity has evolved in the African populations or after D. melanogaster had spread over the world? D. melanogaster migration from Africa to Eurasia might begin after the last glaciations , –, 10–12 thousand years ago, and it has spread with advancements in agriculture and, in recent centuries, with the European colonization of Australia and the Americas. The simple evidence of the African origin of the mitotype diversity is the presence of samples of I-II-III-IV- (M-) and of VI- (S-) clades and the absence only of V- (M-) and VII- (S-) ones in Africa. In addition, to address this question, we calculated the possibly most recent S/M-clade divergence time based on one of the values obtained by direct estimation of the D. melanogaster mtDNA mutation rate . The mitochondrial genome of Oregon-R (14905 bp, AF200828) belongs to the S-clade, and that of Z53 (14916 bp, AF200829), to the M-clade. These genomes differ in 52 single nucleotide mutations and four indels. Assuming that the average mutation rate for every type of a single substitution and an indel is 9.2×10−8 per site per generation  and that D. melanogaster has up to with 20 generations per year in the wild, the time required for this number of mutations to happen in two mitochondrial molecules is more than 1000 years. Of course such value of diversification is too much underestimated because it considers only the fact of mutation but not of fixation in mitochondrial population of the individual. Moreover, it is obvious that reverse and repeat mutations could happen in Oregon-R and Z53 ancestors. It means that the S- and M-clades had diverged long before enhanced human activity promoted the D. melanogaster spread over the Earth and probably even before the end of the last glacial period. It is necessary to note Richardson et al  also conclude that the origin of the global cytoplasmic diversity is in Africa based on a Bayesian phylogenetic analysis. Moreover the estimation of divergence of Wolbachia that are associated with M- and S-clade is 3263–13998 ya  which supports our conclusion on M- and S-clade divergence in Africa. If that scenario is true, then the place where Wolbachia diverged into two genotype groups was Africa, while the wMelCS2 and wMel2 genotypes are likely to have originated in the regions where they were found: Middle Asia and Eastern Europe (wMelCS2) and South, East and Southeast Asia (wMel2). However, it is quite possible that wMel2 exists in Africa because IV-clade mtDNA was found there.
A Hypothesis of Global Wolbachia Replacement
Reigler et al.  suggested the hypothesis of global Wolbachia replacement in D. melanogaster. Their hypothesis is based on the fact that wMelCS was originally present in field collections made before the 1970’s, and later the wMel genotype became dominant. However, the number of wMelCS-infected isofemale lines attributed to the middle of the 20th century is small (n = 14).
The global Wolbachia replacement should result in changes in the mitochondrial variation pattern in uninfected flies, if Wolbachia transmission is imperfect and there is no horizontal transfer. Identical mitochondrial variants have been found in both infected and uninfected D. melanogaster, which is indicative of a recent loss of infection in maternal lineages. Since selection favors infected females, the number of uninfected ones decreases over time. Consequently, the frequency of M-clade occurrences should increase and that of S-clade occurrences should decrease in both infected and uninfected lineages. Nunes et al.  attempted to verify this hypothesis by comparing the ratio of different mitotypes in 10 long-standing isofemale lines (derived before 1955) with the mitochondrial pattern as in modern field collections. Following , we tracked the M- and S-clade dynamics in uninfected isofemale lines by PCR screening for the presence of the 37C/T polymorphism. We found that a considerable number of uninfected flies belonging to the S-clade existed in North Eurasian populations in 1978–1992 (see Results and Figure 2). Therefore, a big contribution to the S-clade lineages that come from the wMelCS-infected cytoplasm (the TC mitotype, JF730695) has been made by lineages that used to harbor the wMelCS2 infection (the CC mitotype, JF730696; they both have 37C), that is confined to Northern Eurasia , . This implies that all other regions of the world could be characterized at that period by fewer S-clade occurrences, and as a consequence a replacement of cytotypes is driven at a different rate in different regions of the world.
Richardson et al.  came to the conclusion that replacement of genotypes is incomplete and it began long before the 20th century, which is confirmed by large M-clade diversity (Figure 3, Supporting Information S3). So the most intriguing question is what is the cause of a notable number of wMelCS laboratory stocks established at the first half of the last century. Further analyses of the S- and M-clade dynamic among uninfected lines is needed to clarify the scale and rate of replacement events.
Horizontal Transmission of Wolbachia
Neither we nor  found evidence for horizontal transmission of MEL or CS genotypes between the clades, it is still possible for such events to occur in the wild. They can be detected by mere comparing diagnostic SNPs in infected flies; however, a low frequency of non-MEL genotypes in field populations poses a challenge.
No detection of Wolbachia strains in D. melanogaster other than those related to the wMel strain were reported earlier. Admittedly, several “undetermined genotypes” of Wolbachia in D. melanogaster were reported . These isolates did not amplify the VNTR-141, VNTR-105 or IS5-WD0615/7 markers; however, they amplified with IS5-WD1310 and looked very similar to the MEL-group entities. Although we have not observed such Wolbachia genotyping profile in D. melanogaster, we have seen it in a different Drosophila species from Thailand, in which the cox1 gene has a high similarity with that in D. ananassae, D. pallidosa and D. papuensis (results not shown). With the exception of any possible methodological difficulties of genotyping, the origin of such “undetermined genotypes” might be accounted for by horizontal transmission of Wolbachia from a different host.
In summary, the modern mitochondrial pattern in D. melanogaster is characterized by a low variation possibly resulting from a selective sweep of Wolbachia. The cytotypes occur at different frequencies: individuals with M-clade cytotypes are most prevalent in the populations of the world. The remainder of individuals should be in the S-clade, although existence of some clades more is but not excluded, for instance, in tropical Africa. Uninfected and Wolbachia-infected flies have identical sets of mitotypes within each of the M- and S-clades. This is a likely indication of a recent infection loss from maternal lineages and an important contribution of Wolbachia selection to the mtDNA pattern of variation in D. melanogaster.
Phylogenetic tree of the 1280-bp coding-region sequence in 43 stocks, derived from a maximum likelihood analysis of Drosophila melanogaster mtDNA. Names, origin, infection status of stocks and bootstrap (1000 replicates) values higher than 75 are provided. The samples infected with identical Wolbachia genotypes are indicated with the same colour.
Phylogenetic tree of the 1515 bp alignment in 327 stocks (“b, d, e” datasets), derived from a maximum likelihood analysis of Drosophila melanogaster mtDNA.
Nucleotide polymorphism in 2757 bp mtDNA of 33 Drosophila melanogaster lines (“a, b” datasets).
We thank Professor Masayoshi Watada (NBRP, Ehime University, Japan) and Elena Kiseleva (Institute of Cytology and Genetics, Russia) for kindly providing fly stocks 10030, 10032 and w1118. We are grateful to Professor Pavel Borodin, Sergei Shekhovtsov (Institute of Cytology and Genetics, Russia), Nellie Provolotskaya for critically reading the manuscript; Casey Bergman (University of Manchester) for helpful discussion and comments on the manuscript; the anonymous reviewer for his thorough analysis; Roman Bykov, Konstantin Gunbin, Andrey Broshkov, and Marina Pakhomova for technical support.
Conceived and designed the experiments: YI. Performed the experiments: YI. Analyzed the data: YI. Wrote the paper: YI.
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