Introduction

Many ant species are socially polymorphic, displaying multiple alternative social structures. A key distinction between the social morphs is the number of egg-laying queens in a colony. In monogyne colonies, only one queen reproduces, making the colony a single family unit. Polygyne colonies, on the other hand, consist of multiple reproducing queens, which are not necessarily related1,2. Social polymorphism has been observed in at least six closely related South-American fire ant species3,4,5, informally known as the Solenopsis socially-polymorphic clade. The clade includes the red imported fire ant and the black imported fire ant, Solenopsis invicta and Solenopsis richteri, that were introduced to North America early in the twentieth century6,7,8.

In S. invicta, the monogyne/polygyne social structure is a Mendelian trait. It is linked to the gene gp-9, which codes for an odorant binding protein3,9,10. All and only monogyne queens are homozygotes gp-9BB, whereas reproductive polygyne queens are almost exclusively heterozygotes gp-9Bb and very rarely gp-9bb11,12,13,14. Gp-9 was thereby implicated as the molecular regulator of the polygyne social structure5,15.

More recently, it was suggested that gp-9 was only one gene in a complex of genes that control social polymorphism14,16. Using a genetic map, Wang et al.17 showed that a non-recombining region of approximately 13.8 megabases, containing gp-9 and over 500 additional genes, is associated with the social dimorphism of S. invicta. The suppression of recombination, probably caused by multiple chromosomal inversions, linked alleles of these genes to the SB and Sb haplotypes of a supergene, appropriately named the ‘social chromosome’. Over time, the two haplotypes diverged, with the Sb variant evolving separately from the SB variant, in a similar fashion to the differentiation taking place between X/Y sex chromosome pairs. While SB haplotypes can still recombine in SB/SB queens (as do the X chromosomes), Sb haplotypes no longer recombine. This resulted in the accumulation of deleterious mutation and repetitive elements in Sb, as in Y chromosomes17,18,19. In comprehensive molecular analyses, each of the six known socially polymorphic Solenopsis species was found to harbor two haplotypes of the supergene19,20. Moreover, the Sb haplotype was found to form a monophyletic clade, suggesting it is a derived haplotype that has evolved in a common ancestor of these species20.

While it seems reasonable to assume that the divergence of the SB and Sb variants predates the divergence of the species in the Solenopsis socially-polymorphic clade, it is also possible that historic admixture carried the polygyne variant across species boundaries. Such an event may have been driven by adaptive introgression as suggested for other loci of high fitness effect21,22,23,24. In a previous study, we used a coalescent-based analysis to date the separation of two social polymorphic species, S. invicta and S. richteri, to 1.1 (0.6–2.1) million years ago, assuming generation time of 6 years25. Additionally, we found no evidence of historic admixture between them, despite the large scale genomic data that was analyzed. This observation is a stark contrast with the prevalent hybridization that is taking place in their introduced range of South-Eastern USA26,27. At odds with this evidence, the divergence of the two social variants was dated to 390,000 (95% confidence level 350,000–420,000) years ago, much later than S. invicta and S. richteri ‘s estimated speciation time. This dating was calibrated by the divergence time of two species of leafcutter ants17, which were the most closely related species to the Solenopsis clade for which genomic sequences were available at the time.

As of late, more and more ant genomes are being sequenced, including the genome of the thief ant Solenopsis fugax28. The divergence of S. fugax and S. invicta was dated to 25 (18–32) million years ago (MYA) using 27 fossil calibration points, in a phylogenetic study that included 251 species in the Myrmicinae sub-family29. For comparison, in the same study the Attini lineage, that includes the two leafcutter ants, was found to have diverged from the Solenopsis lineage approximately 80 MYA. Additionally, reduced genomic sequencing of a large number of S. richteri samples is now available25, allowing the studying of the social chromosome outside of S. invicta.

Here, we take advantage of these new genomic data in order to study the appearance of the polygyne social structure, the evolutionary event that defines the Solenopsis socially-polymorphic clade. We measured the evolutionary distances between seven SB and seven Sb haplotypes in S. invicta individuals of both social variants, and, by comparing them to the homologous haplotype in S. fugax, dated the divergence of SB and Sb haplotypes to the time in which S. invicta and S. richteri speciated. A phylogenetic analysis placed S. richteri SB haplotypes in the same subtree as the Sb haplotypes of both species, suggesting the Sb variant first evolved in S. richteri and then introgressed into S. invicta. These results demonstrate that the dynamic evolutionary path of Solenopsis social chromosome is intertwined with a complicated history of diverging Solenopsis populations ever since it first appeared.

Methods

Aligning social chromosome haplotypes of fully sequenced S. invicta

Using SOAPdenovo230, we de novo assembled 14 whole-genome sequenced haploid S. invicta males: seven SB variants and seven Sb variants, which were seven pairs of sons from seven unrelated SB/Sb queens collected in Athens, Georgia17. The sequencing data for the 14 samples were generated by Wang et al. using short-insert, paired-end, 100 bp reads Illumina sequencing. Our 14 assemblies had an average scaffold N50 size of 9,481 bp (range of 7,010–18,276 bp). We aligned these and three more genome assemblies: the reference genomes of SB and Sb variants of S. invicta (28 version Si_gnH; NCBI accession AEAEQ02000000) and the reference genome of S. fugax (genome assembly version Sf_gnA; NCBI accession QKQZ00000000) as an outgroup.

Using the reference SB genome as the subject, we ran blastn with default parameters (word_size = 11, gapopen = 5, gapextend = 2, reward = 2, penalty = -3) and aligned to it the scaffolds of the other 16 genome assemblies as queries. We then looked for genomic scaffolds in the 16 genomes that were aligned to the non-recombining region of the social chromosome located in chromosome 16 in the reference SB genome (scaffold00008; scaffold00028; scaffold00042; scaffold00045; scaffold00067; scaffold00119; 11,118,233 base-pairs in total). We only accepted alignments with E value ≤ 1 × 10–4 for the top hit. In order to avoid false inference of substitutions from alignments between different copies of repetitive sequences, we rejected alignments for which the second best hit, if such existed, either had E value <  = 1 × 10–4, or had half the number of base matches as the top hit or more, or had half the percentage of mismatches as the first hit or less. This resulted in a list of scaffolds of the 16 genomes which had homology and unique mapping to the social chromosome in the SB reference genome.

Next, we aligned the chosen scaffolds between these genomes, including the scaffolds of the non-recombining region in the reference SB genome, using Mugsy31, a multiple sequence aligner for whole-genome sequences. This resulted in 4,673 separate alignments blocks, each including sequences from all 17 genomes. We filtered out alignments that were less than 60 bp long and improved the remaining alignments further by running GUIDANCE232 with the alignment algorithm PRANK33 and 50 bootstrap repeats. We masked bases that had GUIDANCE2 confidence scores of less than 60%. Eventually, we were left with 4,035 17-way alignments of a total length of 10,326,783 bp (including gaps and ambiguous bases), covering 7,093,489 bases in the non-recombining region, as it was identified in the reference SB genome, which is over 50% of its estimated total length.

We calculated pairwise distances between the 14 S. invicta genomes (not including the two S. invicta reference genomes) and the S. fugax genome by counting the number of substitutions in the alignments and correcting for multiple substitutions in the same site using the Jukes and Cantor (1969) method. We then calculated the distances between each pair of genomes. As a point of reference, we repeated the process detailed above using scaffolds that were aligned to chromosome 16 outside of the non-recombining region of the reference genome (i.e. not linked to gp-9).

Aligning social chromosome haplotypes of RAD sequenced S. invicta and S. richteri

This analysis was based on Restriction site Associated DNA sequencing (RAD-seq) data of S. invicta and S. richteri populations samples from Argentina, which we described in Cohen and Privman25. We used sequences of four homozygote SB/SB and four homozygote Sb/Sb richteri workers from the population of Buenos Aires, Argentina. We also arbitrarily chose four homozygote SB/SB invicta workers from the population of El Recreo, Argentina. We mapped the RAD-seq samples to the reference genome of S. invicta SB using Bowtie234. The results were filtered so that each alignment had no more than 4 mismatches for its top hit. To make sure we were using unique alignments only, we removed alignments that had for their second best hit double the number of mismatches as the top hit, or less. We then looked for loci in the non-recombining region in the invicta SB reference genome that had all 12 RAD-seq samples and the invicta Sb reference genome aligned to them. At each locus, we arbitrarily picked one read from each sample. We calculated distances between the 12 individuals by counting the number of substitutions between them, and applied Jukes and Cantor correction as above. As before, we repeated the analysis with loci outside the non-recombining region.

Constructing phylogenetic trees for both datasets

Finally, using RAxML35 with the GTRCAT model, we built two maximum likelihood trees for the sequences of the non-recombining region in the samples in both datasets (whole genome and RAD-seq). In both cases S. fugax reference genome was used as an outgroup and was aligned to the S. invicta reference genome using Mugsy. For the RAD-seq dataset, we also mapped one of the RAD-seq S. invicta samples to the reference genome of invicta Sb, which allowed us to add this sequence to the multiple sequence alignment and to the RAxML phylogeny. The confidence in the trees were assessed using 100 bootstrap replicas.

Tajima’s D anlalysis of haplotype structure

Tajima’s D36 is used to test a null hypothesis of neutral selection in a population of stable size based on the frequency of rare alleles in the population. Significantly negative (an excess of rare alleles) or positive (deficiency in rare alleles) values mean that the null hypothesis can be rejected. Negative and positive values can be caused by selective pressures or by changes in population size, such as sudden contraction and/or expansion during or after a population bottleneck.

Raw reads of the 14 fully sequenced S. invicta males were analyzed to create a catalogue of single nucleotide polymorphism (SNP) that was used in the analysis. Duplicated reads were removed (i.e., multiple read pairs that were identical in their sequence) and low quality reads were trimmed or removed using Trimmomatic37, leaving sequences of at least 75 bp. The catalogue was then created using HaplotypeCaller and GenotypeGVCFs of Genome Analysis Toolkit (GATK pipeline)38. We filtered out loci that had less than 6 reads in more than one individual. Additionally, we used a SNPs catalogue that was previously created from the RAD-sequencing of the 4 SB/SB and 4 Sb/Sb samples of S. richteri from the native range25. We kept loci that had at least 6 reads in all individuals (as we only have 4 individuals per social haplotype).

Using these datasets, we calculated Tajima’s D separately for each social haplotype in the native and in the introduced ranges using VCFtools39. The values were computed in a sliding window of 100Kbp along chromosome 16, which contains the social supergene. The total number of SNPs used in the analysis were 2,877 and 1,237 in the RAD-seq native SB and Sb haplotypes, respectively, and 24,671 and 21,004 in the fully sequenced introduced SB and Sb haplotypes, respectively. The significance of the results was assessed assuming the β-distribution36.

Results

Evolutionary distances between social chromosome haplotypes were measured from a multiple genome alignment of seven SB and seven Sb S. invicta haplotypes, all from the introduced range in North America, and one S. fugax genome as an outgroup. These were all independent de novo assemblies of full genomes, each sequenced from a single haploid male. The distances were calculated only based on the genomic sequences aligned to the non-recombining region of the social chromosome. A second analysis used RAD sequences of 12 workers, which were homozygous at the social chromosome. These included four SB/SB invicta, four SB/SB richteri and four Sb/Sb richteri, all from the native range in South America. Again, the evolutionary distances were measured only between the sequences that were aligned to the non-recombining region of the social chromosome.

An accelerated evolutionary rate in the Sb social variant

The derived Sb haplotype evolves at an accelerated rate in comparison to the SB variant, as inferred from the maximum likelihood tree constructed from the fully sequenced haplotypes (Fig. 1). The lengths of the paths from the divergence point of SB and Sb to the Sb haplotypes are 2.9 ± 0.01 substitutions per 1,000 sites (average and standard deviation), while the path lengths to the SB haplotypes are 2.1 ± 0.14, which is significantly shorter (p = 0.002 in a two tailed Wilcoxon test). Thus, the evolutionary rate of the Sb sequences is accelerated by the factor of 1.4 ± 0.2 relative to the rate of the SB sequences.

Figure 1
figure 1

A Maximum Likelihood tree constructed from the non-recombining region of the social chromosome (in chromosome 16) in 14 introduced S. invicta samples. The homologous sequence in S. fugax genome was used as an outgroup to root the tree but the branch leading to it is only illustrated in order to maintain the proportional representation of SB and Sb branches. Branch lengths are the average number of substitutions per 100,000 sites.

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Dating the formation of the supergene

The phylogeny reconstructed for the social chromosome haplotypes and the S. fugax outgroup allowed us to calculate a ratio of 1:23 between SB–Sb divergence time and S. invicta and S. fugax speciation time. This was done by comparing the average distance measured from an SB haplotype to the common ancestor of SB and Sb and the average distance from an SB haplotype to the common ancestor of S. invicta and S. fugax. This ratio is not affected by the accelerated evolutionary rate of the Sb variant because it is not part of these paths in the tree (Fig. 1). Given that the two species diverged 25 (95% confidence interval 18–32) MYA29, the divergence between the social morphs is estimated at 1.1 (0.7–1.6) MYA.

Possible introgression of the Sb haplotype across species

In the phylogenetic tree constructed for the RAD sequenced samples (Fig. 2), the first split is between the invicta SB and richteri SB, and the subsequent split is between the SB and Sb haplotype groups, which implies that speciation preceded the formation of the supergene. We also included in this phylogeny the invicta SB and Sb reference genomes to represent both haplotypes in both species. Thereby, we can observe that the Sb haplotypes of both species form a monophyletic clade within the richteri clade. This suggests that the derived Sb haplotype evolved within the richteri lineage and was then introgressed into the invicta lineage.

Figure 2
figure 2

A Maximum Likelihood tree constructed from the RAD-seq of the non-recombining region in 12 native S. invicta and S. richteri samples. The two S. invicta reference genomes (marked INVB-ref and INVb-ref) are also included. The monophyletic subtree of the Sb haplotypes is marked by a blue box. The homologous sequence in S. fugax genome was used as an outgroup to root the tree but is not shown.

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Figure 3 presents the distributions of pairwise distances between haplotype groups in either whole genome or RAD-seq datasets. The analysis of the native S. invicta and S. richteri resulted in slightly longer distances between richteri SB and richteri Sb (5.1 ± 0.2 substitutions per 1,000 sites) than those measured between invicta SB and richteri SB haplotypes (4.8 ± 0.1). However, since the Sb haplotypes evolve 1.4 fold faster than SB, then the inferred time since the speciation is actually longer than the time since haplotype divergence (p < 5 × 10–6 in a two tailed Wilcoxon test). The distance between richteri SB and richteri Sb is composed of two components: 5.1 = t rB + t rb, where t is the time since supergene formation and rB and rb are the evolutionary rates in SB and Sb respectively. This means that 5.1 = t rB + 1.4t rB and so t = 2.1/rB. The time since invicta SB’s and richteri SB’s divergence is 2.4/rB, which is greater than 2.1/rB. This result suggests that the formation of the supergene occurred only after the speciation of S. invicta and S. richteri, which is in agreement with the phylogeny in Fig. 2.

Figure 3
figure 3

Distribution of pairwise evolutionary distances among several groups of haplotypes, summarized in violin plots. The distances were measured between the whole-genome sequences of non-recombining region in 14 S. invicta individuals sampled in the introduced range (green) and between RAD-sequences in the same genomic region of four S. invicta and eight S. richteri individuals sampled in the native range (purple). The evolutionary distances measured in the genomic region of chromosome 16 that is outside the non-recombining region are also shown for comparison (light green and light purple). The number of pairwise comparisons is indicated under each plot. Introduced S. invicta genomes: BB–SB vs. SB; bb–Sb vs. Sb; Bb–SB vs. Sb.; gp-9ul–all 14 individuals compared over their gp-9 unlinked genomic region. RAD sequenced genomes: IBIB–invicta SB vs. invicta SB; RBRB–richteri SB vs. richteri SB; RBRb–richteri SB vs. richteri Sb; IBRB–invicta SB vs. richteri SB; RbRb–richteri Sb vs. richteri Sb; Rgp-9ul–all 8 richteri individuals compared over their gp-9 unlinked genomic region.

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Divergence levels within haplotype groups in the native and introduced ranges

In general, the distances within haplotype groups in the fully sequenced S. invicta, sampled in the introduced range, are shorter than in the equivalent groups in the RAD sequenced S. invicta and S. richteri, sampled in the native range (Fig. 3). These results are consistent with a recent population bottleneck event, as expected with a recently introduced population. Most striking are the differences in the mean and variance of the distances measured within the Sb haplotype group in the native range (RbRb; 3 ± 0.8 substitutions per 1,000 sites) compared to the distances measured within the Sb group in the introduced range (bb; 0.1 ± 0.03), which is more than one order of magnitude. The former group is the most diverse and latter—the least diverse among all haplotype groups. Conversely, the diversity of the SB haplotype group in the introduced range (1.7 ± 0.2) is reduced by only ~ 50% relative to the corresponding native range groups (3.1 ± 0.1; 3.3 ± 0.2 for invicta and richteri, respectively). Pairwise distances and their summary statistics can be found in Supplementary Tables S1A,B and S2A,B.

Tajima’s D statistics over chromosome 16

We calculated Tajima’s D statistics for different haplotype groups in a 100Kbp sliding window along chromosome 16, which contains the social supergene (Fig. 4). In the Sb haplotype group of the introduced range (whole genome data; n = 6), we find a segment of 109 consecutive windows (total length of 10.9 Mbp), with the mean Tajima’s D value of − 1.51, which is significantly different from zero. This segment, which overlaps with all of the scaffolds identified to be linked to the social chromosome, includes 101 windows with negative values, 82 of which were significantly negative. Outside of this region, and for the entire chromosome 16 in the introduced SB haplotype group, there were no negative windows and only a small number of significantly positive windows (2 and 8 respectively). In the same 10.9 Mbp region in the native Sb haplotype group (RAD-seq data; n = 4), 29 windows were significantly negative and the mean Tajima’s D was − 0.842 across it. There were 16 significantly negative windows outside of this region, and a total of 21 significantly negative windows across the entire chromosome of the SB haplotype group. There were no windows with significantly positive Tajima’s D values in either haplotype group in the native range.

Figure 4
figure 4

Tajima’s D statistic for 100 Kb windows across chromosome 16 for native range S. richteri (RAD-seq data) and introduced ranges S. invicta (whole-genome data) for each social haplotype group. Lines and grey zone represent smoothing with the linear model method over 95% confidence levels. A 10.9 Mbp long segment, that for the introduced range Sb haplotypes contains all windows with negative values, including 82 windows with significantly negative values, was smoothed separately in the Sb haplotype groups (positions 7.5–18.4 Mbp in red).

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Discussion

Our estimate of 1.1 (0.7–1.6) MYA is a much older date for the divergence of Solenopsis social variants than the previous estimate of 390,000 (350,000–420,000) years ago17. This previous dating was made relative to the divergence time of two leaf cutter ant species in a different tribe (tribe Attini), whereas our estimate was made relative to the divergence from another species in the Solenopsis genus (tribe Solenopsidini). Additionally, the dating of S. fugax–S. invicta divergence time is based on more robust phylogenetic dating informed by 27 fossil calibration points29,40 relative to only three in the previous study. We previously estimated the number of generations since gene flow between S. invicta and S. richteri ceased at 190,000 (100,000–350,000), using a maximum likelihood coalescence analysis of 15,040 RAD-seq loci25. Assuming a generation time of six years, this means that the species separated 1.1 (0.6–2.1) MYA. This time frame overlaps with the above estimate for the age of the supergene and therefor does not resolve the order of these evolutionary events. We note that there is uncertainty in the generation time parameter, and since it may be as short as three years41, the estimate for the speciation may only be 0.55 MYA.

In our phylogenic analysis of the non-recombining region in native S. invicta and S. richteri samples, the monophyletic clade of Sb haplotype of both species was assigned to the same subtree as the richteri SB haplotypes with bootstrap support of 93%. Thus, it seems likely that S. invicta had obtained its Sb variant through admixture with S. richteri or a species which is closely related to S. richteri. This, combined with the dating results, suggests that the formation of the supergene occurred around the same time in which the process of speciation between S. invicta and S. richteri was taking place. The Sb haplotype may have first evolved in one incipiently speciating population and then introgressed into the other populations before the reproductive barrier between them was completely formed.

Recently, Yan et al.20 conducted a phylogenetic dating analysis of eight Solenopsis species using five conserved genes. They estimated the radiation time of the social polymorphic species at 0.5 (0.1–1.1) MYA, which is in the lower range of our abovementioned estimate for the speciation of S. invicta and S. richteri. They also estimated the divergence time of the Solenopsis socially polymorphic clade from S. saevissima, which does not harbor the Sb haplotype, at 0.7 (0.1–1.5) MYA. This speciation event should be taken as an upper-bound for appearance of the social chromosome, and fits within the lower range of our dating of the supergene. It should be noted that speciation between two incipient species is only complete when a reproductive barrier is formed between them, preventing the exchange of genes (with the possible exception of isolated mating and introgression). This is true for the speciation dating from our previous study. On the other hand, the evolutionary distances inferred from the divergence of genes or haplotype sequences corresponds to the time since the coalescent event for these alleles, which predates the time of reproductive isolation. Taken together, the dating estimates by Yan et al. and our own suggest that the supergene was formed approximately at the same time of the speciation of the socially polymorphic clade.

Gotzek et al.42 anayzed sequences of the gp-9 gene from 14 Solenopsis species, including the six species of the Solenopsis socially polymorphic clade, and found that the monophyletic gp-9b clade is most closely related to gp-9B allele sequences of S. invicta with very high support (bootstrap of 97%), while the gp-9B clade is not monophyletic across the socially polymorphic species. This implies that, contrary to our results, gp-9b has originated in S. invicta and introgressed into the other species. On the other hand, Yan et al. in their analysis of the entire social chromosome, have presented three different social haplotype trees, depending on different filtering criteria. In one, the SB and Sb haplotypes form separate monophyletic clades (albeit with bootstrap of only 47% to the monophyly of SB), which suggests that the Sb haplotype already existed in an ancestral species to the clade. In the other two trees, the SB haplotype clade is paraphyletic with high bootstrap support, and so these trees support the introgression scenario (extended data). Moreover, in one of them, the likely species of origin of the Sb haplotype is S. richteri, similar to our results. Altogether, the inconsistent tree topologies from the various studies indicate that the two evolutionary processes of speciation and social haplotypes divergence, which were probably only established over the course of many generations, most likely overlapped. Fast species radiation and incomplete lineage sorting may also account for the ambiguity of the results. Such evolutionary dynamics would result in different tree topologies for different haplotype samples and for different genes or genomic regions within the supergene.

Cohanim et al.43 investigated natural selection on protein-coding regions of the social chromosome and calculated the ratio of non-synonymous substitutions (dN) to synonymous substitutions (dS). The ratio in the Sb haplotype (dNb/dSb) was estimated at 0.209, while in the SB haplotype (dNB/dSB) it was 0.087, implying that much stronger purifying selection pressures act on the SB haplotype. In our analysis, the 1.4-fold acceleration in the evolutionary rate of the Sb haplotype compared to the SB haplotype was calculated over mostly non-coding genomic regions (92% of the supergene is non-coding sequences according Cohanim et al.). Therefore, the higher substitution rate we report here is probably composed of elevated rate in both amino-acid substitutions and substitutions in non-coding elements (e.g. untranslated regions in exons, promoters, transcription factor binding sites). The elevated rates likely reflect differences in natural selection pressures over the social haplotypes, acting both on amino acid and on mutations in non-coding elements. These differences can be explained by the reduced efficiency of purifying selection on functional sites due to the lack of recombination in Sb, and possibly also by positive selection acting on adaptive traits during the evolution of the polygyne form.

The within-group diversity levels in the native range do not significantly differ between richteri Sb and SB haplotype groups (p > 0.9 in a two tailed Wilcoxon test), or between the gp-9 unlinked genomic region and either of the haplotype groups (p > 0.4 in both cases). However, the evolutionary distances measured among the Sb haplotypes vary greatly, with almost an order of magnitude larger standard deviation in the Sb haplotypes group compared to the SB haplotypes group. As Sb is almost only found in SB/Sb polygyne queens (Sb/Sb queens rarely reach reproductive maturity), recombination between the Sb haplotypes is rare, resulting in increasingly separating Sb lineages. The varying evolutionary distances directly reflect different coalescent times between pairs of Sb haplotypes. Conversely, SB haplotypes do recombine in SB/SB queens, averaging the distances between different haplotypes as shown by the more uniform diversity levels among the SB haplotypes.

In contrast to the diversity of native range Sb haplotypes, in the introduced range, Sb diversity level is much reduced. The distances among the Sb haplotypes in the introduced range are all much lower than the lowest pairwise distance between Sb haplotypes in the native range, which is consistent with a single clone of Sb haplotypes that passed through the bottleneck. This lack of diversity was already pointed out by Pracana et al.44 when analyzing the same introduced S. invicta individuals used in our study. Pracana et al. suggested two possible explanations to the low diversity levels. The first was a recent selective sweep of a single Sb haplotype. As we found that the diversity levels of the native richteri Sb do not differ from those of the native richteri SB, this seems unlikely. The second was that polygyny was only introduced to the USA as late as the 1970s in a secondary introduction event, resulting in severe bottleneck effect of the introduced Sb haplotype. This can account to its much reduced diversity compared to the diversity of introduced SB haplotype, which is the accumulated result of all introductions. Furthermore, the lower fitness of Sb males16,45 means that a mated polygyne queen most likely carried two SB haplotypes and only one Sb haplotype into the introduced range. Even if the founding colony consisted of many polygyne queens, they may have contributed only one clone of Sb haplotypes if they were all maternally related. Negative Tajima’s D values in the supergene, which are the result of increased number of rare alleles, may also be interpreted as the effect of a population bottleneck. We conclude that effectively only a single Sb haplotype passed the bottleneck of the introduction and there is no need to invoke selection to explain the very low diversity and the excess of rare alleles in this haplotype following the recent introduction.

Conclusion

We dated the supergene of the Solenopsis social chromosome at 1.1 (0.7–1.6) million years, which is substantially older than previous estimates. The appearance of the derived polygyne social structure has likely occurred concomitantly with the speciation of S. invicta and S. richteri, during the gradual formation of barriers to gene flow between them. We have compared the evolutionary rate of the two social variants and found that the Sb haplotype evolves at a rate which is 1.4 times that of the SB haplotype, likely due to the suppression of recombination in the Sb haplotypes. While the levels of diversity within the native SB haplotypes and within the native Sb haplotypes are similar, the highly variable evolutionary distances among the latter mark the separately evolving Sb clones. This is in stark contrast to the much lower diversity levels of the Sb haplotype in the introduced range, which is likely the result of a very severe bottleneck that effectively allowed only a single close of Sb haplotypes into the introduced population.