2.6
Gene Duplication and Divergence
The seminal work of Ohno in 1970 popularized the idea of gene duplication and divergence. DNA sequence comparison studies reveal that a large portion…
Gene duplication is a process where a DNA region coding for a gene duplicates, making additional copies of itself within the same genome. These duplicated copies of the gene - called paralogs can later mutate and diverge in one of the following ways.
The first is formation of the pseudogenes. Here, one of the gene paralogs may acquire deleterious mutations and turn into a nonfunctional copy called a pseudogene.
The second is sub-functionalization where both the paralogs acquire mutations in different protein coding domains or exons, thus partitioning the original gene function between them. However, the protein products of the two paralogous genes complement each other and exhibit the original gene function.
For example, in primitive fish and marine animals, a single chain globin protein served as the oxygen carrying molecule in the blood.
During the course of evolution, the globin gene duplicated and sub functionalized into two slightly different genes coding for α- and β-globin proteins, that associate to form the hemoglobin molecule with 4 subunits found in most present day vertebrates.
The third is Neo-functionalization. Here, one paralog acquires novel, advantageous mutations that can lead to the evolution of a new gene. In contrast, the other paralog retains the original function.
For example, the human β-globin gene duplicated and acquired mutations to produce a new gene called fetal β-globin that is expressed exclusively in the human fetus. However, soon after birth, the β-globin gene takes over production of the β-globin proteins.
The evolution of tricolor vision in humans is another interesting example of neofunctionalization. Much before the evolution of modern apes, the early primates had dichromatic vision due to the presence of the Blue and Green opsin genes.
Later on, the Green opsin gene duplicated and neo-functionalized into a novel red opsin gene.
Therefore, the species which evolved after the duplication event, such as the old world monkeys, apes, and humans have three opsin genes which impart tricolor vision.
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Q1: What is gene duplication and how does it occur in genomes?
Gene duplication is a process where a DNA region coding for a gene duplicates, creating additional copies within the same genome. Duplication can arise through unequal crossing over during meiosis, replication slippage where DNA polymerase dissociates and realigns incorrectly, or retrotransposition where mRNA is reverse transcribed into DNA copies called retrogenes that reinsert into the genome.
Q2: What are paralogs and how do they relate to gene families?
Paralogs are duplicated copies of genes with similar sequences and functions. When paralogs share enough similarity, they form a gene family. Gene families are widespread across species; for example, the trypsin gene family in D. melanogaster has over 111 members, while the olfactory receptor gene family in mammals contains around 1000 member genes.
Q3: How does sub-functionalization differ from neo-functionalization after gene duplication?
Sub-functionalization occurs when both paralogs acquire mutations in different protein domains, partitioning the original gene function between them while their protein products complement each other. Neo-functionalization happens when one paralog acquires novel mutations leading to a new gene function, while the other retains the original function, as seen with fetal β-globin evolution.
Q4: What is a pseudogene and how does it form from gene duplication?
A pseudogene is a nonfunctional copy of a gene that forms when one paralog acquires deleterious mutations after duplication. Pseudogenes commonly arise from retrotransposition, where inserted retrogenes lack promoters and regulatory elements necessary for transcription, causing them to lose function over time.
Q5: How did hemoglobin evolution demonstrate sub-functionalization in vertebrates?
In primitive fish and marine animals, a single globin gene encoded oxygen-carrying proteins. During evolution, the globin gene duplicated and sub-functionalized into α-globin and β-globin genes. These proteins associate to form hemoglobin with four subunits found in modern vertebrates, with each protein complementing the other to maintain oxygen transport function.
Q6: What role did gene duplication play in the evolution of primate color vision?
Early primates had dichromatic vision from Blue and Green opsin genes. The Green opsin gene later duplicated and neo-functionalized into a novel Red opsin gene. Species evolving after this duplication, including old world monkeys, apes, and humans, gained three opsin genes enabling tricolor vision.
Q7: What is whole-genome duplication and how does it differ from single gene duplication?
Whole-genome duplication occurs when entire chromosomes or genomes duplicate, often through chromosome segregation failures during meiosis. Unlike single gene duplication, this creates multiple copies of all genes simultaneously. For example, wheat genomes have duplicated six times, creating a hexaploid organism with multiple chromosome sets.