This blog post explores how radioactivity, initially just an experimental subject, became a core tool in life sciences.
Radioactivity is dangerous. After the Fukushima disaster, it caused Japanese seafood to vanish from dinner tables, and people grew wary of nuclear power plants and radioactive waste disposal facilities being built near their homes. The quiet Belarusian city of Chernobyl remains an uninhabitable death zone. Yet the effects of radiation on living organisms—radiation that claimed the lives of Marie Curie, a two-time Nobel laureate; the workers at the factory making luminous watches (known as the ‘Radium Girls’); and countless others too numerous to list—were surprisingly poorly understood until the early 20th century. It was thanks to Hermann Joseph Muller that radiation could transition from a physicist’s toy to a crucial subject of life science research. He understood the effects of radiation on living organisms and, furthermore, proposed methods for utilizing radiation in life science research.
Hermann Joseph Muller’s most outstanding scientific achievement was his paper “The Problem of Genetic Modification,” presented at the International Congress of Genetics (ICG) in Berlin in 1927. In this paper, he proved that radiation induces mutations in the DNA of fruit flies. To understand this method, one must first understand the crucial device used in the experiment: the ClB chromosome.
After observing numerous fruit flies and their offspring, Muller secured fruit flies possessing a special X chromosome with several key mutations. Compared to normal fruit flies, this chromosome had at least three major differences. The first mutation was in a vital gene essential for life. If both of a female’s X chromosomes carried this mutation, or if the sole X chromosome of a male (males possess one X and one Y chromosome) carried it, the fly would die before birth. However, if only one of the female’s X chromosomes carried the mutation, the fly could survive. Because this mutation does not function when present alongside the normal gene, it is called a recessive lethal gene. The second mutation was in a gene determining eye shape. If even one copy of this mutation is present, the eyes become flatter rather than round. The final mutation was in a gene that helps mix chromosomes during egg formation. Genetic diversity is a crucial factor enabling a species to persist over long periods. Multicellular organisms, including humans and fruit flies, secure genetic diversity by mixing chromosomes received from parents through a process called crossing over. However, this mutation prevents crossing over from occurring in Drosophila cells. This allowed Muller to prevent the chromosome he discovered from being destroyed during the crossing-over process. This chromosome is called the ClB chromosome, named after the initial letters of each mutation’s English name (Cross-compressor, lethal, Bar-eye).
In his paper, Muller demonstrated that this uniquely and cleverly constructed chromosome could be used to induce unintended mutations via radiation. The experimental method was straightforward. First, healthy males without mutations were crossed with females carrying one ClB chromosome among their two X chromosomes (females with two ClB chromosomes could not survive). The females could survive because the paternal X chromosome lacked the lethal gene; thus, even if they inherited the ClB chromosome from their mother, they remained viable. The probability of receiving either a normal chromosome or a ClB chromosome was exactly 1/2. Consequently, females with rod-shaped eyes would constitute half of all female offspring. Next, males were exposed to X-rays of varying intensities before the experiment, and the same procedure was performed. Surprisingly, the proportion of female offspring with rod-shaped eyes decreased as the intensity of X-ray exposure increased.
Muller interpreted this result as X-rays inducing recessive lethal gene mutations on the normal X chromosome of the father fruit fly. If X-rays create a recessive lethal gene on the father’s X chromosome, a fruit fly with a rod-shaped eye—receiving the ClB chromosome from the mother and the recessive lethal gene from the father—cannot survive. However, if the fly received a normal X chromosome from the mother, it could be born normally even if the father’s X chromosome carried the recessive lethal gene mutation. Therefore, Muller’s interpretation accurately explained the result: the number of flies with normal eyes remained almost unchanged, but only the number of flies with rod-shaped eyes decreased. This proved that X-rays cause mutations.
Muller’s research brought a revolutionary change to biology. Mutations are the most fundamental tools for studying DNA. To determine what function a specific DNA segment performs, comparing individuals with mutations in that segment is the simplest approach. Indeed, both Mendel’s pea experiments and those of Muller’s mentor, Morgan, utilized naturally occurring mutations.
However, naturally occurring mutations are extremely rare. In fruit flies, approximately one in 360 million base pairs mutates during each DNA replication. Assuming the average gene size is 1,600 base pairs (based on human hemoglobin), this means a mutation in the desired gene occurs in only one out of roughly 220,000 individuals. Furthermore, not all mutations affect the phenotype, making the probability of obtaining a desired mutant individual even lower. Morgan himself took over two years of experimentation to discover a white-eyed fruit fly. Considering that a fruit fly generation lasts about two weeks, utilizing natural mutations for research was extremely difficult.
In contrast, Muller’s method allowed biologists to dramatically shorten experimental time. By artificially increasing the mutation rate using radiation, it became easier to find individuals with mutations in the desired DNA segment. Radiation mutagenesis techniques were used in many early genetics experiments, including Beadle and Tatum’s one gene-one enzyme theory experiment, and this greatly contributed to the advancement of genetics.
After his fruit fly experiments, Muller continued his work using diverse species like corn and wasps, along with various mutagens such as mustard gas. He also warned of the potential dangers to humans posed by mutagenic substances in his 1941 paper “Role of Radiation Mutations in Mankind” and numerous other articles. In 1946, the year after two atomic bombs were dropped on Japan, Muller was awarded the Nobel Prize.
As discussed above, Muller’s achievements significantly contributed to several monumental experiments in biology. Of course, today new technologies like restriction enzymes or TALEN/CRISPR-Cas9 gene scissors, which can precisely cut and paste specific sites in DNA, have emerged, making the use of radiation in biological experiments rare. However, his contribution to dramatically improving the efficiency of mutation research—which previously relied on extremely low probabilities—through artificial intervention is still highly valued.
