Background Neutron Radiation Correlation With Mutation Rate

When cosmic rays hit the upper atmosphere, they send high-energy particles, such as neutrons, showering down towards the surface. So an interesting question is how these neutrons influence things on the ground.

Researchers have long known that high-energy neutrons can smash into atomic nuclei causing all kinds of damage to the structure of materials. This is one cause of errors in computer memories.

Indeed, back in 2004, a group of researchers from IBM measured the flux of neutrons from cosmic ray collisions and used their results to predict the error rate in computer memories another electronic logic devices. Their predictions closely matched the observed rate of error, suggesting that neutrons are indeed an important source of problems in computing.

That finding has captured the imagination of Augusto González at the Institute of Cybernetics, Mathematics and Physics in Havana, Cuba. If background neutron radiation can cause errors in computer circuitry, then it ought to have a similarly destructive effect on another much more common information processing system–life.

Evolutionary biologists have long known that spontaneous mutations occur at a rate that has a crucial influence on the nature of evolution. But exactly what causes spontaneous mutation has never been properly understood.

Now González says that same approach that the IBM researchers used to predict errors in computer memory also explains the rate of spontaneous mutations in living things.

González bases his work on a fascinating experiment carried out by Richard Lenski and his team at Michigan State University that has been ongoing since 1988. These guys have been growing the bacteria, E. coli, since then and monitoring the mutations that occur between generations.

Every day, a small amount of bacteria is taken from one culture and allowed to grow in a new dish of glucose over the course of the following day. The bacteria reproduce until the glucose runs out, usually within eight hours or so. A small amount of bacteria is taken from this dish and allowed to grow in a new one and so on.

Since 1988, the team has observed how the bacteria have evolved over 60,000 generations. And they’ve discovered that the number of point mutations in the bacteria after 20,000 generations is about 300 million. That’s a rate of about 1 per second.

The question that González addresses is whether this mutation rate can be explained by the background neutron radiation. He does this by creating a mathematical model of the environment in which bacteria grow, which is essentially water.

He calculates that high-energy neutron would enter the water on a bacterial sample about once every 125 seconds. This high-energy neutron would then transfer its energy to water molecules creating a relatively short track of ions. He says that a single neutron would generate some 300 ions over a track length of about 100 nanometers and about 30 ions at a distance of 0.1 mm

“The bacteria touched by this ion shower could be destroyed or experience a permanent damage, especially in their DNA, which can be later inherited by the descendants,” he says.

The question then is how often this happens. And he calculates that this rate is consistent with the frequency of deleterious mutations measured in Lenski’s experiments. “In this way, we are indicating the probable origin of a class of “spontaneous” mutations,” he says.

Of course, González is pointing out a correlation between background neutron radiation and the observed rate of deleterious mutations in E. coli. That’s only part of what is required for scientific confirmation.

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Dads: You're the Reason You're kid is a Mutant Freak (ahem)

The offspring of chimpanzees inherit 90% of new mutations from their father, and just 10% from their mother, a finding which demonstrates how mutation differs between humans and our closest living relatives, and emphasises the importance of father's age on evolution.

Published today in Science, researchers from the Wellcome Trust Centre for Human Genetics and the Biomedical Primate research Centre in the Netherlands looked at whether, in chimpanzees, there was a heightened risk of fathers passing on mutations to their children compared to humans.

In humans, each individual inherits, on average, about 70 new mutations from their parents. However, this number is influenced by paternal age such that older fathers tend to result in more mutations – in humans each extra year of age results in two extra mutations.

Mutation risk is linked to father's age because the sperm lineage in males keeps dividing, while females have all the eggs they are ever going to produce present at birth. Paternal age is an established risk factor in a number of disorders including schizophrenia and autism.

The study found that the number of new mutations inherited by chimpanzees from their parents is, on average, very similar to that in humans, but that the effect of the father's age is much stronger – each additional year of father's age results in three extra mutations.

The results suggest that sexual selection can influence the rate of evolution through its effect on the male mutation rate.

Professor Gil McVean, from the Wellcome Trust Centre for Human Genetics at the University of Oxford said: "In humans, a father's age is known to affect how many new mutations he passes on to his children, and is also an established risk factor in a number of mental health disorders.

"This study finds that in chimpanzees the father's age has a much stronger effect on mutation rate – about one and a half times that in humans. As a consequence, a greater fraction of new mutations enter the population through males, around 90 per cent, compared to humans, where fathers account for 75 per cent of new mutations."

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Blond Hair Caused by a Very Simple Genetic Mutation

A single-letter change in the genetic code is enough to generate blond hair in humans, in dramatic contrast to our dark-haired ancestors. A new analysis by Howard Hughes Medical Institute (HHMI) scientists has pinpointed that change, which is common in the genomes of Northern Europeans, and shown how it fine-tunes the regulation of an essential gene.

"This particular genetic variation in humans is associated with blond hair, but it isn't associated with eye color or other pigmentation traits," says David Kingsley, an HHMI investigator at Stanford University who led the study. "The specificity of the switch shows exactly how independent color changes can be encoded to produce specific traits in humans." Kingsley and his colleagues published their findings in the June 1, 2014, issue of the journal Nature Genetics.

Kingsley says a handful of genes likely determine hair color in humans, however, the precise molecular basis of the trait remains poorly understood. But Kingsley's discovery of the genetic hair-color switch didn't begin with a deep curiosity about golden locks. It began with fish.

For more than a decade, Kingsley has studied the three-spined stickleback, a small fish whose marine ancestors began to colonize lakes and streams at the end of the last Ice Age. By studying how sticklebacks have adapted to habitats around the world, Kingsley is uncovering evidence of the molecular changes that drive evolution. In 2007, when his team investigated how different populations of the fish had acquired their skin colors, they discovered that changes in the same gene had driven changes in pigmentation in fish found in various lakes and streams throughout the world. They wondered if the same held true not just in the numerous bodies of water in which sticklebacks have evolved, but among other species.

Genomic surveys by other groups had revealed that the gene – Kit ligand – is indeed evolutionarily significant among humans. "The very same gene that we found controlling skin color in fish showed one of the strongest signatures of selection in different human populations around the world," Kingsley says. His team went on to show that in humans, different versions of Kit ligand were associated with differences in skin color.

Furthermore, in both fish and humans, the genetic changes associated with pigmentation differences were distant from the DNA that encodes the Kit ligand protein, in regions of the genome where regulatory elements lie. "It looked like regulatory mutations in both fish and humans were changing pigment," Kingsley says. Kingsley's subsequent stickleback studies have shown that when new traits evolve in different fish populations, changes in regulatory DNA are responsible about 85 percent of the time. Genome-wide association studies have linked many human traits to changes in regulatory DNA, as well. Tracking down specific regulatory elements in the vast expanse of the genome can be challenging, however. "We have to be kind of choosy about which regulatory elements we decide to zoom in on," Kingsley says. "We thought human hair color was at least as interesting as stickleback skin color." So his team focused its efforts on a human pigmentation trait that has long attracted attention in history, art, and popular culture.

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The Story of European Lactase Persistence

In the 1970s, archaeologist Peter Bogucki was excavating a Stone Age site in the fertile plains of central Poland when he came across an assortment of odd artefacts. The people who had lived there around 7,000 years ago were among central Europe's first farmers, and they had left behind fragments of pottery dotted with tiny holes. It looked as though the coarse red clay had been baked while pierced with pieces of straw.

Looking back through the archaeological literature, Bogucki found other examples of ancient perforated pottery. “They were so unusual — people would almost always include them in publications,” says Bogucki, now at Princeton University in New Jersey. He had seen something similar at a friend's house that was used for straining cheese, so he speculated that the pottery might be connected with cheese-making. But he had no way to test his idea.

The mystery potsherds sat in storage until 2011, when Mélanie Roffet-Salque pulled them out and analysed fatty residues preserved in the clay. Roffet-Salque, a geochemist at the University of Bristol, UK, found signatures of abundant milk fats — evidence that the early farmers had used the pottery as sieves to separate fatty milk solids from liquid whey. That makes the Polish relics the oldest known evidence of cheese-making in the world.

Roffet-Salque's sleuthing is part of a wave of discoveries about the history of milk in Europe. Many of them have come from a €3.3-million (US$4.4-million) project that started in 2009 and has involved archaeologists, chemists and geneticists. The findings from this group illuminate the profound ways that dairy products have shaped human settlement on the continent.

I wonder what the Americas look like if they had been included on that map...