Whole Genome Duplication (WGD) Explained: Mechanisms, Evolutionary Fate, 2R Hypothesis and Polyploidy

Table of Contents

• Introduction to Whole Genome Duplication
• Discovery of Whole Genome Duplication
• Mechanism of Gene Duplication and Whole-Genome Duplication
• Evolutionary Fate of Duplicated Genes
• Whole Genome Duplication in Evolution
• The 2R Hypothesis
• Occurrence of Whole Genome Duplication
• References

Introduction to Whole Genome Duplication

• Whole genome duplication (WGD), also known as polyploidy, is a genomic event in which an organism acquires additional complete copies of its entire genome.
• Polyploidy can result from non-disjunction during meiosis, when chromosomes fail to separate properly, producing cells with an abnormal chromosome number.
• The effects of whole genome duplication can vary: it may provide evolutionary advantages to an organism or, in some cases, have detrimental effects.
• Polyploidy is particularly common in plants, where it has played an important role in genome evolution and diversification.
• In animals, whole genome duplication has also occurred; evidence indicates that it happened at least twice during early vertebrate evolution.
• WGD produces multiple additional copies of genes across the genome. Over evolutionary time, many of these duplicated genes may be lost, allowing the genome to return toward its original or default state.
• However, some duplicated genes are retained and may acquire new functions or contribute to adaptive changes.
• The Hox genes are a notable example of genes that have been retained following genome duplication and subsequently contributed to evolutionary and adaptive changes.

Discovery of Whole Genome Duplication

• In 1997, Wolfe and Shields provided evidence for whole genome duplication in the genome of the yeast Saccharomyces cerevisiae.
• They identified numerous homologous gene sequences located in different regions of the yeast genome, including 32 pairs of homologous chromosomal regions.
• Based on these observations, they proposed that the S. cerevisiae genome had undergone an ancient whole genome duplication event after its evolutionary divergence from Kluyveromyces, a genus of ascomycetous yeasts.
• The yeast whole genome duplication event is estimated to have occurred approximately 100–110 million years ago (around 108 million years ago).
• Over evolutionary time, many of the duplicated genes were lost or became non-functional.
• The duplicated chromosomes subsequently underwent rearrangements and fragmentation, leaving behind the homologous chromosomal regions observed in the present-day yeast genome.

Mechanism of Gene Duplication and Whole-Genome Duplication

• Whole-genome duplication (WGD) generally occurs when chromosomes fail to segregate properly during cell division, resulting in a cell receiving an additional complete set of chromosomes.
• Abnormal chromosome segregation can occur because of errors during meiosis or mitosis.
• During meiosis, failure of chromosomes to separate properly (non-disjunction) can produce cells with abnormal chromosome numbers.
• Under appropriate biological conditions, these cells may develop further or fuse with other cells, allowing the organism to acquire additional complete copies of its genome, resulting in polyploidy.
• Unlike single-gene duplication, WGD duplicates the entire genome simultaneously, generating additional copies of numerous genes.
• Following duplication, these extra gene copies may be lost, become non-functional, or acquire new functions.

Major Mechanisms of Gene Duplication and Whole-Genome Duplication

1. Ectopic Recombination

• Ectopic recombination is a crossing-over event between homologous or similar DNA sequences located at different positions rather than at corresponding positions on homologous chromosomes.
• It can produce chromosomal rearrangements, which may be harmful to the organism.
• However, under certain conditions, ectopic recombination can be beneficial because it may result in gene duplication.
• Ectopic recombination can occur during both mitosis and meiosis.

2. Replication Slippage

• Slipped-strand mispairing (SSM), also called replication slippage, is a mutation mechanism that occurs during DNA replication.
• During replication, DNA polymerase may temporarily dissociate from the DNA template.
• When DNA polymerase reattaches, the newly synthesized strand may become misaligned with the template strand.
• This misalignment can cause a DNA sequence to be copied more than once, potentially producing a duplication.
• Replication slippage occurs particularly frequently in regions containing tandem repeats, which consist of repetitive nucleotide sequences.
• These repetitive regions are relatively unstable and can lead to insertions and deletions (indels).
• In some cases, this instability can contribute to the duplication of genes or gene segments.

3. Retrotransposition

• Retrotransposition is a mechanism in which mobile genetic elements or RNA-derived sequences are copied and inserted into new locations within the genome.
• During this process, an RNA molecule is reverse-transcribed into DNA, which is then integrated into another genomic location.
• Retrotransposable elements can therefore amplify themselves and become abundant in eukaryotic genomes.
• Retrotransposition can also generate retrogenes, which are gene copies derived from processed RNA.
• Retrogenes typically lack introns and may retain a poly(A) sequence originating from the RNA template.
• Because retrogenes can be inserted into new genomic environments, their gene regulation and expression patterns may differ from those of their original genes.
• These changes can sometimes contribute to the evolution of novel functions.

4. Polyploidy

• Polyploidy occurs when an organism possesses more than two complete sets of chromosomes.
• During meiosis, errors such as non-disjunction can cause homologous chromosomes or chromatids to fail to separate properly.
• This can result in cells or gametes containing additional sets of chromosomes.
• Under suitable conditions, the additional chromosome sets can become established in an organism, resulting in whole-genome duplication.
• Following WGD, the duplicated genome may initially be genetically and structurally unstable.
• Over evolutionary time, this can be accompanied by extensive gene loss, increased mutation, and changes in gene regulatory networks.

Evolutionary Fate of Duplicated Genes

• Following genome duplication, most additional gene copies are eventually lost through evolutionary processes.
• Some duplicated genes remain in the genome but accumulate mutations that make them non-functional, resulting in pseudogenes.
• Other duplicated genes are retained and may undergo neofunctionalization or subfunctionalization.
• These different evolutionary outcomes allow genome duplication to contribute to genetic diversity and adaptation.

Neofunctionalization

• Neofunctionalization occurs when one copy of a duplicated gene acquires a novel function while the other copy retains the original function.
• The duplicated copy can accumulate mutations more freely because the original gene continues to perform the essential function.
• Over successive generations, some mutations may alter the protein or its regulation and produce a new biological function.
• When the new function provides an advantage, natural selection can favor its retention.
• Changes in regulatory regions and protein features, including alterations in protein phosphorylation motifs, can contribute to the evolution of novel functions.

Subfunctionalization

• Subfunctionalization occurs when the functions of an ancestral gene become partitioned between two duplicated copies.
• Both copies may accumulate degenerative mutations, but each retains a subset of the original gene's functions.
• As a result, both gene copies may become necessary for the organism because neither copy alone performs the complete ancestral function.
• Unlike neofunctionalization, subfunctionalization generally does not require the evolution of an entirely new function.
• In some cases, retaining both copies can nevertheless provide an adaptive advantage by allowing more specialized or regulated gene functions.

Whole-Genome Duplication in Evolution

1. Genome Diversity

• Whole-genome duplication (WGD) can significantly increase an organism’s genomic and genetic diversity by creating additional copies of the entire genome.
• The increased number of gene copies can alter gene expression, cellular processes, and cell size, producing substantial changes at the cellular and organismal levels.
• Newly duplicated genes provide additional genetic material that can undergo mutation and evolutionary divergence without necessarily disrupting the function of the original gene copies.
• Over time, duplicated genes may be lost, become pseudogenes, or acquire new or specialized functions through processes such as neofunctionalization and subfunctionalization.
• Therefore, WGD can produce both short-term and long-term evolutionary effects, influencing how organisms respond to environmental conditions and selective pressures.

2. Speciation

• Polyploidy can contribute directly to speciation by producing organisms with chromosome numbers that differ from those of their parental or non-polyploid populations.
• Because of these differences in chromosome number and pairing during meiosis, polyploid organisms may have reduced fertility or reproductive compatibility with their non-polyploid relatives.
• This reduction in gene flow can create reproductive isolation, an important step in the formation of a new species.
• Following polyploidization, duplicated genes may undergo various evolutionary changes, including gene deletion, pseudogenization, neofunctionalization, and subfunctionalization.
• These genetic changes can produce new phenotypic and adaptive traits that help polyploid organisms survive under different environmental conditions.
• When such differences become established and reproductive isolation is maintained over generations, polyploid populations can diverge from their ancestral populations and contribute to the emergence of new species.

The 2R Hypothesis

• The two-round (2R) hypothesis was proposed in 1970 by Susumu Ohno, a Japanese evolutionary biologist and geneticist.
• The hypothesis proposes that the early vertebrate genome experienced two rounds of whole-genome duplication (WGD) during vertebrate evolution.
• These duplication events are generally proposed to have occurred approximately 550–450 million years ago, during the early evolution of vertebrates.
• The 2R hypothesis was based on comparisons of genome size, gene copy numbers, and karyotypes across different organisms, particularly fishes and amphibians, in which polyploid species are naturally found.
• According to the hypothesis, the genome of the early vertebrate lineage underwent two successive complete genome duplications after the emergence of urochordates and before the evolution and diversification of jawed vertebrates.
• The proposed sequence is:
• First round (1R): WGD occurred in the common ancestor of jawed and jawless vertebrates.
• Second round (2R): Another WGD occurred later in the common ancestor of jawed vertebrates.
• Evidence supporting the 2R hypothesis comes from the organization of HOX gene clusters. For example, the cephalochordate amphioxus has a single HOX gene cluster located on one chromosome, whereas humans have four HOX gene clusters distributed across different chromosomes.
• The presence of additional copies of genes located near the HOX clusters provides further evidence that large genomic regions—and potentially the genome as a whole—underwent repeated duplication.
• Two successive rounds of genome duplication could theoretically increase the number of gene copies from one to two and then from two to four, explaining the presence of multiple corresponding gene families in vertebrates.
• Not all duplicated genes were retained. Over evolutionary time, many copies were lost or became non-functional, whereas others were preserved and acquired new or specialized functions.
• The retention of duplicated genes provided vertebrates with additional genetic material that could contribute to biological complexity, adaptation, and diversification.

2R Hypothesis and the Evolution of the Immune System

• Whole-genome duplication may have contributed to the evolution of the vertebrate immune system by generating additional copies of genes involved in immune functions.
• One important example is the Major Histocompatibility Complex (MHC), a critical genetic region in vertebrates.
• MHC genes encode molecules that present peptide fragments derived from pathogens to T cells, allowing the immune system to recognize and respond to foreign antigens.
• Genome duplication could have produced additional copies of MHC-related genes, which were subsequently retained and diversified.
• These duplicated genes could undergo subfunctionalization or neofunctionalization, resulting in new or specialized roles within MHC class I and class II pathways.
• Thus, WGD is considered one potential contributor to the increased genetic and functional complexity of the vertebrate immune system.

Occurrence of Whole Genome Duplication

• Whole-genome duplication (WGD), or polyploidy, is particularly common in plants, and repeated genome duplication has contributed to the large and complex genomes observed in many plant species.
• The vertebrate lineage is thought to have experienced two major rounds of WGD, known as 1R and 2R.
• A subsequent duplication occurred specifically in the lineage leading to teleost fishes. This event is known as the teleost-specific genome duplication (TSGD) or 3R.
• WGD has occurred repeatedly across different organisms, but the number and timing of duplication events vary among species and lineages.

Examples of Whole-Genome Duplication

Organism/Group	Occurrence of Whole-Genome Duplication (WGD)
Saccharomyces cerevisiae (Yeast)	One round
Vertebrate lineage	Two rounds (1R and 2R)
Teleost fishes	One additional round (3R; Teleost-Specific Genome Duplication, TSGD)
Zea mays (Maize)	Two rounds
Arabidopsis thaliana	Three rounds
Triticum aestivum (Wheat)	Three rounds

• These repeated genome duplication events have played an important role in genome evolution, gene diversification, adaptation, and the emergence of new biological traits.

References

• 18.4C: Whole-Genome Duplication. (2018, July 13). Biology LibreTexts.
• https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/18%3A_Evolution_and_the_Origin_of_Species/18.04%3A_Evolution_of_Genomes/18.4C%3A_Whole-Genome_Duplication
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