Complex fitness landscape shapes variation in a hyperpolymorphic species

Abstract

It is natural to assume that patterns of genetic variation in hyperpolymorphic species can reveal large-scale properties of the fitness landscape that are hard to detect by studying species with ordinary levels of genetic variation. Here, we study such patterns in a fungus Schizophyllum commune, the most polymorphic species known. Throughout the genome, short-range linkage disequilibrium (LD) caused by attraction of minor alleles is higher between pairs of nonsynonymous than of synonymous variants. This effect is especially pronounced for pairs of sites that are located within the same gene, especially if a large fraction of the gene is covered by haploblocks, genome segments where the gene pool consists of two highly divergent haplotypes, which is a signature of balancing selection. Haploblocks are usually shorter than 1000 nucleotides, and collectively cover about 10% of the S. commune genome. LD tends to be substantially higher for pairs of nonsynonymous variants encoding amino acids that interact within the protein. There is a substantial correlation between LDs at the same pairs of nonsynonymous mutations in the USA and the Russian populations. These patterns indicate that selection in S. commune involves positive epistasis due to compensatory interactions between nonsynonymous alleles. When less polymorphic species are studied, analogous patterns can be detected only through interspecific comparisons.

Introduction

Alleles do not affect fitness and other phenotypic traits independently and, instead, engage in epistatic interactions (de Visser et al., 2011; de Visser and Krug, 2014; Gillespie, 1994; Good and Desai, 2015; Kryazhimskiy et al., 2011; Maynard Smith, 1970; McCandlish et al., 2013; Povolotskaya and Kondrashov, 2010). Epistasis is pervasive at the scale of between-species differences, where it is saliently manifested by Dobzhansky-Muller incompatibilities and results in low fitness of interspecific hybrids (Callahan et al., 2011; Corbett-Detig et al., 2013; Dobzhansky, 1936; Kondrashov et al., 2002; Orr, 1995; Taverner et al., 2020). By contrast, at the scale of within-population variation, the importance of epistasis remains controversial (Crow, 2010; Hill et al., 2008; Hivert et al., 2021). This may look like a paradox, because such variation provides an opportunity to detect epistasis through linkage disequilibrium (LD), non-random associations between alleles at different loci (Beissinger et al., 2016; Boyrie et al., 2021; Garcia and Lohmueller, 2021; Wang et al., 2012; Zan et al., 2018). In the case of positive epistasis, a situation when a combination of alleles confers higher fitness than that expected from selection acting on these alleles individually, it can maintain favorable coadapted combinations of alleles at interacting sites, increasing linkage disequilibrium (LD) between them (Barton, 2010; Boyrie et al., 2021; Kouyos et al., 2007; Pedruzzi et al., 2018; Takahasi and Tajima, 2005). In sexual populations, recombination competes with epistasis, disrupting such coupling LD (Neher and Shraiman, 2009; Pedruzzi et al., 2018). Nevertheless, within a single gene, physical proximity alone may suffice to limit recombination, so sets of coadapted variants may evolve (Dobzhansky, 1950; Lewontin and Kojima, 1960). Such positive within-gene epistasis has been proposed to affect variation in natural populations (Arnold et al., 2020; Ragsdale, 2021), but conditions for this are expected to be restrictive (Hansen, 2013; Mäki-Tanila and Hill, 2014; Sackton and Hartl, 2016).

Perhaps, the fitness landscape is complex macroscopically but is more smooth microscopically or, in other words, epistasis is genuinely more pronounced at a macroscopic scale (Ochs and Desai, 2015). If so, studying epistasis in hyperpolymorphic populations, where differences between genotypes can be as high as those between genomes of species from different genera or even families, holds a great promise because variation within such a population can cover multiple fitness peaks or a sizeable chunk of a curved ridge of high fitness (Bateson, 1909; Dobzhansky, 1937; Gavrilets, 1997; Kondrashov et al., 2002; Muller, 1942; Appendix 1).

Results

Elevated LD between nonsynonymous polymorphisms

In a vast majority of species, nucleotide diversity π, the evolutionary distance between a pair of randomly chosen genotypes, is, at selectively neutral sites, of the order of 0.001 (as in Homo sapiens) or 0.01 (as in Drosophila melanogaster) (Leffler et al., 2012). Still, a few hyperpolymorphic species with π>0.1 are known, of which the wood-decaying fungus Schizophyllum commune is the most extreme, where π=0.20 or 0.13 in the USA or the Russian populations, respectively (Baranova et al., 2015; Appendix 3—figure 1). The two populations of S. commune are highly divergent (dS between populations ≈ 0.34, F_st = 0.58), but there is essentially no structure within either of them (Appendix 3—figure 2). We studied 34 haploid genotypes from the USA and 21 from Russia, each obtained by sequencing and de novo assembly of a haploid culture originated from a single haplospore. The use of haploid samples and de novo assembly of each sample ensures robust identification of haplotypes. We then compared the LD between nonsynonymous SNPs (LD_nonsyn) to that between synonymous SNPs (LD_syn).

In both S. commune populations, at sites with minor allele frequency (MAF) >0.05, LD_nonsyn is much higher than LD_syn at the same nucleotide distance (Figure 1A, Figure 1—figure supplement 1A). This excess of LD_nonsyn is much stronger for pairs of SNPs located within the same gene, compared to pairs of SNPs from adjacent genes at the same distance. By contrast, the excess of LD_nonsyn is independent of whether the two SNPs are located within the same or in different exons of a gene (Figure 1—figure supplement 2). In S. commune, the recombination rate is higher within exons (Seplyarskiy et al., 2014), which may affect the patterns of LD; however, this factor could only reduce within-gene LD, and in any case cannot explain the difference between LD_nonsyn and LD_syn. For S. commune, the excess of LD_nonsyn over LDsyn holds when we explicitely control for MAFs (Figure 1—figure supplement 3), indicating that differences in MAFs between synonymous and nonsynonymous polymorphisms cannot explain the excess of LD_nonsyn.

Figure 1.

The efficiency of epistatic selection in populations with different levels of genetic diversity.

(A–C) LD in natural populations for SNPs with MAF >0.05. (A) USA population of S. commune, (B) Zambian population of D. melanogaster, (C) African superpopulation of H. sapiens. Filled areas in (A)-(C) indicate SE of LD calculated for each chromosome or scaffold separately. (D–F) A hyperpolymorphic population (D) may occupy a sizeable chunk of a complex fitness landscape, leading to pervasive positive epistasis, while variation within less polymorphic populations (E and F) is confined to smaller, and approximately linear, portions of the landscape, so that no strong epistasis and LD can emerge. The area of the landscape covered by the population is shown in green.

Figure 1—figure supplement 1.

The efficiency of epistatic selection in populations with different levels of genetic diversity.

LD between nonsynonymous SNPs is shown in orange, and LD between synonymous SNPs is shown in blue. (A) Russian population of S. commune, (B) European super-population of H. sapiens. Solid lines indicate LD between pairs of SNPs located within the same gene; dashed lines correspond to pairs of SNPs located in different genes. Only SNPs with minor allele frequency > 0.05 are analysed. Filled areas indicate SE of LD calculated for each chromosome (for human) or scaffold (for S. commune) separately.

Figure 1—figure supplement 2.

Linkage disequilibrium within and between exons in S. commune.

LD between nonsynonymous SNPs is shown in orange, and LD between synonymous SNPs is shown in blue. Solid lines indicate LD between pairs of SNPs located within the same exon of the gene; dashed lines correspond to pairs of SNPs located in different exons of the gene. (A) USA population of S. commune, (B) RUS population of S. commune. Only SNPs with minor allele frequency > 0.05 are analysed. Filled areas indicate SE of LD calculated for each scaffold separately.

Figure 1—figure supplement 3.

Comparison of LD_nonsyn and LD_syn in S. commune populations with exact matching of both MAFs and distance.

For each possible minor allele count and nucleotide distance, the number of corresponding pairs of nonsynonymous variants and LD_nonsyn between them is calculated. Then, the same number of synonymous variants on the same nucleotide distance and with the same minor allele count is randomly chosen to calculate LD_syn. Subsampling is performed for 100 times. Filled areas show 95% intervals of LD_syn in the subsamples. (A) All SNPs, (B) SNPs with MAF >0.05.

Figure 1—figure supplement 4.

LD between SNPs with different MAF in S. commune.

LD between nonsynonymous SNPs is shown in orange, and LD between synonymous SNPs is shown in blue. Filled areas indicate SE of LD calculated for each scaffold separately. (A, B) LD between all pairs of SNPs pooled together. Solid lines indicate LD between pairs of SNPs located within the same gene; dashed lines correspond to pairs of SNPs located in different genes. (C, D) Pairs of SNPs split by MAF.

Figure 1—figure supplement 5.

LD between SNPs with different MAF in D. melanogaster.

LD between nonsynonymous SNPs is shown in orange, and LD between synonymous SNPs is shown in blue. Filled areas indicate SE of LD calculated for each chromosome separately. (A) LD between all pairs of SNPs pooled together. Solid lines indicate LD between pairs of SNPs located within the same gene; dashed lines correspond to pairs of SNPs located in different genes. (B) Pairs of SNPs with MAF <0.05 (large scale). (C) Pairs of SNPs split by MAF.

Figure 1—figure supplement 6.

LD between SNPs with different MAF in H. sapiens.

LD between nonsynonymous SNPs is shown in orange, and LD between synonymous SNPs is shown in blue. Filled areas indicate SE of LD calculated for each chromosome separately. (A) LD between all pairs of SNPs pooled together. Solid lines indicate LD between pairs of SNPs located within the same gene; dashed lines correspond to pairs of SNPs located in different genes. (B) Pairs of SNPs with MAF <0.05 (large scale). (C) Pairs of SNPs split by MAF.

A much weaker excess of LD_nonsyn over LD_syn for MAF >0.05 is also observed in the less genetically diverse D. melanogaster population (Figure 1B). In the still less polymorphic human populations, LD_nonsyn is indistinguishable from LD_syn at the same distances (Figure 1C, Figure 1—figure supplement 1B).

The excess of LD_nonsyn over LD_syn corresponds to the attraction between minor nonsynonymous alleles. This attraction can only appear due to positive epistasis between such alleles - higher-than-expected fitness of their combinations (Appendix 2). Positive epistasis can be expected to cause stronger LD in more polymorphic populations (Figure 1D–F, Appendix 1) and must be more common for pairs of sites located within the same gene, which are more likely to interact with each other.

For rare SNPs with MAF <0.05 taken alone, LD_nonsyn is similar or lower to LD_syn for all three species, consistent with the effects of random drift, Hill-Robertson interference, and/or negative epistasis (Figure 1—figure supplements 4–6; Appendix 2). Decreased LD between negatively selected polymorphisms is expected due to Hill-Robertson interference between deleterious alleles (Hill and Robertson, 1966; Roze and Barton, 2006); this effect has been described previously for H. sapiens (Garcia and Lohmueller, 2021) and D. melanogaster (Sandler et al., 2021) and is observed in our simulations (Appendix 2—figure 4). In addition, LD_nonsyn can be reduced by negative epistasis between deleterious alleles (Garcia and Lohmueller, 2021), similarly to the negative LD detected among loss-of-function polymorphisms in humans, flies and plants (Sandler et al., 2021; Sohail et al., 2017).

Elevated LD between interacting sites

Natural selection acting on physically interacting amino acids that are located close to each other within the three-dimensional structure of a protein is characterized by strong epistasis which leads to their coevolution at the level of between-species differences (Marks et al., 2011; Ovchinnikov et al., 2014; Sjodt et al., 2018). Genome-wide elevated LD between amino acid sites within structural domains was recently demonstrated in human populations (Ragsdale, 2021). Extraordinary diversity of S. commune makes it possible to observe an analogous phenomenon at the level of individual genes in within-population variation.

To test this, we aligned S. commune proteins to the PDB database of protein structures. In the obtained set of 5188 genes with a good match to a protein with known structure, we identified pairs of codons in the S. commune genome encoding amino acid residues positioned near each other (within 10 Å) in the protein structures, and calculated the average LD between SNPs in such pairs of codons. Naturally, pairs of physically interacting sites are more likely to be closely spaced in the gene sequence and therefore to be under a higher LD than non-interacting ones. To account for this, we discarded pairs of SNPs within five amino acids from each other, and used a controlled permutation test (see Materials and methods) to compare the LD between physically close pairs of sites to that between distant pairs of sites.

In both S. commune populations, pairs of nonsynonymous SNPs are in stronger LD when they are located at codons encoding physically close than distant amino acids (Figure 2A; permutation test p-value <1e-3). This is not the case for pairs of synonymous SNPs (Figure 2A; permutation test p-value = 0.58).

Figure 2.

Excessive LD between physically interacting protein sites.

(A) Within pairs of SNPs that correspond to pairs of amino acids that are colocalized within 10 Å in the protein structure, the LD is elevated between nonsynonymous, but not between synonymous, variants. Dashed lines show the average LD between colocalized sites. Permutations were performed by randomly sampling pairs of non-interacting SNPs while controlling for genetic distance between them, measured in amino acids; pairs of SNPs closer than 5 aa were excluded. (B–D) Examples of proteins with LD patterns matching their three-dimensional structures. Heatmaps show the physical distance between pairs of sites in the protein structure; only positions carrying biallelic SNPs are shown. Black dots correspond to pairs of sites with high LD (>0.9 quantile for the gene). Dashed lines in (c) structure show high LD between physically close SNPs from different segments of high LD. In these examples, LD is calculated in the Russian population of S. commune.

Figure 2—figure supplement 1.

Examples of proteins with LD patterns matching the three-dimensional structure in the RUS population of S. commune.

Heatmaps show the physical distance between pairs of sites in the protein structure; only positions carrying biallelic SNPs are shown. Black dots correspond to pairs of sites with high LD (>0.9 quantile for the gene). Grey regions indicate the exon structure of the genes. (A) cog1523 (5Y1B:A); (B) cog2779 (1SXJ:B); (C) cog5375 (1RGI:G); (D), cog5725 (1TA3:B); (E) cog18092 (4QJY:A); (F) cog7878 (4TYW:A). LD statistics and p-values for each gene are listed in Appendix 3—table 1.

Figure 2—figure supplement 2.

Examples of proteins with LD patterns matching the three-dimensional structure in the USA population of S. commune.

Heatmaps show the physical distance between pairs of sites in the protein structure; only positions carrying biallelic SNPs are shown. Black dots correspond to pairs of sites with high LD (>0.9 quantile for the gene). Grey regions indicate the exon structure of the genes. (A) cog1536 (6AHR:E); (B) cog5725 (1TA3:B); (C) cog8253 (6F87:A); (D) cog9241 (1YCD:A). LD statistics and p-values for each gene are listed in Appendix 3—table 1.

Moreover, it is possible to identify individual proteins with significant associations between the patterns of LD and of physical interactions between sites. At a 5% FDR, we found 22 such proteins in the USA population, and 87 proteins in the Russian population (Appendix 3—table 1); three examples are shown in Figure 2B–D (see also Figure 2—figure supplements 1 and 2). The alignment of ADAT2 protein contains two segments (teal and red in Figure 2B) characterized by high within-segment LD. The boundaries of these segments match those of structural units of the protein, but not the exon structure of its gene. In RadB protein, a similar pattern is observed, and LD is also elevated between pairs of SNPs from different segments on the interface of the corresponding structural units (Figure 2C). The alignment of 4CL protein can be naturally split into four high-LD segments, which also match its structure (Figure 2D).

Distinct regions of high LD

The magnitude of LD varies widely along the S. commune genome. Visual inspection of the data shows a salient pattern of regions of relatively low LD, alternating with mostly short regions of high LD (haploblocks, Figure 3—figure supplement 1). We calculated LD along the genome in a sliding window of 250 nucleotides and regarded as a haploblock any continuous genomic region with LD values that belong to the heavy tail of its distribution (see Materials and methods).

In the USA population, 8.4% of the genome is occupied by 5,316 such haploblocks, 56% consist of regions with background LD level, and the rest cannot be analyzed due to poor alignment quality or low SNP density. Eighty-eight percent of the haploblocks are shorter than 1000 nucleotides, although the longest haploblocks spread for several thousand nucleotides (Figure 3—figure supplement 2). In the Russian population, there are 10,694 haploblocks, occupying 15.9% of the genome, and regions of background LD cover 39% of it. There is only a modest correlation between the USA and Russian haploblocks: the probability that a genomic position belongs to a haploblock in both populations is 2.3% instead of the expected 1.3%, indicating their relatively short persistence time in the populations (examples shown in Figure 3—figure supplement 1).

LD within a haploblock is usually so high that most genotypes can be attributed to one of just two distinct haplotypes, which carry different sets of alleles (Figure 3—figure supplement 3). This results in a bimodal distribution of the fraction of minor alleles in a genotype within a haploblock, because some genotypes belong to the major haplotype and, thus, carry only a small fraction of minor alleles, and other genotypes belong to the minor haplotype and, thus, possess a high fraction of minor alleles (Figure 3A). Polymorphic sites within haploblocks are characterized by higher MAF than that at sites that reside in non-haploblock regions (t-test p-value <2e-16), and in the USA population MAFs within a haploblock are positively correlated with its strength of LD (Figure 3B, Pearson correlation estimate = 0.07, p-value <2e-6).

Figure 3.

Patterns of linkage disequilibrium in the USA population of S. commune.

(A) Distribution of the fraction of polymorphic sites that carry minor alleles in a genotype within haploblocks. Black line shows the distribution of fraction of minor alleles in genotypes in non-haploblock regions. (B) Distributions of the average MAF within a haploblock for haploblocks with different average values of LD. The average MAF in non-haploblock regions is shown as a horizontal black line for comparison. (C) LD between nonsynonymous and synonymous SNPs within individual genes. Linear regression of LD_nonsyn on LD_nsyn is shown as the red line. To control for the gene length, only SNPs within 300 nucleotides from each other were analyzed. Genes with fewer than 100 such pairs of SNPs were excluded. (D,E) The positive correlation between p_n/p_s of the gene and its average LD (D) or the difference between LD_nonsyn and LD_syn (E). Here, the data on the USA population of S. commune are shown; similar patterns in the Russian population are shown in Figure 3—figure supplement 4.

Figure 3—figure supplement 1.

Examples of haploblocks in two populations of S. commune.

The heatmaps show LD between polymorphic SNPs in the same genomic regions in the USA and RUS populations of S. commune. Only biallelic polymorphic sites with minor allele frequency >1 are shown, the number of such sites can differ between populations.

Figure 3—figure supplement 2.

Distribution of haploblock lengths (nt) in the two populations of S. commune.

Figure 3—figure supplement 3.

Example of the S. commune alignment within a haploblock.

Region 3097200–3097500 of scaffold 4 in the USA population of S. commune is shown. The top line shows the consensus sequence based on 34 genotypes; dot indicates match with the consensus.

Figure 3—figure supplement 4.

Patterns of linkage disequilibrium in the RUS population of S. commune.

(A) Bimodal distribution of the fraction of polymorphic sites carrying minor alleles per genome within the haploblocks. Each count corresponds to a genotype within a haploblock. Black line shows the background distribution of minor alleles in the non-haploblock regions. (B) The increased average minor allele frequency within haploblocks as compared to the non-haploblock regions (dashed line, t-test p-value <2e-16). (C) LD between nonsynonymous and synonymous SNPs within single genes. Each dot represents an individual gene. Linear regression of LD_nonsyn over LD_nsyn is shown as the red line. To control for the gene length, only SNPs within 300 bp from each other were analyzed. Genes with fewer than 100 such pairs of SNPs were excluded. (D,E) The positive correlation between p_n/p_s of the gene and its average LD (Spearman correlation p-value = 4e-16) (D) or the difference between LD_nonsyn and LD_syn (Spearman correlation p-value = 2e-5) (E).

Figure 3—figure supplement 5.

Comparison of LD_nonsyn and LD_syn in the genes of S. commune.

(A) The USA population, (B) the RUS population. The genes are stratified by their average LD (the panels) and by the p_n/p_s. Only pairs of SNPs within 300 bp from each other are analyzed; genes with less than 100 such pairs of nonsynonymous or synonymous SNPs are excluded. Spearman correlation p-values are shown.

Figure 3—figure supplement 6.

The difference between LD_nonsyn and LD_syn under pairwise epistasis and balancing selection.

(A) The excess of LD_nonsyn over LD_syn under different models of epistasis between two deleterious mutations A → a and B → b without balancing selection and in the presence of negative frequency-dependent selection (NFDS) or associate overdominance (AOD) acting in the linked sites. The height of columns shows fitness of the corresponding genotypes. (+) indicate simulations where the excess of LD_nonsyn is reproduced. (B) The average LD in the simulations. (C) The difference between LD_nonsyn and LD_syn in the simulations.

Figure 3—figure supplement 7.

Criteria for haploblocks in S. commune.

Red lines show the distribution of LD (r²) in windows of 250 nucleotides in two populations. Black line corresponds to the lognormal distribution with the same mean and variance. The windows with LD higher than the threshold value defined as the intersection point of the two lines (dashed) are attributed to haploblocks.

There is no one-to-one correspondence between haploblocks and genes, which are, on average, longer. Still, different genes are covered by haploblocks to different extent, which leads to wide variation in the strength of LD and other characteristics among them. Genes with high LD, that is those that contain haploblocks, have the largest excess of LD_nonsyn over LD_syn (Figure 3C). Positive correlation between the overall LD within the gene and the excess of LD_nonsyn in this gene indicates that the attraction between nonsynonymous variants, driven by epistasis, is stronger if combinations of epistatic alleles are persisting within population for a long time, comprising haplotypes within a haploblock. Since both haplotypes tend to be common in a haploblock (Figure 3), this excess is much stronger for loci with MAF >0.05.

LD between alleles of all kinds is higher within genes with large ratios of nonsynonymous and synonymous polymorphisms p_n/p_s (Spearman correlation p-value <2e-16, Figure 3D). Genes with elevated p_n/p_s also have a stronger excess of LD_nonsyn over LD_syn (Figure 3E, Spearman correlation p-value = 4.4e-17). This excess is the strongest for genes with high overall LD, but its correlation with p_n/p_s holds even when the overall LD is controlled for (Figure 3—figure supplement 5).

There can be multiple non-exclusive mechanisms by which epistasis could lead to the observed positive associations between pn/ps, overall LD, and excess LD_nonsyn. First, genes under weaker selection, and therefore higher pn/ps, could be characterized by a higher overall amount and/or strength of epistasis. Second, epistasis, as estimated by excess LD_nonsyn, can contribute to increased pn/ps by allowing nonsynonymous polymorphisms to segregate in the population when maintained in coadapted combinations, therefore weakening negative selection against them. Third, epistasis can be more potent in genes with lower overall recombination rate due to competition between epistasis and recombination: recombination breaks positively interacting combinations of alleles, disrupting linkage between them and interfering with epistasis. Fourth, existence of cosegregating combinations of mutually beneficial alleles could select for reduced local recombination rate.

Excess of LD_nonsyn requires stable polymorphism

Simulations show that positive epistasis alone cannot lead to the observed large excess LD_nonsyn over LD_syn, for which two extra conditions need to be satisfied. The general reason for this is simple: in order for a substantial LD between not-too-rare alleles to appear, these alleles must persist in the population for a long enough time.

First, positive epistasis must lead to a full compensation of deleterious effects of individual alleles. In other words, the fitnesses of at least two most-fit genotypes that are present in the population at substantial frequencies must be (nearly) the same (Figure 3—figure supplement 6). If this is not the case, selection favoring the only most-fit genotype leads to a too low level of genetic variation, which persists only due to recurrent mutation. It is natural to assume that the two major haplotypes that are common within a haploblock correspond to high-fitness genotypes. High-fitness genotypes can represent either isolated fitness peaks of equal heights (corresponding to a situation when two out of the four allele combinations confer high fitness) or a flat, curved ridge of high fitness (corresponding to a situation when three out of four combinations confer high fitness). The available data are insufficient to distinguish between these two options. Of course, with complete selective neutrality of all allele combinations there is no reason for LD_nonsyn >LD_syn, so that at least some mixed genotypes, carrying alleles from different high-fitness genotypes, must be maladapted.

Second, there must be some kind of balancing selection that specifically works to maintain variation, because otherwise random drift does not allow genetic variation to persist for a long enough time even if some, or even all, genotypes are equally fit (Figure 3—figure supplement 6). Here, there are at least two options. On the one hand, a ‘real’ negative frequency-dependent selection (NFDS) can act either directly at loci that display high LD or at some other tightly linked loci (Charlesworth, 2006; Olendorf et al., 2006). On the other hand, variation can be maintained due to associative overdominance (AOD), resulting from selection against recurrent deleterious mutations at linked loci (Gilbert et al., 2020; Ohta, 1971; Zhao and Charlesworth, 2016).

Balancing selection is also neccessary for the presence of haploblocks, because a pair of divergent haplotypes can evolve in a panmictic population only if they coexist for a considerable time. A single locus under NFDS is enough to maintain a haploblock comprising the region of the genome around it. By contrast, if variation is maintained by AOD, it is more likely that selection against recessive mutations acts at a number of tightly linked loci (Gilbert et al., 2020). Long coexistence of diverged haplotypes that comprise a haploblock enables accumulation of co-adapted combinations of nonsynonymous alleles within them. Thus, it is not surprising that a pronounced excess of LD_nonsyn over LD_syn in S. commune is observed primarily within haploblocks and that this excess is higher in genes with higher p_n/p_s.

Correlated LDs in two populations

Although a high excess of LD_nonsyn is observed only within haploblocks, a signature of epistasis can also be seen outside of them in the form of a correlation between LDs in the two populations. This correlation can be high even if LDs by themselves are low.

The USA and the Russian populations share a large proportion of their SNPs. Given the high divergence between the two populations, few such shared SNPs are expected to have common origin in the ancestral population, and instead they are likely to have arisen from recurrent mutation. Since the haploblocks show little correlation between the two populations, we assume that they arose after their divergence. The high prevalence of coincident SNPs is not surprising because SNPs comprise 0.28 and 0.13 of all the aligned nucleotide sites in the USA and Russian populations, respectively (Baranova et al., 2015, Appendix 3—figure 2). We identified pairs of shared biallelic SNPs located within 2 kb from one another and calculated the LD between them in both populations. To avoid the effects of strong within-population linkage and the occasional co-ocсurrence of haploblocks between populations, we excluded SNPs located within haploblocks or within genes under high LD (>0.8 LD quantile for the corresponding population) in either population.

The values of LD in the two populations are strongly correlated only for pairs of nonsynonymous SNPs located within the same gene, and only if both populations carry the same pairs of amino acids in the same sites (Figure 4). The correlation of LDs is the strongest if shared SNPs carry the same pairs of nucleotides, but is also observed if they encode the same amino acids by different nucleotides (Figure 4—figure supplement 1). The contrast between correlations within pairs of sites that reside in the same vs. different genes and the correlation of LDs observed for different nucleotides encoding the same amino acid cannot be explained by inheritance of LD from the common ancestral population. Moreover, synonymous SNPs are expected to be on average older than nonsynonymous ones, so that this mechanism should lead to a higher correlation of LDs for pairs of synonymous mutations. Thus, the observed pattern indicates that epistatic selection is shared between the two populations.

Figure 4.

Correlation of LD values between pairs of shared SNPs in the two S. commune populations.

(A) Pairs of SNPs with the same alleles in both sites, (B) pairs of SNPs differing by at least one allele. Asterisks indicate Spearman correlation p-values <0.001.

Figure 4—figure supplement 1.

Association of LD values between pairs of shared nonsynonymous SNPs encoding the same amino acids in the two S. commune populations.

(A) All pairs of SNPs pooled together. Pair of SNPs is considered to carry different alleles if at least one allele differs in at least one site. (B) Pairs of SNPs stratified by distance between them. Asterisks indicate Spearman correlation p-values <0.01.

Figure 4—figure supplement 2.

Association of LD values between pairs of shared SNPs within haploblocks in the two S. commune populations.

(A) Pairs of SNPs with the same major and minor alleles in both sites, (B) pairs of SNPs differing by at least one allele. Asterisks indicate Spearman correlation p-values <0.001.

The correlation of LDs between SNPs located within haploblocks in both populations is high regardless of whether they reside in the same or different genes, apparently because of occasional coincidence of haploblocks between populations (Figure 4—figure supplement 2).

Discussion

On top of its most salient property, an exceptionally high π, genetic variation within S. commune possesses two other pervasive features. The first is a high prevalence of mostly short haploblocks, genome segments comprising two or occasionally three distinct haplotypes, which is a signature of balancing selection. The overall fraction of the genome covered by haploblocks is ~10%, which is about an order of magnitude higher than the fraction covered by detectable signatures of balancing selection in genomes of other species (DeGiorgio et al., 2014; Leffler et al., 2013; Rasmussen et al., 2014).

The second feature is the excessive attraction between nonsynonymous alleles polarized by frequency. This pattern is much stronger within haploblocks, indicating that they were shaped by both balancing and epistatic selection, so that amino acids common within a haplotype together confer a higher fitness. Polymorphisms that involve haplotypes that comprise many interacting genes, such as inversions (Charlesworth and Charlesworth, 1973; Dobzhansky and Pavlovsky, 1958; Singh, 2008; Sturtevant and Mather, 1938) and supergenes (Joron et al., 2011; Mather, 1950), are known from the dawn of population genetics, but here we are dealing with an analogous phenomenon at a much finer scale, because haploblocks are typically shorter than genes. Thus, instead of coadapted gene complexes (Dobzhansky and Pavlovsky, 1958), haplotypes represent coadaptive site complexes within genes.

In our simulations, equally high fitnesses of two or more genotypes was a necessary condition for a large excess of LD_nonsyn, because otherwise the polymorphism did not live long enough for any substantial LD to evolve. However, epistasis between loci responsible for real or apparent balancing selection and those involved in compensatory interactions probably abolished the need for this fine-tuning of fitnesses. For example, if each haploblock carries its own complement of partially recessive deleterious mutations, together with alleles engaged in compensatory interactions with each other which also make these recessive mutations less deleterious, AOD can be expected to cause stable coexistence of these alleles.

Why are haploblocks and positive LD between minor nonsynonymous alleles so common in S. commune, but not in other, less polymorphic, species? There may be several, not mutually exclusive, reasons for this. Regarding haploblocks, real or apparent balancing selection may be more common in S. commune due to its higher polymorphism. Also, the same balancing selection may protect polymorphism in a huge population of S. commune, but not in populations with lower N_e. Finally, an excess of haploblocks in S. commune may be at least due to better detection of signatures of balancing selection in a species with an extraordinary density of SNPs.

Excessive LD_nonsyn in S. commune is also likely to be due to its hyperpolymorphism which increases the probability that mutually compensating alleles at a pair of interacting sites achieve high frequency and encounter each other in the same haplotype before being eliminated by selection. In other words, even if the fitness landscape remains the same, it results in more epistatic selection and, thus, in stronger LD in a species whose genetic variation covers a larger chunk of this landscape (Figure 1).

In a vast majority of species, π is a small parameter <<1. This imposes a severe constraint on operation of selection and obscures signatures of its particular modes. Thus, hyperpolymorphic species where π is ~1 provide a unique opportunity to study phenomena which are traditionally viewed as belonging to the domain of macroevolution through data on within-population variation.

Materials and methods

S. commune sampling, sequencing, and assembly

Haploid cultures of 24 isolates, each originated from a single haplospore, were obtained from fruit bodies collected in Ann Arbor, MI, USA by T. James and A. Kondrashov and in Moscow and Kostroma regions, Russia by A. Kondrashov, A. Baykalova and T. Neretina in 2009–2015. Specimen vouchers are stored in the White Sea Branch of Zoological Museum of Moscow State University (WS). Herbarium numbers are listed in Appendix 3—table 2. To obtain isolates, wild fruit bodies were hung on the top lid of a 10 cm petri dish with agar medium. Petri dish was set at an angle of 60–70 degrees to the horizontal surface for 32 hr. A germinated spore was excised together with a square-shaped fragment (approximately 0.7 × 0.7 mm) of the medium from the maximally rarefied area of the obtained spore print under a stereomicroscope with 100 x magnification. The obtained isolates were cultured in Petri dishes on 2% malt extract agar for a week. For storage, cultures were subcultured into 1.5 ml microcentrifuge tubes with 2% malt extract agar. To obtain sufficient biomass for DNA isolation, isolates were cultured in 20 ml 0.5% malt extract liquid medium in 50 ml microcentrifuge tubes in a horizontal position on a shaker at 100 rpm in daylight for 5–10 days. The tubes with the cultures were then centrifuged at 4000 rpm, and the supernatant was decanted. The resulting mycelium was lyophilized. DNA was extracted using Diamond DNA kit according to the manufacturer’s recommendations.

DNA libraries were constructed using the NEBNext Ultra II DNA Library Prep Kit kit by New England Biolabs (NEB) and the NEBNext Multiplex Oligos for Illumina (Index Primers Set 1) by NEB following the manufacturer’s protocol. The samples were amplified using 10 cycles of PCR. The constructed libraries were sequenced on Illumina NextSeq500 with paired-end read length of 151. The genomes were assembled de novo using SPAdes (v3.6.0) (Bankevich et al., 2012); possible contaminations were removed using blobology (Kumar et al., 2013). Average N50 was ~165 kb for USA samples and ~70 kb for Russian samples (assembly statistics are provided in Appendix 3—table 2).

Together with the 30 samples sequenced previously (Baranova et al., 2015; Bezmenova et al., 2020), the obtained haploid genomes were aligned with TBA and multiz (Blanchette et al., 2004) and projected onto the reference scaffolds (Ohm et al., 2010). Ortholog sequences were extracted on the basis of the reference genome annotation (Ohm et al., 2010) and realigned using macse codon-based aligner (Ranwez et al., 2011). The alignments are available at https://makarich.fbb.msu.ru/astolyarova/schizophyllum_data/. Only the gap-free columns of the whole-genome alignment and the orthologs that were found in all 55 genomes were used for analysis. The total number of detected SNPs was 5.8 million for the USA population (82% of them biallelic) and 2.7 million for the Russian population (93% biallelic). 25% of the USA SNPs were shared with the Russian population (11% with the same major and minor alleles), and 53% of the Russian SNPs were shared with the USA population (23% with the same major and minor alleles, Appendix 3—figure 1).

The phylogeny of the sequenced genomes was reconstructed with RAxML (Stamatakis, 2014; Appendix 3—figure 2). Nucleotide diversity (π) was estimated as the average frequency of pairwise nucleotide differences; π for different classes of sites is shown in Appendix 3—figure 1B. Two samples from Florida (USA population) were excluded from the further analysis to minimize the possible effect of population structure.

Genome sequence data are deposited at DDBJ/ENA/GenBank under accession numbers JAGVRL000000000-JAGVSI000000000, BioProject PRJNA720428. Sequencing data are deposited at SRA with accession numbers SRR14467839-SRR14467862.

Data on H. sapiens and D. melanogaster populations

We used polymorphism data from 347 phased diploid human genomes from African and 301 genomes from European super-populations sequenced as part of the 1000 Genomes project (1,000 1000 Genomes Project Consortium et al., 2015). If several individuals from the same family were sequenced, we included only one of them. As a D. melanogaster dataset, we used 197 haploid genomes from the Zambia population (Lack et al., 2015). Only autosomes were analyzed in both datasets.

Estimation of LD

As a measure of linkage disequilibrium between two biallelic sites, we used r², calculated as follows:

$r^2=\frac{\left(p\right(AB)-p(A\left)p\right(B\left)\right){}^2}{p\left(A\right)(1-p(A\left)\right)p\left(B\right)(1-p(B\left)\right)}$, where p(A) and p(B) are the minor allele frequencies at these sites, and p(AB) is the frequency of the genotype that carries minor alleles at both sites.

Singletons (sites with minor allele present only in one genotype) were excluded from the analysis if not stated otherwise.

Haploblocks annotation

In order to annotate the haploblocks, we calculated LD along the S. commune genome in a sliding window of 250 nucleotides with a step of 20 nucleotides (only non-singleton SNPs are analyzed; the windows with less than 10 SNPs were excluded). Any continuous sequence of overlapping windows with r² larger than the threshold value was merged together in a haploblock. The LD threshold value was defined independently for each S. commune population as the heavy tail of the within-window LD distribution, as compared with the lognormal distribution with the same mean and variance as in the data (Figure 3—figure supplement 7).

Estimation of LD between physically interacting amino acid sites

Of 16,319 annotated protein-coding genes of S. commune (Ohm et al., 2010) 9,941 were found in all 55 aligned genomes. We blasted the protein sequences of these orthologous groups against the PDB database of protein structures. About 52% of them (5,188) had a match (e-value threshold = 1e-5) amongst the proteins with the known structure. We realigned the sequences of S. commune protein and the matching PDB protein with clustal and calculated within-population LD and physical distance (Å) for each pair of aligned positions. A pair of amino acid sites was considered physically adjacent if they were located within 10 Å from each other.

To compare LD between pairs of physically close and distant sites, we used the controlled permutation test (Figure 2A): for each pair of physically close amino acid sites (within 10 Å) we sampled a pair of physically distant amino acids on the same genetic distance (measured in aa). Pairs of sites closer than 5 aa were excluded from the analysis.

To examine LD patterns within individual protein structures, we calculated contingency tables of pairs of SNPs being located in codons encoding physically close amino acids and having high LD (no less than 90% quantile for a given gene). Pairs of amino acid sites located closer than 30 aa or more distant than 100 aa from each other were excluded; genes with less than five pairs of physically close sites under high or low LD were also excluded. From these contingency tables, we calculated the odds ratio (OR) and chi-square test p-value for each gene. p-values were adjusted using BH correction. We identified 22 genes with pairs of adjacent sites having significantly higher LD in the USA population (out of 1286 eligible genes in total), and 87 genes in the Russian population (out of 967) under 5% FDR (Appendix 3—table 1). Examples of such genes are shown in Figure 2 and Figure 2—figure supplements 1 and 2.

Simulations of epistasis

To simulate evolution of populations with or without epistasis and balancing selection (Figure 3—figure supplement 6), we used an individual-based model implemented by SLiM (Haller and Messer, 2019). Simulations were performed with diploid population size N=1000 and recombination rate 0. To achieve the level of genetic diversity π similar to S. commune, mutation rate μ was scaled as μ=π/2N=5e-5. The length of the simulated sequence was 100 nt. Each simulation started with a monomorphic population and proceeds for 100 N generations. For calculations of synonymous and nonsynonymous LD, random 100 haploid genotypes were sampled from the population. Only SNPs with minor allele frequency >5% in the sample were analyzed.

We modelled two types of mutations, depending on whether they are neutral (with selection coefficient s_syn = 0) or weakly deleterious (s_nonsyn ≤0), representing synonymous and nonsynonymous variants correspondingly. There were twice as many nonsynonymous as synonymous sites. Under the non-epistatic model, s was independent of the genetic background. We assumed s_nonsyn = −0.01 with the dominance coefficient h of 0.5.

Under the pairwise positive epistasis model, we assumed that one nonsynonymous mutation can be partially or fully compensated by a mutation at another site. In this model, all nonsynonymous sites were split into pairs. Each mutation of a pair individually occurring within a genotype was assumed to be deleterious, with selection coefficient s_nonsyn = −0.01; however, the fitness of the double mutant is larger than expected under the additive (non-epistatic) model. We used several models of epistasis, with different strengths of epistasis strength and landscape shapes (Figure 3—figure supplement 6).

In the NFDS model of balancing selection, a single mutation at a random position was subjected to frequency-dependent selection (so that it is positively selected at frequencies below 0.5, and negatively selected at frequencies above 0.5). In the AOD model, mutations in 10 random positions were fully recessive (h=0) and weakly deleterious (s=−0.0025).

To simulate evolution of populations with different levels of genetic diversity under epistasis (Appendix 1), we used FFPopSim (Zanini and Neher, 2012). To achieve different levels of genetic diversity π, mutation rate μ was scaled as μ=π/2N. The calculations were performed the same way as in SLiM, but In this case, we used haploid population size N=2000, population-scaled recombination rate 0.01 and the simulated sequence length of 300 nucleotides.