High-resolution comparative analysis of great ape genomes
Most nonhuman primate genomes generated to date have been “humanized” owing to their many gaps and the reliance on guidance by the reference human genome. To remove this humanizing effect, Kronenberg et al. generated and assembled long-read genomes of a chimpanzee, an orangutan, and two humans and compared them with a previously generated gorilla genome. This analysis recognized genomic structural variation specific to humans and particular ape lineages. Comparisons between human and chimpanzee cerebral organoids showed down-regulation of the expression of specific genes in humans, relative to chimpanzees, related to noncoding variation identified in this analysis.
INTRODUCTION
Understanding the genetic differences that make us human is a long-standing endeavor that requires the comprehensive discovery and comparison of all forms of genetic variation within great ape lineages.
RATIONALE
The varied quality and completeness of ape genomes have limited comparative genetic analyses. To eliminate this contiguity and quality disparity, we generated human and nonhuman ape genome assemblies without the guidance of the human reference genome. These new genome assemblies enable both coarse and fine-scale comparative genomic studies.
RESULTS
We sequenced and assembled two human, one chimpanzee, and one orangutan genome using high-coverage (>65x) single-molecule, real-time (SMRT) long-read sequencing technology. We also sequenced more than 500,000 full-length complementary DNA samples from induced pluripotent stem cells to construct de novo gene models, increasing our knowledge of transcript diversity in each ape lineage. The new nonhuman ape genome assemblies improve gene annotation and genomic contiguity (by 30- to 500-fold), resulting in the identification of larger synteny blocks (by 22- to 74-fold) when compared to earlier assemblies. Including the latest gorilla genome, we now estimate that 83% of the ape genomes can be compared in a multiple sequence alignment.We observe a modest increase in single-nucleotide variant divergence compared to previous genome analyses and estimate that 36% of human autosomal DNA is subject to incomplete lineage sorting. We fully resolve most common repeat differences, including full-length retrotransposons such as the African ape-specific endogenous retroviral element PtERV1. We show that the spread of this element independently in the gorilla and chimpanzee lineage likely resulted from a founder element that failed to segregate to the human lineage because of incomplete lineage sorting.The improved sequence contiguity allowed a more systematic discovery of structural variation (>50 base pairs in length) (see the figure). We detected 614,186 ape deletions, insertions, and inversions, assigning each to specific ape lineages. Unbiased genome scaffolding (optical maps, bacterial artificial chromosome sequencing, and fluorescence in situ hybridization) led to the discovery of large, unknown complex inversions in gene-rich regions. Of the 17,789 fixed human-specific insertions and deletions, we focus on those of potential functional effect. We identify 90 that are predicted to disrupt genes and an additional 643 that likely affect regulatory regions, more than doubling the number of human-specific deletions that remove regulatory sequence in the human lineage. We investigate the association of structural variation with changes in human-chimpanzee brain gene expression using cerebral organoids as a proxy for expression differences. Genes associated with fixed structural variants (SVs) show a pattern of down-regulation in human radial glial neural progenitors, whereas human-specific duplications are associated with up-regulated genes in human radial glial and excitatory neurons (see the figure).
CONCLUSION
The improved ape genome assemblies provide the most comprehensive view to date of intermediate-size structural variation and highlight several dozen genes associated with structural variation and brain-expression differences between humans and chimpanzees. These new references will provide a stepping stone for the completion of great ape genomes at a quality commensurate with the human reference genome and, ultimately, an understanding of the genetic differences that make us human.
ABSTRACT
Genetic studies of human evolution require high-quality contiguous ape genome assemblies that are not guided by the human reference. We coupled long-read sequence assembly and full-length complementary DNA sequencing with a multiplatform scaffolding approach to produce ab initio chimpanzee and orangutan genome assemblies. By comparing these with two long-read de novo human genome assemblies and a gorilla genome assembly, we characterized lineage-specific and shared great ape genetic variation ranging from single– to mega–base pair–sized variants. We identified ~17,000 fixed human-specific structural variants identifying genic and putative regulatory changes that have emerged in humans since divergence from nonhuman apes. Interestingly, these variants are enriched near genes that are down-regulated in human compared to chimpanzee cerebral organoids, particularly in cells analogous to radial glial neural progenitors.
