Friday, June 19, 2009

EVOLUTION CAN OCCUR IN LESS THAN 10 YEARS

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How fast can evolution take place? In just a few years, according to a new study on guppies led by UC Riverside's Swanne Gordon, a graduate student in biology.

Gordon and her colleagues studied guppies — small fresh-water fish biologists have studied for long — from the Yarra River, Trinidad. They introduced the guppies into the nearby Damier River, in a section above a barrier waterfall that excluded all predators. The guppies and their descendents also colonized the lower portion of the stream, below the barrier waterfall, that contained natural predators.

Eight years later (less than 30 guppy generations), the researchers found that the guppies in the low-predation environment above the barrier waterfall had adapted to their new environment by producing larger and fewer offspring with each reproductive cycle. No such adaptation was seen in the guppies that colonized the high-predation environment below the barrier waterfall.

"High-predation females invest more resources into current reproduction because a high rate of mortality, driven by predators, means these females may not get another chance to reproduce," explained Gordon, who works in the lab of David Reznick, a professor of biology. "Low-predation females, on the other hand, produce larger embryos because the larger babies are more competitive in the resource-limited environments typical of low-predation sites. Moreover, low-predation females produce fewer embryos not only because they have larger embryos but also because they invest fewer resources in current reproduction."

Study results appear in the July issue of The American Naturalist.

Natural guppy populations can be divided into two basic types. High-predation populations are usually found in the downstream reaches of rivers, where they coexist with predatory fishes that have strong effects on guppy demographics. Low-predation populations are typically found in upstream tributaries above barrier waterfalls, where strong predatory fishes are absent. Researchers have found that this broad contrast in predation regime has driven the evolution of many adaptive differences between the two guppy types in color, morphology, behavior, and life history.

Gordon's research team performed a second experiment to measure how well adapted to survival the new population of guppies were. To this end, they introduced two new sets of guppies, one from a portion of the Yarra River that contained predators and one from a predator-free tributary to the Yarra River into the high-and low-predation environments in the Damier River.

They found that the resident, locally adapted guppies were significantly more likely to survive a four-week time period than the guppies from the two sites on the Yarra River. This was especially true for juveniles. The adapted population of juveniles showed a 54-59 percent increase in survival rate compared to their counterparts from the newly introduced group.

"This shows that adaptive change can improve survival rates after fewer than ten years in a new environment," Gordon said. "It shows, too, that evolution might sometimes influence population dynamics in the face of environmental change."

(Photo: Paul Bentzen)

UC Riverside

MOBILE DNA ELEMENTS IN WOOLLY MAMMOTH GENOME GIVE NEW CLUES TO MAMMALIAN EVOLUTION

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The woolly mammoth died out several thousand years ago, but the genetic material they left behind is yielding new clues about the evolution of mammals. In a study published online in Genome Research, scientists have analyzed the mammoth genome looking for mobile DNA elements, revealing new insights into how some of these elements arose in mammals and shaped the genome of an animal headed for extinction.

Interspersed repeats, also known as transposable elements, are DNA sequences that can "jump" around the genome, causing mutations in the host and contributing to expansion of the genome. Interspersed repeats account for a significant fraction of mammalian genomes, and some of these elements are still actively mobile. In humans, interspersed repeats account for approximately 44% of the entire genome sequence. Even more extreme is the opossum genome, where more than half of the sequence is composed of repetitive elements.

Scientists recently sequenced the woolly mammoth genome, using DNA samples obtained from preserved specimens. Dr. Stephan Schuster and his research group at Penn State University, who were involved in the sequencing and analysis of the mammoth genome, are now looking deeper into the sequence for interspersed repeats. The mammoth genome is an excellent candidate for comparative analysis of interspersed repeats in mammals, as it had a remarkably large genome of approximately 4.7 billion bases, 1.5 times larger than the human genome. Using the mammoth genome sequence and sequences of other mammals for comparison, Schuster's group found that the mammoth genome contained the largest proportion of interspersed repeats of any other mammal studied. In fact, a single class of elements, known as the BovB long interspersed repeat, accounted for nearly 12% of the mammoth genome alone.

Dr. Fangqing Zhao, a postdoctoral researcher in Schuster's group and primary author of the work, emphasized that the BovB family of repeats is particularly interesting, because while this family has been identified in other mammalian genomes, such as ruminants, snakes, opossum, and now the mammoth, its distribution in the mammalian lineage is inconsistent. Zhao explained that this finding in mammoth further supports the hypothesis that BovB may have been acquired "horizontally," meaning that vertebrate genomes attained the element from another organism, rather than inherited from ancestors.

Many species within the Afrotheria group of mammals, which includes the woolly mammoth, are at high risk for extinction or are already extinct. "Further analyses examining if the genomes of extinct and endangered Afrotherians contain more repetitive elements than non-endangered mammals may elucidate whether there is an interplay between repetitive elements and extinction," Zhao noted, underscoring the need to study genomes of species on the brink of extinction.

CSHL

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