Thursday, May 20, 2010


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A team of scientists in the United Kingdom and the Netherlands are reporting disturbing evidence that soil microbes have become progressively more resistant to antibiotics over the last 60 years. Surprisingly, this trend continues despite apparent more stringent rules on use of antibiotics in medicine and agriculture, and improved sewage treatment technology that broadly improves water quality in surrounding environments. Their report appears in ACS' bi-weekly journal Environmental Science and Technology.

David Graham and colleagues note that, although scientists have known for years that resistance was increasing in clinical situations, this is the first study to quantify the same problem in the natural environment over long time-scales. They express concern that increased antibiotic resistance in soils could have broad consequences to public health through potential exposure through water and food supplies. Their results "imply there may be a progressively increasing chance of encountering organisms in nature that are resistant to antimicrobial therapy."

The study involved an analysis of 18 different antibiotic resistance genes (ARGs) to four different classes of antibiotics in soil samples collected in the Netherlands from 1940 to 2008. ARGs are genes chosen to assess potential changes in resistance in microbes. Using data from sites around the Netherlands, the scientists found increasing levels in 78 percent of the ARG tested, clearly indicating increased potential for resistance over time. Because soil samples were only collected from the Netherlands, the scientists conclude their report by suggesting that further studies need be performed around the world so that the scope and possible ramifications of their results can be better understood.

(Photo: iStock)

American Chemical Society


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An experienced fruit fly researcher can tell at a glance whether the fly she is observing is male or female; a distinct pigmentation pattern on a fly's body (a type of bristle found only on the legs of males) and differences in the genitalia are dead giveaways. But most of the fly's body parts look identical in males and females, and until now, scientists had no idea whether "maleness" or "femaleness" extended to all of the insect's cells and tissues. In a study publishing today in the online, open access journal PLoS Biology, researchers at the Janelia Farm Research Campus of the Howard Hughes Medical Institute find that most cells in flies' bodies are identical, regardless of whether they are in a male or a female.

This is because the influence of sex-determining genes is restricted to specific tissues, says Bruce Baker, a group leader who conducted the research with colleagues Carmen Robinett, Alex Vaughan, and Jon Michael Knapp. Baker has been studying the genetics of fruit fly sex for 30 years. By the early 1980s, it was known that a cascade of genes determines how tissues and organs develop in a sex-specific manner. In this bureaucratic chain of command, each gene tells the next one how and when to act. The doublesex gene, the master gene that determines sexual characteristics, is the last gene in the cascade. In flies, females express one form of doublesex, and males another. This gene is viewed as a master switch that tells female cells to create female sexual characteristics and male cells to make male ones. Until now, fly geneticists clung to the notion that the male version of doublesex was switched on in every cell of male flies, while the female version was present in every cell in female flies.

Prompted by other labs reporting that doublesex is turned on only in small areas in the fly embryo and only in specific parts of the brain, Robinett and her colleagues employed a genetic engineering technique to detect doublesex expression in the cells of developing flies. The technique attaches a reporter gene to doublesex that makes cells expressing the gene glow green. Tracking doublesex then becomes as easy as watching for green spots in fly embryos, larvae, and adults. With this approach, Baker's group found that during fly development, doublesex switches on and off at different times and in different tissues. As expected, they saw doublesex protein in the genitalia and in other body parts that display differences between males and females. But in body parts where there are not overt differences between males and females, they found that doublesex was expressed only in certain cells and not expressed in many other types of cells. "It's a simple observational study with profound implications," Baker says. This study demonstrates that only a subset of cells is likely to know whether they are male or female. Robinett and her colleagues found that male and female fruit flies are really a mixture of cells that are sex-specific and cells that are sexless. Many of the fly's cells never turn on the doublesex gene. So while, some cells "know" their sex and take on sex-appropriate characteristics during development, those that do not express doublesex remain identical in males and females.

Because doublesex or analogous genes are present in myriad organisms, including humans, this theme may apply to more than fruit flies. "It may be broadly true that males and females are made up of a mosaic of cells that know their sex and cells that don't," Baker says. This could prompt other researchers to check for similar doublesex patterns in other model organisms such as roundworms, zebrafish, and mice. If similar findings emerge, it could shake up the entire notion of sex differences in the animal kingdom. And if the finding applies to humans – a big "if" at this point – the implications could be even more significant. "If it applies to people, it's certainly going to change our sociological view of what we think of as maleness and femaleness."



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Trees and other plants help keep the planet cool, but rising levels of carbon dioxide in the atmosphere are turning down this global air conditioner. According to a new study by researchers at the Carnegie Institution for Science, in some regions more than a quarter of the warming from increased carbon dioxide is due to its direct impact on vegetation. This warming is in addition to carbon dioxide’s better-known effect as a heat-trapping greenhouse gas. For scientists trying to predict global climate change in the coming century, the study underscores the importance of including plants in their climate models.

“Plants have a very complex and diverse influence on the climate system,” says study co-author Ken Caldeira of Carnegie’s Department of Global Ecology. “Plants take carbon dioxide out of the atmosphere, but they also have other effects, such as changing the amount of evaporation from the land surface. It’s impossible to make good climate predictions without taking all of these factors into account.”

Plants give off water through tiny pores in their leaves, a process called evapotranspiration that cools the plant, just as perspiration cools our bodies. On a hot day, a tree can release tens of gallons of water into the air, acting as a natural air conditioner for its surroundings. The plants absorb carbon dioxide for photosynthesis through the same pores (called stomata). But when carbon dioxide levels are high, the leaf pores shrink. This causes less water to be released, diminishing the tree’s cooling power.

The warming effects of carbon dioxide as a greenhouse gas have been known for a long time, says Caldeira. But he and fellow Carnegie scientist Long Cao were concerned that it is not as widely recognized that carbon dioxide also warms our planet by its direct effects on plants. Previous work by Carnegie’s Chris Field and Joe Berry had indicated that the effects were important. “There is no longer any doubt that carbon dioxide decreases evaporative cooling by plants and that this decreased cooling adds to global warming,” says Cao. “This effect would cause significant warming even if carbon dioxide were not a greenhouse gas.”

In their model, the researchers doubled the concentration of atmospheric carbon dioxide and recorded the magnitude and geographic pattern of warming from different factors. They found that, averaged over the entire globe, the evapotranspiration effects of plants account for 16% of warming of the land surface, with greenhouse effects accounting for the rest. But in some regions, such as parts of North America and eastern Asia, it can be more than 25% of the total warming. “If we think of a doubling of carbon dioxide as causing about four degrees of warming, in many places three of those degrees are coming from the effect of carbon dioxide in the atmosphere, and one is coming from the direct effect of carbon dioxide on plants.”

The researchers also found that their model predicted that high carbon dioxide will increase the runoff from the land surface in most areas, because more water from precipitation bypasses the plant cooling system and flows directly to rivers and streams. Earlier models based on greenhouse effects of carbon dioxide had also predicted higher runoff, but the new research predicts that changes in evapotranspiration due to high carbon dioxide could have an even stronger impact on water resources than those models predict.

“These results really show that how plants respond to carbon dioxide is very important for making good climate predictions,” says Caldeira. “So if we want to improve climate predictions, we need to improve the representation of land plants in the climate models. More broadly, it shows that the kind of vegetation that’s on the surface of our planet and what that vegetation is doing is very important in determining our climate. We need to take great care in considering what kind of changes we make to forests and other ecosystems, because they are likely to have important climate consequences.”

(Photo: Carnegie I.)

Carnegie Institution of Washington




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