¶¶ÒõÂÃÐÐÉä

Mutations produce new genetic forms all the time, in all species, in all genes

At this time of year, firemoths (Schinia masoni) can be seen on blanketflower (Gaillardia aristata), a colorful native wildflower.

This is a fine example of crypsis, for the moth must sit with its yellow head over the ray flowers (look like petals) and its wings over the disc flowers (central disc) to be camouflaged. I was photographing firemoths on blanketflowers near Walker Ranch and Meyers Gulch when I saw a very strange blanketflower. 

A single flower had ray flowers emanating from two sides of the bloom and they seemed to be packed too tightly and projecting irregularly. Between the clusters of ray flowers the disc flowers wrapped tightly around the bases of the ray flowers, forming a band or crest. I spent hours looking at blanketflowers but found only one of these mutant forms. 

A normal blanketflower occupied by moths and a fasciate blanketflower. Photo by Jeff Mitton.

This bizarre growth form is a general phenomenon called fasciation, from a Greek root meaning band or stripe. Fasciate flowers, also called crested or cristate flowers, have been reported in about 100 species. Fasciation can be caused by bacterial, fungal or viral infection, or a genetic mutation. Horticulturalists have developed fasciate varieties of some species for their interesting and unusual flowers. Gregor Mendel demonstrated the simple inheritance of a fasciate mutant when he was crossing peas to reveal the laws of inheritance.

Fasciate flowers are rare in natural populations. It is likely that the mutants do not produce seed as efficiently as the normal forms, so the genes producing fasciate flowers disappear. 

A firemoth sitting on a blanketflower near Walker Ranch and Meyers Gulch. Photo by Jeff Mitton.

In contrast to selection against fasciate mutations, we are embedded in a striking example of favorable selection of a new mutation in SARS-CoV-2 (hereafter SARS), the virus that drives the epidemic of COVID-19 (hereafter COVID). Since COVID was announced to the world on the last day of 2019, genome banks have received more than 10,000 sequences of the entire SARS genome. Already, those data have provided important insights.

Is SARS a single form, just one genotype? The SARS virus is mutating as it spreads and analyses of genomes reveals that about 200 sites have mutated repeatedly as the virus moved between continents and spread among populations. Many other mutations have occurred, but geneticists are focusing on mutations that have appeared repeatedly, to learn whether these are fortuitous changes that allow the virus to infect humans more efficiently.

When did SARS first infect humans? Numerous sequences and substantial amounts of genetic diversity among viruses has allowed evolutionary biologists to create an evolutionary tree showing the relationships among all the different SARS genotypes. All lineages converge at the base, revealing the single genotype that first infected humans. Furthermore, they are able to use rates of accumulation of mutants in this tree to infer time elapsed from the present to the base of the tree. They concluded that SARS entered humans late in 2019.

As SARS moved and spread, a specific mutation appeared -- that is, it has arisen anew, many times. The name of the original gene for this protein is D614 while the new form is D614G and it is referred to as the G form of the gene. This mutation is in a gene that produces a protein incorporated into the spikes that reach out from the virus to make contact with a cell. If the spike and the host cell fit together, SARS fuses with the host cell, injecting its genetic material. 

When the G form appears as a new mutation, it is a single copy in an immense population of particles, so its frequency is very low. But once it is in a population, it rises to moderate and then high frequencies very quickly. For example, prior to March, it had not been found in Taiwan, but it appeared with a low frequency in the first 10 days of March. Before March was over, the G form had increased in frequency and completely replaced the original form of the gene. Similarly, G increased from a low frequency to 100% in France, Italy, Germany, the Netherlands and Spain. Globally, G rose from 10% to about 80% in March. This repeated pattern of meteoric rise in frequency must be driven by a form of selection that greatly favors G over the original form of the gene. 

Because the G form produces a spike protein and spikes play a primary role in the process of infection, biologists are considering the hypothesis that the G form makes SARS more infectious. 

Mutations produce new genetic forms all the time, in all species, in all genes. Mutants that enhance reproduction rise in frequency, while new mutants that diminish reproduction disappear. Fasciation mutants undoubtedly reproduce poorly and consequently they are either rare or absent in populations. The G mutation in SARS increases frequency very quickly, perhaps due to an advantage in the process of infection. This advantage produces natural selection, defined as reproductive differences among genotypes in a population. Evolution is the change of genetic forms in a population. We are watching SARS evolve, adapting to humans.