Reconstruction and evolutionary history of eutherian chromosomes

Abstract

Whole-genome assemblies of 19 placental mammals and two outgroup species were used to reconstruct the order and orientation of syntenic fragments in chromosomes of the eutherian ancestor and six other descendant ancestors leading to human. For ancestral chromosome reconstructions, we developed an algorithm (DESCHRAMBLER) that probabilistically determines the adjacencies of syntenic fragments using chromosome-scale and fragmented genome assemblies. The reconstructed chromosomes of the eutherian, boreoeutherian, and euarchontoglires ancestor each included >80% of the entire length of the human genome, whereas reconstructed chromosomes of the most recent common ancestor of simians, catarrhini, great apes, and humans and chimpanzees included >90% of human genome sequence. These high-coverage reconstructions permitted reliable identification of chromosomal rearrangements over ∼105 My of eutherian evolution. Orangutan was found to have eight chromosomes that were completely conserved in homologous sequence order and orientation with the eutherian ancestor, the largest number for any species. Ruminant artiodactyls had the highest frequency of intrachromosomal rearrangements, and interchromosomal rearrangements dominated in murid rodents. A total of 162 chromosomal breakpoints in evolution of the eutherian ancestral genome to the human genome were identified; however, the rate of rearrangements was significantly lower (0.80/My) during the first ∼60 My of eutherian evolution, then increased to greater than 2.0/My along the five primate lineages studied. Our results significantly expand knowledge of eutherian genome evolution and will facilitate greater understanding of the role of chromosome rearrangements in adaptation, speciation, and the etiology of inherited and spontaneously occurring diseases. chromosome evolution | ancestral genome reconstruction | genome rearrangements C hromosome rearrangements are a hallmark of genome evolution and essential for understanding the mechanisms of speciation and adaptation (1). Determining chromosome rearrangements over evolutionary time scales has been a difficult problem, primarily because of the lack of high-quality, chromosome-scale genome assemblies that are necessary for reliable reconstruction of ancestral genomes. For closely related species with good map-anchored assemblies, such as human, chimpanzee, and rhesus, it is possible to infer most inversions, translocations, fusions, and fissions that occurred during evolution by simple observational comparisons (2). However, for sequence-based genome-wide comparisons that require resolving large numbers of rearrangements of varying scale, determining ancestral chromosomal states is challenging both methodologically and computationally because of the complexity of genomic events that have led to extant genome organizations, including duplications, deletions, and reuse of evolutionary breakpoint regions (EBRs) flanking regions of homologous synteny (3, 4).A variety of methods have been used for resolving the evolutionary historie...

Genome diversity and species richness in mammals

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