Friday, November 6, 2009

SNAIL FOSSILS SUGGEST SEMIARID EASTERN CANARY ISLANDS WERE WETTER 50,000 YEARS AGO

0 comentarios

Fossil land snail shells found in ancient soils on the subtropical eastern Canary Islands show that the Spanish archipelago off the northwest coast of Africa has become progressively drier over the past 50,000 years.

Isotopic measurements performed on fossil land snail shells resulted in oxygen isotope ratios that suggest the relative humidity on the islands was higher 50,000 years ago, then experienced a long-term decrease to the time of maximum global cooling and glaciation about 15,000 to 20,000 years ago, according to new research by Yurena Yanes, a post-doctoral researcher, and Crayton J. Yapp, a geochemistry professor, both in the Roy M. Huffington Department of Earth Sciences at Southern Methodist University in Dallas, Texas.

With subsequent post-glacial climatic fluctuations, relative humidity seems to have oscillated somewhat, but finally decreased even further to modern values.

Consequently the eastern Canary Islands experienced an overall increase in dryness during the last 50,000 years, eventually yielding the current semiarid conditions. Today the low-altitude eastern islands are characterized by low annual rainfall and a landscape of short grasses and shrubs, Yanes says.

The research advances understanding of the global paleoclimate during an important time in human evolution, when the transition from gathering and hunting to agriculture first occurred in the fertile Middle East and subsequently spread to Asia, North Africa and Europe.

"In the Canary Archipelago, land snails are one of the rare 'continuous' records of paleoclimatic conditions over the last 50,000 years," Yanes says. "The results of this study are of great relevance to biologists and paleontologists investigating the evolution of plants and animals linked to climatic fluctuation in the islands."

The researchers' isotopic evidence reflects changing atmospheric and oceanic circulation associated with the waxing, waning and subsequent disappearance over the past 50,000 years of vast ice sheets at mid- to high latitudes on the continents of the Northern Hemisphere.

The research also is consistent with the observed decline in diversity of the highly moisture-sensitive land snails.

Land snail shells are abundant and sensitive to environmental change and as fossils they are well-preserved. Measurement of variations in oxygen isotope ratios of fossil shells can yield information about changes in ancient climatic conditions.

The shells are composed of the elements calcium, oxygen and carbon, which are combined to form a mineral known as aragonite. Oxygen atoms in aragonite are not all exactly alike. A small proportion of those atoms is slightly heavier than the majority, and these heavier and lighter forms of oxygen are called isotopes of oxygen.

Small changes in the ratio of heavy to light isotopes can be measured with a high degree of accuracy and precision. Variations in these ratios are related to climatic variables, including relative humidity, temperature and the oxygen isotope ratios of rainwater and water vapor in the environments in which land snails live.

(Photo: SMU)

Southern Methodist University

RESEARCHERS DISCOVER MECHANISM THAT PREVENTS 2 SPECIES FROM REPRODUCING

0 comentarios

When two populations of a species become geographically isolated from each other, their genes diverge from one another over time.

Eventually, when a male from one group mates with a female from the other group, the offspring will die or be born sterile, as crosses between horses and donkeys produce sterile mules. At this point, they have become two distinct species.

Now, Cornell researchers report in the October issue of Public Library of Science Biology (Vol. 7, No. 10) that rapidly evolving "junk" DNA may create incompatibilities between two related species, preventing them from reproducing. In this case, the researchers studied crosses between closely related fruit flies, Drosophila melanogaster and D. simulans. Nearly 100 years ago, scientists discovered that when male D. melanogasters mate with female D. simulans, normal males survive, but the female embryos die.
"It has remained an unsolved problem," said Patrick Ferree, the paper's lead author and a postdoctoral researcher in the lab of co-author Daniel Barbash, an assistant professor of molecular biology and genetics. "The question is, what are the elements that are killing these female hybrids and how are they doing that?"

The researchers found that the female hybrid embryos died very early in development. In most species, when the male's sperm (carrying either an X or Y chromosome) fertilizes the female's egg (containing an X chromosome), a new cell forms with a single nucleus containing a sex chromosome from each parent. If the offspring inherits its father's X chromosome, it becomes female; if it inherits a Y chromosome, it becomes male. Ferree and Barbash found that a unique segment of DNA in the father's X chromosome leads to embryo death of hybrid females.

The segment of DNA was found in the chromosome's heterochromatin, a densely packed region of highly repetitive sequences of junk DNA near the chromosome's center.

During the embryo's initial divisions, the researchers found, a specific segment of heterochromatin gets "sticky" and halts the process, preventing the entire X chromosome from separating properly; the result is that the early embryo dies.

Researchers have known that DNA in heterochromatin evolves faster than in other parts of the genome. Also, during early development, the proteins required for cell division come from the mother. The researchers speculate that the heterochromatin of the male D. melanogaster's X chromosome has rapidly evolved, such that after mating, the machinery involved in DNA packaging from a D. simulans mother no longer recognizes the D. melanogaster father's "junk" DNA, Ferree said.

The problematic region of D. melanogaster's X chromosome contains about 5 million base pairs of DNA, while the same region of D. simulans' X chromosome contains only about 100,000 base pairs, a 50-fold difference, said Ferree.

"It points to a species-specific difference in heterochromatin between these two species," he added. "This could explain other instances when you have female hybrid lethality," Ferree said.

Cornell University

FITNESS LEVELS DECLINE WITH AGE, ESPECIALLY AFTER 45

0 comentarios
Men and women become gradually less fit with age, with declines accelerating after age 45, according to a report in the October 26 issue of Archives of Internal Medicine, one of the JAMA/Archives journals. However, maintaining a healthy body mass index (BMI), not smoking and being physically active are associated with higher fitness levels throughout adult life.

"The U.S. population is aging and is becoming more obese and sedentary," the authors write as background information in the article. "It is well documented that the cardiorespiratory fitness of men and women declines with age and that body composition and habitual physical activity are related to cardiorespiratory fitness." Low fitness levels increase the risk of diseases and interfere with older adults' ability to function independently.

Andrew S. Jackson, P.E.D., of the University of Houston, and colleagues studied 3,429 women and 16,889 men age 20 to 96 who participated in the Aerobics Center Longitudinal Study (ACLS) between 1974 and 2006. During the study, participants completed between two and 33 health examinations that included counseling about diet, exercise and other lifestyle factors along with a treadmill exercise to assess fitness.

Statistical models showed that while fitness levels declined continuously over time, the decrease was not linear or steady—cardiorespiratory fitness declined more rapidly after age 45. The decline for men was greater than that for women.

The results also "showed that being active, keeping a normal BMI and not smoking were associated with substantially higher levels of cardiorespiratory fitness during the adult life span studied," the authors write. "Being inactive and having a high BMI were associated with a lower age at which an individual could be expected to reach threshold cardiorespiratory fitness levels associated with substantially higher health risks."

Given the high rates of obesity and low levels of physical activity previously observed in the general population, the results also suggest that more men and women will reach the fitness level designated by the Social Security Administration as representing disability at a younger age, the authors note. "These data indicate the need for physicians to recommend to their patients the necessity to maintain their weight, engage in regular aerobic exercise and abstain from smoking," they conclude.

JAMA

TREES FACILITATE WILDFIRES AS A WAY TO PROTECT THEIR HABITAT

0 comentarios

Fire is often thought of something that trees should be protected from, but a new study suggests that some trees may themselves contribute to the likelihood of wildfires in order to promote their own abundance at the expense of their competitors.

The study, which appears in the December 2009 issue of the journal The American Naturalist, says that positive feedback loops between fire and trees associated with savannas can make fires more likely in these ecosystems.

"We used a mathematical model to show that positive feedback loops between fire frequency and savanna trees, alone or together with grasses, can stabilize ecological communities in a savanna state, blocking conversion of savannas to forest," said the study's leading author Brian Beckage, associate professor in the Department of Plant Biology at the University of Vermont. The study's co-authors are William Platt, professor of biology at Louisiana State University, and Louis Gross, director of the National Institute for Mathematical and Biological Synthesis and professor of ecology and evolutionary biology and mathematics at the University of Tennessee. Beckage was a short-term visitor conducting research at NIMBioS in 2009 and will be on sabbatical at NIMBioS in 2010.

The promotion of fire by the savanna trees increases their own abundance by limiting the establishment and growth of tree species that are better competitors for resources and that might ultimately displace the savanna trees. The research results suggest that some trees may modify or "engineer" their environment, including the characteristic fire frequencies in a landscape, to facilitate their own persistence at the expense of their competitors, Beckage said.

The research proposes a scenario for the development of savannas in landscapes that would otherwise become closed forests. Examples of savanna trees that facilitate frequent low-intensity fires include the longleaf pine and the south Florida slash pine, both of which frequently shed their needles providing fodder for wildfires. The savanna tree initially invades grassland, but by facilitating frequent fires, it limits its own density and thus prevents conversion to a forest.

(Photo: Brian Beckage/UVM)

National Institute for Mathematical and Biological Synthesis

ANSWERING THAT AGE-OLD LAMENT: WHERE DOES ALL THIS DUST COME FROM?

0 comentarios

Where does it come from? Scientists in Arizona are reporting a surprising answer to that question, which has puzzled and perplexed generations of men and women confronted with layers of dust on furniture and floors. Most of indoor dust comes from outdoors.

In the study, David Layton and Paloma Beamer point out that household dust consists of a potpourri that includes dead skin shed by people, fibers from carpets and upholstered furniture, and tracked-in soil and airborne particles blown in from outdoors. It can include lead, arsenic and other potentially harmful substances that migrate indoors from outside air and soil. That can be a special concern for children, who consume those substances by putting dust-contaminated toys and other objects into their mouths.

The scientists describe development and use on homes in the Midwest of a computer model that can track distribution of contaminated soil and airborne particulates into residences from outdoors. They found that over 60 percent of house dust originates outdoors. They estimated that nearly 60 percent of the arsenic in floor dust could come from arsenic in the surrounding air, with the remainder derived from tracked-in soil. The researchers point out the model could be used to evaluate methods for reducing contaminants in dust and associated human exposures.

(Photo: Wikimedia Commons)

American Chemical Society

PHYSICIST MAKES NEW HIGH-RES PANORAMA OF MILKY WAY

0 comentarios

Cobbling together 3000 individual photographs, a physicist has made a new high-resolution panoramic image of the full night sky, with the Milky Way galaxy as its centerpiece. Axel Mellinger, a professor at Central Michigan University, describes the process of making the panorama in the November issue of Publications of the Astronomical Society of the Pacific.

"This panorama image shows stars 1000 times fainter than the human eye can see, as well as hundreds of galaxies, star clusters and nebulae," Mellinger said. Its high resolution makes the panorama useful for both educational and scientific purposes, he says.

Mellinger spent 22 months and traveled over 26,000 miles to take digital photographs at dark sky locations in South Africa, Texas and Michigan. After the photographs were taken, "the real work started," Mellinger said.

Simply cutting and pasting the images together into one big picture would not work. Each photograph is a two-dimensional projection of the celestial sphere. As such, each one contains distortions, in much the same way that flat maps of the round Earth are distorted. In order for the images to fit together seamlessly, those distortions had to be accounted for. To do that, Mellinger used a mathematical model—and hundreds of hours in front of a computer.

Another problem Mellinger had to deal with was the differing background light in each photograph.

"Due to artificial light pollution, natural air glow, as well as sunlight scattered by dust in our solar system, it is virtually impossible to take a wide-field astronomical photograph that has a perfectly uniform background," Mellinger said.

To fix this, Mellinger used data from the Pioneer 10 and 11 space probes. The data allowed him to distinguish star light from unwanted background light. He could then edit out the varying background light in each photograph. That way they would fit together without looking patchy.

The result is an image of our home galaxy that no star-gazer could ever see from a single spot on earth. Mellinger plans to make the giant 648 megapixel image available to planetariums around the world.

(Photo: Dr. Axel Mellinger)

Followers

Archive

 

Selected Science News. Copyright 2008 All Rights Reserved Revolution Two Church theme by Brian Gardner Converted into Blogger Template by Bloganol dot com