The genetic architecture of gene expression levels in wild baboons

Figure 1 and 3 should not be main figures. The pipeline should be moved to the supplement. Also the ASE and eQTL effect sizes is a general technical issue and does not relate specifically to the study of the genetics of gene expression in baboons. The text around these sections is long and generally distracting.

We have removed the original Figure 1 from the manuscript, as it provided an overview version of the more detailed pipelines provided in the original Figure 1–figure supplements 1-6. These supplements are now attached to the original Figure 2 (now Figure 1), along with other figures that were previously included as Figure 1’s supplements. The plots in former Figure 3 have also been changed to Figure 1 supplements and removed from the main text. In addition, we have streamlined our discussion of the power to detect ASE versus eQTL in the main text, largely removing this entire section.

There is a comment regarding the lack of an association between effect size and minor allele frequency compared to Battle et al., 2014. However, Battle et al. had 922 individuals compared to 66 individuals here (among several technical differences). We are not sure that these studies can be easily compared to provide a definitive statement regarding negative selection in baboons here. A caveat regarding study differences might suffice.

We agree and have added such a caveat to this section of the Results.

The authors bring up the possibility of admixture in the baboon population. This is slightly concerning as it could increase the number of hets and has the potential to increase the length of haplotypes (both observations of the study). It further could explain many of the observations of the study. Three possible suggestions to address this are to test whether: (i) one sees an excess of trans-associations on the same chromosome compared to across chromosomes for cis-eQTL in baboons versus YRI, (ii) apply a surrogate variable or hidden factor correction to eQTL analysis on a single chromosome or (iii), our reviewers’ favorite, test if ASE is correlated over a longer distance (more independent genes) than in humans—in this case, since there is no biological basis for why the locations of causal variants should be further away from the genes they regulate between baboons and humans, the decay of the correlation of ASE measured in proximal genes should be similar. We realize the authors have a model to control for individual relatedness and population structure, but this is derived from the entire genetic data set and does not address local patterns of admixture.

Thanks for these helpful suggestions. We agree that the evolutionary history of this population could have influenced our findings. In the revised manuscript, we investigate this possibility in substantially greater detail (eleventh paragraph in the Results section and under the heading “Testing the contribution of admixture to eQTL detection”, in a new Materials and methods section). Overall, we found little evidence that admixture drove our results. These conclusions are based on three sets of findings:

1) Distance between sites tested for ASE predicts the magnitude of the difference between ASE estimates, as expected; however, this relationship does not differ between baboons and YRI (new Figure 1–figure supplement 10). This analysis, motivated by the reviewers’ third suggestion, suggests that ASE is not correlated over a longer distance in baboons than in humans (note that we could not directly test how correlations between ASE estimates decay with distance because our ASE estimates are site-specific, not individual-specific).

2) Controlling for local structure in addition to global ancestry (suggestion 2) modestly reduces the number of eQTL discoveries in the baboon data set, but not more so than the same procedure in YRI. When controlling for local structure using the top two principal components for variants on the same chromosome, the number of detectable eQTL drops from 1787 to 1583. However, this number is still 5.4-fold larger than the number detected in the YRI. A drop also occurs when running a parallel model in YRI (from 290 to 216), so that the number of eQTL detected in baboons is ∼7-fold larger when controls for local structure are used in both. Thus, more extensive local structure does not appear to explain the increased power in baboons.

3) In support of this idea, the spatial distribution of baboon eQTLs relative to genes is almost identical between the models with and without local structure controls (new Figure 1–figure supplement 11). If admixture drove most of the signal in the data set, we would expect to observe greater enrichment of eQTL within or near genes when adding controls for local structure; we do not.

The gene expression data was quantile normalized. Why was a hidden factor correction not applied? Typically, these types of corrections dramatically improve eQTL discovery. Our concern is if there is some structure to the data that is both present and correlated in genotype and gene expression space, the number discoveries will be artificially inflated.

This comment reflects our mistake in writing the original version of our Methods. We indeed corrected for hidden factors by regressing out the first 10 principal components of the overall gene expression data. As the reviewers note, this process greatly improved our ability to detect eQTL and minimized the possibility that the eQTL we detected reflect global structure in gene expression space (we did the same for the YRI data, so our methods remained comparable). We explain our procedure in the revised Methods (tenth paragraph) and have also added a figure supplement (new Figure 1–figure supplement 12) showing the relationship between the number of PCs we removed and eQTL detection.

For ASE analyses: (i) the authors assume no recombination, this is not stated, (ii) how is beta in theta∼beta (alpha, beta) estimated, and (iii) detection of ASE correlates with expression level (Figure 3D), this is not a surprise, but given their model we are concerned whether this estimate is more extreme, because ASE has different variances in effect size when it is estimated from a few individuals for very highly expressed genes (2 het individuals with 150 reads = 300 total) compared to lots of estimates from intermediately expressed genes (10 het individuals with 30 reads = 300 total). For robustness, the authors should show whether detection of ASE in their study is independent of the number of input individuals once a testable site has been selected using their criterion.

With regards to (i), we did not make any explicit assumptions about recombination in the ASE analysis. Recombination between the exonic SNPs we used to test for ASE and true causal regulatory SNPs would decrease our power to detect ASE; we make this point clear in the revision (in the subsection “ASE detection” of the Materials and methods). For (ii), we used a maximum likelihood approach to estimate both the alpha and beta parameters in the beta component of the beta-binomial model; we have clarified this point in the same subsection of the Materials and methods of the revised manuscript. To address point (iii), we have now tested whether ASE is more likely to be detected for sites with more heterozygous individuals, conditional on total read depth (overall, number of hets and total read depth are correlated: r = 0.266, p < 10^-100). We find no evidence for such an effect across the ten deciles of total read depth values we tested (please see the aforementioned subsection and Figure 1–figure supplement 14).

The authors should discuss the possibility that the negative correlation between conservation and probability of eQTL in a gene in baboons at least may be driven by the technical issue that only coding SNPs were tested and therefore conserved genes will tend to have low MAF and therefore low power.

Thanks for pointing out this possible interpretation. We have now calculated the correlation between levels of conservation and average minor allele frequency for each gene, for SNPs used in our analysis. The correlation between a gene’s average phyloP score (from the 46-way primate comparison) and the average MAF of all SNPs tested in association with that gene is nominally significant (p = 0.002) but explains a very small fraction of the variance (r = -0.037). Thus, while more conserved genes do tend to have lower average MAFs (among SNPs tested), this relationship is weak, probably because we filter out sites with very low MAFs and because many sites occur in non-protein coding regions of the transcript or in the transcribed regions of other genes. Further, we observe no significant correlation between a gene’s Homologene score and average minor allele frequency among SNPs tested (p = 0.38). These analyses suggest that the relationship between conservation and eQTL discovery is probably not driven by a relationship between conservation and MAF (at least within the data set we ultimately analyzed). We have discussed this point in the revised manuscript (under the heading “Mixed evidence for nature selectin of gene expression levels”, in the Results section). We also cite a recent paper by Popadin et al. (third paragraph of the Discussion) that also suggests that fewer cis-eQTL are found in older genes, similar to our findings for the Homologene analysis.

The authors indicate a large component of expression variability is in trans. Is trans defined as on other chromosomes? In particular, the authors should clarify what goes into the p_trans matrix.

Because the number of sites that are included in p_trans is almost equal to p (the total number of sites in the genome), we calculated the p_trans matrix based on all SNPs typed. However, in the revision we also have run p_trans based on all other chromosomes except the chromosome containing the focal gene. These results are almost identical to the results obtained when using all SNPs; we include them as a new supplemental figure (please see the subsection “Estimation of genetic contributions to gene expression”, in the Materials and methods, and new Figure 4–figure supplement 3).