March 22, 2018

Last month, in February, scientists published the DNA sequence of an invasive species of crustacean known as the marbled crayfish. The animal’s DNA sequence is unique, and the all-female crayfish species can reproduce without mating. The marbled crayfish, Procambarus virginalis, started as an aquarium species. A few of the animals were released into the wild, however, and the species is now threatening many native animals in the Czech Republic, Japan, Madagascar, the United States, and other nations.

The marbled crayfish threatens to crowd out seven native crayfish species in the east African island of Madagascar. Credit: © Ranja Andriantsoa

The story of the marbled crayfish began in the 1990’s. Its origin is not completely clear, but somehow this species carries three copies of each chromosome. Usually organisms carry two sets of chromosomes, one set from each parent. Procambarus virginalis apparently evolved from a species known as the slough crayfish, Procambarus fallax, which lives only in the tributaries of the Satilla River in the southern U.S. states of Georgia and Florida. Scientists believe that the new species was created after two slough crayfish mated in an aquarium setting, but one of them had a mutation in a sex cell. In crayfish, this can happen in response to sudden changes in temperature. If the cell was then fertilized by another individual in the aquarium, it would have resulted in an embryo with three copies of its genome. And just like that, a drastic mutation in a single crayfish produced the marbled crayfish.

Because the marbled crayfish has an additional set of chromosomes, it does not need a mate to reproduce. It basically clones itself through a type of reproduction called parthenogenesis. All offspring of marbled crayfish are female, and they are all very aggressive.

The marbled crayfish reproduced so quickly and prolifically that the animals were doled out to pet shops, given away to private individuals, or, unfortunately, released into the wild. Other species of crayfish cannot compete with the reproductive abilities and aggressive nature of the marbled crayfish, which quickly dominates any habitat it enters. Release into the wild, then, has been disastrous for other crayfish species, whose numbers have been drastically reduced or completely wiped out when faced with marbled crayfish competition. People have taken marbled crayfish to lands far from Georgia and Florida, and now the crustaceans are wreaking havoc on native crayfish populations in parts of Africa, Asia, and Europe. In the European Union and parts of the United States, bans on marbled crayfish aim to curb the animal’s increasing dominance, but the bans may have come too late.

Some scientists consider the marbled crayfish one of the most remarkable species known to science. Discovering a species with such a recent origin and such remarkable reproductive ability is rare. However, ecologists consider it a pest and a nuisance. Marbled crayfish may have some potential benefits, however. They could possibly become a significant food source, and the marbled crayfish’s unique reproductive talents may help scientists understand how cancer tumors adapt and develop resistance to drug treatments.

June 1, 2015

Genetic clues from a 35,000-year-old fossil provide evidence that the dog became our best friend thousands of years earlier than most archaeologists had previously thought. Until recently, scientists believed that dogs—as we know them today—and people had lived with each other for at least 14,000 years. That date made the dog the oldest known domesticated animal. However, this latest research pushes that date back even further. The findings suggest that dogs may have been first domesticated as long as 40,000 years ago.

Scientists know that modern domestic dogs are descended from wolves. Earlier genetic studies of wolves and ancient and modern dogs placed the dog’s domestication in Europe, China, or the Middle East. The oldest archaeological sites where human and dog remains occur together are dated from 11,000 to 12,000 years old. However, archaeologists now suspect that dogs may have been domesticated much earlier.

Geneticist Pontus Skoglund of the Harvard Medical School in Boston, Massachusetts, analyzed DNA obtained from the fossilized rib of a Siberian wolf discovered in 2010. The fossil was part of a collection recovered by a joint Swedish and Russian team on the Taymyr Peninsula in Russian Siberia in 2010. The frozen permafrost of this region preserves the remains of long-extinct prehistoric animals in such good condition that scientists can extract DNA from the fossilized bones.

Skoglund’s team compared the DNA they obtained from the 35,000-year-old rib bone of an ancient wolf, labeled Taimyr 1, to DNA sequences from ancient and living wolves and dogs. They found that the Taimyr 1 wolf belonged to a lineage that diverged from the ancestors of dogs and existing wolves at roughly the same time that dog and wolf lineages split from each other. From this, they used various techniques to calculate when the earliest dogs split from the ancestors of modern wolves. The results suggests that the two groups split some time between about 40,000 and 27,000 years ago.

Some dogs, including huskies, preserve DNA from a line of wolves that lived 35,000 years ago in the area that is now Siberia. (copyright melis/ShutterStock)

Some dogs today, including Siberian huskies and Greenland sled dogs, still preserve some DNA inherited from the wolf lineage that included Taimyr 1. The researchers involved believe that the first domestic dogs might have been hunting companions for people who settled Europe and Asia during the last Ice Age.

Other World Book articles:

• DNA

• Genetics

• Prehistoric people

May 4, 2015

Genetic differences between humans help determine why mosquitoes appear to prefer the scent of some people over others, according to scientists at the London School of Hygiene and Tropical Medicine. The research may explain why some people can be eaten alive by mosquitoes while their companions remain untouched. The results were published last week in the online journal PLOS One.

Recent research shows that mosquitoes have preferences about which humans to feed upon based on genetic factors in humans. © Dmitry Knorre, Dreamstime

More than 3,000 species (kinds) of mosquitoes live worldwide. Only female mosquitoes bite, and only a few species attack humans. They sip the victim’s blood, which they need for the development of their eggs, usually leaving an itchy welt. However, certain mosquitoes spread some of the world’s worst diseases, including dengue, encephalitis, malaria, and yellow fever. Scientists know that mosquitoes locate their victims through smell, but they do not understand why some people are bitten more than others.

The London scientists observed 37 sets of twins in an experiment to test the attractiveness of skin odors to captive populations of Aedes aegypti, an aggressive species of mosquito known for its fondness for human blood. Each twin placed one hand into one side of a Y-shaped glass tube as 20 hungry mosquitoes were released into the far end of the tube. The shape of the tube allowed each mosquito to fly toward the hand with a scent they preferred and away from a hand they disliked. A screen prevented the mosquitoes from actually biting the subjects. The scientists ran 40 trials with each set of twins to determine if the mosquitoes favored one twin over another or if they favored or disliked both twins equally.

The scientists observed that the mosquitoes often had a clear preference or clear dislike of certain subjects. However, the experiments showed that the overlap in the mosquitoes preference or dislike was about double among identical twins than it was for fraternal twins. Since identical twins share 100 percent of their genetic makeup compared to fraternal twins, who share only about half their genetic makeup, the researchers determined a person’s degree of mosquito attractiveness is determined by their genes.

The experiment demonstrates that some people have genetic makeups that are more attractive to mosquitoes than others. In addition, scientists saw that some people also seemed to have genetic makeups that are naturally repellant to mosquitoes. The researchers know that genetic makeup has a role in a person’s individual body odor. However, many other factors, including diet and the billions of bacteria that make up a person’s microbiome (the microbes that live on and in a human body), contribute to that odor. The scientists hope to learn exactly what factors are responsible for skin odors that mosquitoes dislike in order to produce more effective mosquito repellants.

Other World Book articles:

• Gene
• Multiple birth

September 2, 2014

Present-day Inuit people have virtually no genetic relationship with the earliest populations to inhabit the region, a surprising study of genetic material from prehistoric and modern Arctic peoples have shown. The analysis was conducted by scientists at the University of Copenhagen in Denmark. Archaeologists use the term Paleo-Eskimo to describe the earliest Arctic peoples who migrated into Arctic North America about 4,000 years ago. Many scientists assumed that these Paleo-Eskimos contributed to the ancestry of modern Inuit people as they were assimilated into Inuit society through intermarriage. However, genetic and archaeological evidence now strongly indicates that although Inuit ancestors and Pale-Eskimos shared the territory for a time, intermarriage and interbreeding was rare if it occurred at all.

The earliest Paleo-Eskimo people are called the Saqqaqs by archaeologists. They lived in small bands that hunted seal and caribou in the region beginning more than 4,000 years ago. The Tuniit people followed the Saqqaqs into Arctic North America in a series of migrations from across the Bering Strait. The Inuit refer to  the people who inhabited the Arctic before they arrived as the Tunitt. Archaeologists refer to the Tuniit as the Dorset culture. The Tuniit/Dorset people hunted seals, walruses, and narwhals. They spread across Canada to Greenland by about 500 B.C. But the Tuniit/Dorset people disappeared soon after another new culture called the Thule spread across the region about 1,000 years ago. The Thule people lived in villages. Whaling was the cornerstone of their culture. They also hunted on land with dog sleds and bow and arrow. By about 1700, the Thule culture had become the modern Inuit culture.

An Inuk fisher uses a pronged spear called a leister to catch his fish. Recent studies have revealed that modern Inuit are not related to the earliest inhabitants of Arctic North America. © Bryan & Cherry Alexander, Photo Researchers

In their study, the University of Copenhagen scientists collected bone, teeth, and hair samples from the preserved bodies of 169 ancient Paleo-Eskimo bodies from North America. The scientists isolated DNA from these samples and compared it to genomes sequenced from living Inuit and other Native American peoples. Native American groups are often reluctant to provide biological samples for genetic studies, but special tribal permission was given for this study.

The scientists found that the Paleo-Eskimo DNA samples were remarkably similar to each other yet genetically distinct from modern Inuit. The high degree of similarity in Paleo-Eskimo DNA suggests their populations were quite small. Over thousands of years, Paleo-Eskimo groups, each perhaps no more than 50 related individuals, spread out across the vast Arctic expanse. However, they apparently did not  interact with the Thule once they migrated into the region. The Paleo-Eskimos disappear from the archaeological record within a period of perhaps decades after the first Thule arrival. The abrupt disappearance of the Tuniit/Dorset people soon after this event remains mysterious. Archaeologists have not found any evidence of violent conflicts between Tuniit and the newly-arrived Thule. Some researchers suspect that the technologically advanced Thule may simply have out-competed the Paleo-Eskimo cultures in the rugged environment and pushed them towards extinction.

July 3, 2014

Recent findings are sure to disappoint people who believe that the Yeti, or Abominable Snowman, stalks the Himalaya and that Bigfoot prowls the Pacific Northwest in the United States. In a study published last week, British scientists who examined samples of “Yeti hair” sent in by museums and cryptozoologists (people who study and search for legendary creatures) reported finding no Yeti yet. The study appeared online in the Proceedings of the Royal Society B.

The Yeti is a creature said to live on Mount Everest and other mountain ranges of Asia. According to legend, the Yeti is a hairy beast with a large, apelike body and a face that resembles that of a human being. (World Book illustration by Ted Lewis).

Many people are fascinated by the idea that a large primate like the Yeti—or its American cousin, Bigfoot, or Russian relative, Almasty—could survive in remote wilderness areas unexplored by modern science. Cryptozoologists have investigated the Yeti for decades. Sensational reports of Yeti sightings have been exposed as hoaxes and fakes in the past, which has led many skeptics to dismiss the creature as pure fantasy.

The study on Yeti hair was led by Bryan Sykes, a geneticist at Oxford University. In 2012, Sykes requested samples of hair from Yeti enthusiasts,  saying, “I’m challenging and inviting the cryptozoologists to come up with the evidence instead of complaining that science is rejecting what they have to say.” Scientists on the study ended up analyzing the DNA of more than 30 of the 57 “Yeti hair” samples submitted. The remaining samples were eliminating after the scientists found that they were not hair but such substances as fiberglass. The researchers discovered that most of the 30 analyzed hair samples came from such common animals as cows, horses, raccoons, deer, and coyotes. None of the samples came from a new primate species.

Two of the samples, however, were less straightforward: one was found in Ladakh—a region in Jammu and Kashmir, the northernmost state in India; and the other came from Bhutan to the east. The tests revealed that the hair was from a mammal, but it did not match samples from known primates. Surprisingly, the DNA from the two samples matched up almost exactly with DNA from the 40,000-year-old jawbone of a Norwegian polar bear. The researchers do not believe that the “Himalayan Yeti” actually represents an isolated population of polar bears. Instead, they theorize that the region is home to a hybrid bear species unknown to science that developed from matings between polar bears and brown bears somewhere in northern Asia during the last Ice Age, when the ranges of those bears overlapped.

Sykes admits that the study does not disprove the existence of the Yeti or other large primates unknown to scientists; it only means that the samples submitted for analysis did not come from such creatures. Enthusiasts need to keep looking for evidence that stands up to scientific scrutiny.

Additional World Book articles:

• Folklore
• Hillary, Sir Edmund Percival
• Journey to the Top of the World by Sir Edmund P. Hillary (a special report)

October 8, 2012

Biologists John Gurdon of the Gurdon Institute in the United Kingdom and Shinya Yamanaka of Kyoto University in Japan won the 2012 Nobel Prize in physiology or medicine for their experimental findings on how adult animal cells can be transformed into stem cells. A stem cell is a cell that has the ability to develop into any of the different cell types that make up the tissues and organs of the body. This development process is called differentiation. Stem cells have the ability to divide endlessly, producing more stem cells or other types of cells.

In experiments with frogs conducted in the 1960’s, Gurdon demonstrated that the genetic material from a single cell of an adult frog contained all the information necessary to create a whole frog. He took the genetic material from the intestines of an adult frog and placed it inside an unfertilized frog egg. The resulting cell began to divide and developed into a tadpole as if it were a stem cell. Working independently in Japan, Shinya Yamanaka used a different approach to turn adult cells into stem cells. He altered individual genes in mouse skin cells to transform the cells into stem cells. These stem cells later differentiated into several different kinds of cells.

Being able to transform adult cells into stem cells could eliminate the need for controversial therapies that rely on embryonic stem cells. Scientists first succeeded in isolating (separating) and growing stem cells from a human embryo (developing human) in a laboratory in 1998. Such stem cells are called embryonic stem cells. These cells can differentiate into nerve, liver, muscle, blood, and all other cells that make up an organism.

This discovery led to a debate over whether it is morally acceptable to use cells taken from human embryos for research. The embryos are destroyed in the process of isolating the stem cells. Some people consider embryos as human beings with legal rights and believe it is wrong to destroy them. Other people believe that the potential medical benefits of embryonic stem cells justify their use. Learning how to control the creation and differentiation of stem cells will help scientists develop new treatments for many diseases, including Alzheimer’s disease, diabetes, and heart disease.

Additional World Book articles:

• Bone marrow transplant
• Genetic engineering
• Stem Cells: Seeds of Hope (a Special Report)
• Medicine (2007) (a Back in Time article)

Nov. 4, 2011

Weakened muscles and certain other signs of aging could be reversed or even prevented if a technique used in mice can be applied to humans, says a team of researchers at the Mayo Clinic in Rochester, Minnesota. The technique involves targeting worn-out cells, called senescent cells, for destruction. Senescence is the process of growing old.

Scientists have long known that all cells in the body divide during growth and development. But cells can divide only a limited number of times. Aging cells often die. Only about 5 to 15 percent of the cells in the body of a typical elderly adult become senescent. But these few cells have a dramatic effect on the body.  In people, age-related changes include graying or loss of hair, weakened muscles, wrinkled skin, and hearing and vision problems. One way senescent cells produce such changes is by secreting a variety of harmful compounds that cause inflammation. In healthy individuals, inflammation is the body’s normal response to injury or infection. However, inappropriate or uncontrolled inflammation can damage healthy tissue. Scientists believe inflammation is an underlying cause of many age-related diseases in people, including arthritis, cataracts, and dementia.

Exercise can help elderly people maintain or improve physical fitness. © Chuck Savage, Corbis.

The Mayo scientists experimented with a strain of mice that were genetically engineered so their cells produced a molecule called caspase 8 as they aged. They then injected the mice with a drug that specifically targets the molecule and causes those cells that contain it to commit suicide. The drug has no effect on normal cells, so healthy tissues were spared and there were no side effects. The drug did not affect the life expectancy of the mice. This kind of genetic engineering cannot be used in people, but researchers hope to develop other ways of targeting human senescent to produce the same beneficial effects. However, such techniques are probably years away.

Additional World Book articles:

• Antioxidant
• Death
• Telomere