Genetics – Page 6

02.06.2013

too many chromosomes

Oxytricha trifallax

This tiny pond creature has 15,600 chromosomes. From Ed Yong:

Within its cell, Oxytricha contains two nuclei, which enclose its DNA. One of these—the micronucleus— contains the complete edition of Oxytricha’s genome, just like the single nucleus within our own cells. That’s the tidy encyclopaedia shelf. But while the material in our nucleus must be constantly decoded and transcribed so that we can live, Oxytricha’s micronucleus is largely inactive. The encyclopaedia’s are barely read.

Instead, it relies on a second structure called the macronucleus. That’s the messy drawer. All of the DNA in the micronucleus is copied thousands of times over, and shunted into the macronucleus. In the process, it is broken up at tens of thousands of places, rearranged, and pruned. What’s left is a collection of thousands of “nanochromosomes” that contain all the information Oxytricha needs to survive. This is the stuff that gets decoded and transcribed, used and reused while the originals gather dust.

Sequencing this almighty mess must have been a devilish task, but Etienne Swart from Princeton University rose to the challenge. Leading a team of US and Swiss scientists, he has sequenced Oxytricha’s complete macronuclear genome. …

The team found around 15,600 of these nanochromosomes. On average, each is around 3,200 DNA ‘letters’ long, and around 80 percent of them contain just a single gene.

Domesticated dog breeds evolved long ago from the wolf. In a study published in Nature, researchers examined differences in the genomes of modern domesticated breeds and wolves and made some interesting discoveries. They expected differences in the nervous systems since the species behave differently. They also found many differences in genes effecting metabolism, especially digestion of carbohydrates. Domesticated breeds digest starches much better than wolves. From Science News:

The new study focuses on genetic differences between 60 dogs representing 14 breeds and 12 wolves from around the world. Those changes, the researchers reasoned, could identify genes that were important in separating dogs from wolves.

The researchers determined the genetic makeup of groups of dogs and compared the results to those from wolves, concentrating on parts of the genetic instruction book that differ between the two species. As they had expected, the researchers uncovered differences in many genes relating to the brain. But the search also revealed lots of genes involved in starch digestion and metabolism, and in the use of fats. Dogs, the team found, have more copies than wolves do of the AMY2B gene, which produces an enzyme that breaks starch into easily digestible sugars.

Other genetic variants seem to contribute to dogs’ increased ability to convert a sugar called maltose to glucose, the sugar that cells prefer to burn for energy. Yet other genetic changes improve dogs’ ability to move glucose into their cells. Combined, the tweaks alter dogs’ metabolism so they can get more energy out of a carbohydrate-rich diet than wolves can, the researchers conclude. The scientists confirmed the effect of the genetic variants by identifying biochemical differences in starch metabolism in blood and tissue samples from dogs and wolves.

11.27.2012

tracing the origins of chickens

Photograph of a chicken. Image from PLOS via Scienceblogs

Researchers are interested in tracking down the genetic origins of today’s domesticated chickens. From Science:

Their genes, and those of other isolated populations, are now being sequenced (see sidebar, p. 1022) as part of a larger effort to understand the world’s most common bird and biggest source of animal protein. In 2009, Americans ate 36 billion pounds of chicken, and the numbers keep growing, especially in developing countries in Asia and Africa. That importance is highlighted by the fact that the chicken was the first farm animal to have its genome published, back in 2004. Since then, the proliferation of factory farms, mass bird deaths from avian influenza, and dwindling diversity in chickens have raised concerns about this critical source of food.

A key thrust of research in the past decade has been to track the genetic changes that turned a remarkably shy creature into today’s meat-and-eggs dynamo, with an eye to protecting and improving breeds. But this research has also given scientists the opportunity to unravel a long-standing mystery that fascinated Charles Darwin: Where, when, and how was the chicken domesticated?…

The advent of sequencing tools in the 1990s promised a new line of evidence that went beyond physical characteristics. The results, however, have only heightened the controversy. A draft of the chicken genome, for example, isn’t enough to trace the bird’s evolution: Researchers need ancestral birds for comparison. Geneticists first used mitochondrial DNA (mtDNA) to trace the female line of the species back to its origin. Akishinomiya Fumihito, an ornithologist and prince in Japan’s royal family, extracted sections of mtDNA from Thai red jungle fowl and asserted in a 1994 paper that the findings suggested a single domestication in Thailand. Eight years later, another team used mtDNA from native Chinese chickens to support that idea.

In 2006, however, a team led by Yi-Ping Liu of China’s Kunming Institute of Zoology found nine separate clades—that is, groups descended from a common ancestor—in the mtDNA of a large sample of wild and domestic modern birds. The distribution of the clades suggests a distinct and separate expansion of lineages in southern China, Southeast Asia, and the Indian subcontinent, supporting a multiple origins theory. Another team published a study this week in Heredity based on nuclear DNA, which is not limited to the maternal line, supporting that view.

Much more at the source link. [Subscription may be required.]

GOLLYGEE!

Category: Genetics (page 6 of 8)

too many chromosomes

dogs, wolves and carbs

tracing the origins of chickens