Posted: 22 Feb 2010 12:42 PM PST
How all-female species avoid the shrinkage of their gene pool is among the animal kingdom's great mysteries. Now biologists think they've discovered the trick.
According to a study published Sunday in Nature, egg-producing cells in a ladies-only species of whiptail lizard contain double the standard genetic complement. They pick the healthiest set of chromosomes, preventing the loss of vital variation.
In asexually reproducing species, "there's an absence of sperm, and genetic information is never provided by another source. Anything that's lost is lost for good," said Peter Baumann, a University of Kansas cell biologist. "If there's a way to prevent the loss, then how is that accomplished? That's what our paper explains."
In all animal cells, genes are contained in DNA packages called chromosomes. Cells have two copies of each chromosome: in sexually reproducing species, one copy comes from mom, and the other from dad.
During reproduction, germ cells duplicate their chromosome set, then go through two rounds of cell division. The result is a sperm or egg cell with one copy of each chromosome. Some genetic material is lost in the process, but it doesn't matter, since sperm and egg soon fuse. Any gaps are filled during their union.
But in asexual reproducers — the whiptail species studied by Baumann, formally known as Aspidoscelis tellesata, plus about 70 other fish, reptile and bird species — both chromosome copies come from mom. Genetic gaps ought to ensue, and to accumulate in subsequent generations, eventually producing an animal unable to survive.
But as Baumann's group shows, A. tesselata germ cells start with four sets of chromosome copies, not two. When the cells finish dividing, the resulting eggs contain a standard set —one that's assembled, they found, from two loss-free copies.
"This is an elegant mechanism," said Baumann. And though they don't yet know how A. tesselata's germ cells evolved this trick, "you can imagine that it happens by a relatively simple change." That could explain why asexual reproduction has emerged in so many species.
As for whether asexual reproduction in vertebrates is an evolutionary aberration or viable strategy remains debated. Even if these species maintain their gene pool, they still lack the genetic mixing that gives sexual reproducers a steady flow of new adaptations.
According to Baumann, A. tesselata is lucky: it appears to be descended from a union of two related species, giving it a hybrid vigor. As for populations lacking built-in durability, he said that asexual reproduction may be a useful short-term strategy. It could maintain lineages through periods of isolation, with species reverting to sexual reproduction when suitable partners were available.
However, Baumann cautioned against assuming that all asexually reproducing species used the mechanisms seen in A. tesselata.
"I think it's going to be widespread, but nature often surprises us. We think we know how something works, then find out that nature came up with many ways of doing it," he said.
Images: 1. Whiptail lizards/Peter Baumann 2. A comparison of chromosomes in the gametes of a related, sexually-producing lizard species (above) and the whiptail (below).
Citation: "Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards." By Aracely A. Lutes, William B. Neaves, Diana P. Baumann, Winfried Wiegraebe & Peter Baumann. Nature, Advance Online Publication, February 21, 2010.
Posted: 22 Feb 2010 12:27 PM PST
NASA has released its first iPhone game as the agency continues its relentless conquest of new media
Starting Monday, you can virtually drive a fictional Lunar Electric Rover on a future lunar outpost. The game is free and available through the iTunes store.
Noted for its use of Twitter and educational iPhone apps, NASA has been at the forefront of government engagement with new media of all types. This one grew out of the agency's video podcast show, NASA Edge.
"We wanted to make this a cool game instead of an app where you just retrieve information," said Chris Giersch, the host of NASA EDGE.
The game is very simple. As the game review site Krapps notes, the gameplay is a bit "Pacmanish." Beyond driving around the rover, you can also see images from the proposed lunar outpost and learn more about what life on the Moon might be like.
For the first iteration, NASA decided not to go too extravagant. "We thought about going high-tech and going really jazzy, but for this first version, let's just keep it basic," Giersch said.
The Lunar Electric Rover in the game is based on a prototype tested at the Black Point Lava Flow in Arizona. It would have been part of a planned lunar outpost under the old NASA Constellation solar system exploration plan.
The game was developed by Analytical Mechanics Association, Inc, at the NASA Langley Research Center in Hampton, Virginia.
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