Discovery of viruses: Where did viruses come from?
The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose
There are three main hypotheses that try to explain the origins of viruses:
1. Regressive hypothesis:
- Viruses may have once been small cells that parasitised larger cells
- Over time, genes not required by their parasitism were lost
- The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells
- This support the theory
- This is also called the degeneracy hypothesis, or reduction hypothesis
2. Cellular origin hypothesis:
- Some viruses may have evolved from bits of DNA or RNA that “escaped” from the genes of a larger organism
- The escaped DNA could have come from plasmids or transposons
- Once called “jumping genes”, transposons are examples of mobile genetic elements and could be the origin of some viruses
- They were discovered in maize by Barbara McClintock in 1950
- This is sometimes called the vagrancy hypothesis, or the escape hypothesis
3. Coevolution (virus-first) hypothesis:
- Proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on Earth and would have been dependent on cellular life for billions of years
- Virusoids of plants and the hepatitis delta virus of humans have an RNA genome but depend on other viruses for their replication (thus referred to as satellites)
- These satellites may represent evolutionary intermediates of viroids and viruses
- Perhaps the first written record of a virus infection consists of a hieroglyph from Memphis, the capital of ancient Egypt, drawn in approximately 3700 BC, which depicts a temple priest showing typical clinical signs of paralytic poliomyelitis.
- Pharaoh Ramses V, who died in 1196 BC and whose extraordinarily well-preserved mummified body is now in a Cairo museum, is believed to have succumbed to smallpox—a comparison between the pustular lesions on the face of the mummy and those of more recent patients is startling.
- Generally, virology as a science is approximately 100 years old – but virus diseases have been known for millennia
- Mesopotamian laws concerning rabid dogs date from before 1,000 B.C
- Smallpox was endemic in the Ganges river basin by the 5th century B.C
- The invention of the microscope by van Leuwenhoek’ in the 17th C
- Jacob Henle (1840)- 1st person with an idea of a microorganism too small to be seen by a microscope
- Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected using a microscope
- In 1884, the French microbiologist Charles Chamberland invented a filter (known today as the Chamberland filter or Chamberland-Pasteur filter) with pores smaller than bacteria
- In 1892, the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus
- His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration
- Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea
Koch’s postulates
- Explanation by Robert Koch 1890 —-for bacterial disease
At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium – this was part of the germ theory of disease
In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that the filtered solution contained a new form of infectious agent
He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus
In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages
Koch’s postulates as modified by Rivers (1937) —-for viral disease
- The virus must be isolated from the diseased host
- The virus must be grown in host cell cultures
- Proof of filterability
- Production of a comparable disease in the original animal host, or a related one
- Re-isolation of the virus
- Detection of a specific immune response to the virus
The history series of virology
The history of virology is a fascinating journey that spans centuries. Here is a brief overview of key developments and milestones in the history of virology:
- Late 19th Century: Discovery of Tobacco Mosaic Virus (TMV):
- In 1886, Adolf Mayer and Dmitri Ivanovsky independently discovered the infectious nature of the agent causing the mosaic disease in tobacco plants. This marked the first identification of a virus, later named Tobacco Mosaic Virus (TMV).
- Early 20th Century: Development of the Filterable Agent Concept:
- In the early 20th century, researchers such as Martinus Beijerinck and Wendell Stanley demonstrated that the agent causing certain diseases could pass through filters that retained bacteria. This led to the realization that a new type of infectious agent, smaller than bacteria, was responsible for these diseases.
- 1930s: Electron Microscopy and Visualizing Viruses:
- The development of electron microscopy in the 1930s allowed scientists to visualize viruses for the first time. In 1935, Wendell Stanley crystallized the tobacco mosaic virus, providing further evidence that viruses were distinct entities.
- 1940s-1950s: The Birth of Molecular Virology:
- The discovery of DNA as the genetic material by James Watson and Francis Crick in 1953 paved the way for molecular studies of viruses. In 1956, Heinz Fraenkel-Conrat and Robley Williams showed that the genetic information of TMV was contained in its RNA.
- 1950s-1960s: Development of Tissue Culture Techniques:
- The development of tissue culture techniques allowed researchers to grow and study viruses in vitro. This breakthrough facilitated the isolation and study of many viruses, contributing to a deeper understanding of viral replication.
- 1960s-1970s: Discovery of RNA Viruses and Reverse Transcriptase:
- David Baltimore and Howard Temin independently discovered the enzyme reverse transcriptase in the early 1970s. This enzyme allowed the conversion of RNA into DNA, challenging the central dogma of molecular biology. The discovery had profound implications for the study of RNA viruses, including retroviruses like HIV.
- 1970s-1980s: Recombinant DNA Technology and Genetic Engineering:
- The advent of recombinant DNA technology in the 1970s allowed scientists to manipulate and clone viral genes. This led to the development of vaccines and the production of therapeutic proteins using genetically engineered viruses.
- 1980s-Present: Molecular Virology Advances and Antiviral Therapies:
- Advances in molecular virology, including techniques like PCR (Polymerase Chain Reaction), have enabled rapid detection and characterization of viruses. The development of antiviral drugs, such as protease inhibitors for HIV, has revolutionized the treatment of viral infections.
- 21st Century: Genomics and Vaccine Development:
- The sequencing of viral genomes and the use of genomics have accelerated the identification and characterization of viruses. Rapid vaccine development, as seen in response to the COVID-19 pandemic, showcased the power of modern molecular virology.
The history of virology is marked by continuous discoveries and technological advancements that have deepened our understanding of these unique infectious agents. It has also played a crucial role in the development of vaccines and antiviral therapies, contributing to public health and medical advancements.
Landmarks in the study of animal viruses
(Flint, S. Jane et al. Principles of virology, 4th Edition)