2.4
Synteny and Evolution
John H. Renwick first coined the term “synteny” in 1971, which refers to the genes present on the same chromosomes, even if they are not genetically l…
Synteny in classical terms refers to the genes present on the same chromosome, like beads on the same string.
For example, consider species X, which has genes A, B, and C present on one chromosome. These genes are said to be syntenic in species X.
Then consider another species, Y, with alleles A1, B1, and C1 on another chromosome. These genes are also said to be syntenic in species Y.
In evolutionary perspective, synteny refers to the phenomenon of co-localization of genes on a chromosome of two or more different species.
Therefore, the genes A and A1, B and B1 and C and C1 represent a synteny between the two species, X and Y.
In addition, the genes present in common chromosomal regions in two or more species, like the genes A, B and C, represent a conserved syntenic segment or conserved syntenic blocks.
During the course of evolution, species undergo random mutations, survive natural selection and evolve into several lineages all while mostly maintaining conserved syntenic blocks over many chromosomes.
For example, around 85 million years ago, humans and tree shrews shared a common mammalian ancestor. Even after millions of years of divergence, several genes present on the long arm of human chromosome 10 and the tree shrew’s chromosome 16 show conserved synteny in some regions.
This indicates that an ancestral mammalian chromosome evolved into chromosome 16 in the tree shrew, and also the long arm of chromosome 10 in humans.
Based on synteny analysis and high resolution DNA data from all known modern primates, scientists have proposed a karyotype for the common ancestor of all of these species.
The matching colors shown in the human and ancestral chromosomes indicate the conserved syntenic blocks and ancestral chromosomes from which human chromosomes evolved.
These ancestral chromosomes would have undergone several rounds of chromosome rearrangement, fusion or breakage, facilitating diversity and speciation into the primate species of today.
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Q1: What does synteny mean in genetics?
Synteny refers to genes present on the same chromosome, like beads on a string. In evolutionary perspective, it describes the co-localization of genes on chromosomes across two or more different species. For example, genes A, B, and C on one species' chromosome correspond to alleles A1, B1, and C1 on another species' chromosome, representing synteny between those species.
Q2: How do conserved syntenic blocks reveal evolutionary relationships?
Conserved syntenic blocks are genes present in common chromosomal regions across multiple species, indicating shared ancestry. Humans and tree shrews, which diverged 85 million years ago, show conserved synteny in some regions of human chromosome 10 and tree shrew chromosome 16. This pattern demonstrates that an ancestral mammalian chromosome evolved into both modern chromosomes, revealing evolutionary relationships through genome comparisons.
Q3: What chromosomal events created diversity among primate species?
Ancestral chromosomes underwent several rounds of chromosome rearrangement, fusion, and breakage to facilitate diversity and speciation in primates. For example, the fusion of ancestral chromosomes 9 and 11 formed human chromosome 2, while reciprocal translocation of ancestral chromosomes 14 and 21 led to human chromosomes 12 and 22. These large-scale rearrangements occur rarely, approximately once every 5 million years.
Q4: How many chromosomal changes separated humans and mice from their common ancestor?
Approximately 180 chromosomal breakage-and-rejoining events occurred as ancestral chromosomes evolved into human and mouse chromosomes after their lineages diverged around 80 million years ago. Despite these extensive rearrangements, several chromosome regions in both species maintained common gene order or synteny. For instance, over 510,000 base pairs of mouse chromosome 12 share syntenic blocks with human chromosome 14.
Q5: How do scientists reconstruct ancestral primate chromosomes?
Scientists use synteny analysis combined with high-resolution DNA data from all known modern primates to propose ancestral karyotypes. By comparing conserved sequences and evolutionary relationships across primate species, researchers identified which ancestral chromosomes gave rise to modern chromosomes. Matching colors in human and ancestral chromosome diagrams indicate conserved syntenic blocks derived from specific ancestral chromosomes.
Q6: Why does synteny decrease as evolutionary distance between species increases?
As evolutionary time increases between species, more chromosomal breakage-and-rejoining events accumulate, disrupting the original gene order. Over millions of years, these rearrangements gradually eliminate conserved syntenic blocks. Therefore, species that diverged more recently show greater synteny, while distantly related species show less synteny due to accumulated chromosomal changes.
Q7: What role did John Renwick play in synteny research?
John Renwick coined the term synteny in 1971, defining it as genes present on the same chromosomes, even if they are not genetically linked. His terminology enabled scientists to describe evolutionary relationships between species based on gene organization. Today, synteny analysis remains a fundamental tool for understanding how chromosomes evolved and how species are related through their genomic architecture.