Posted: 09 Dec 2010 04:33 PM PST
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Taking photos from the same vantage point years apart has been used to study changes in the landscape since the late 19th century. The technique got its start as a way to document the retreat of European glaciers.
For 50 years, the U.S. Geological Survey has been building an archive old photos of desert landscapes and revisiting the sites to take new photos. The result is the largest collection of repeat photography in the world.
Some of these sets of photos appear in a new book about the technique and the effects of climatic variation and land-use that it can document. We have a few of the most interesting repeat photographs in this gallery that show changes such as the retreat of glaciers seen above, the birth and death of cactus forests, the excavation of ruins and the shifting of a river channel.
Captions are modified from USGS information about the photos.
Above: Muir and Riggs Glaciers, Muir Inlet, Alaska: 1941, 1950 and 2004
In 1941 (left), Muir Glacier was a large 2,300-foot-thick tidewater glacier that had been retreating since the mid-eighteenth century. Riggs Glacier can be seen in the right of the photo flowing into Muir Glacier, which ends just out of the frame on the right. In the center photo, taken nine years later, Muir Glacier has retreated almost two miles and lost well over 300 feet of ice thickness. It is still connected to Riggs Glacier, and the ridge in the foreground is still bare of vegetation.
When the photo on the right was taken after another 54 years, Muir Glacier has retreated out of the photo more than four miles and Riggs has moved back around 2,000 feet and thinned more than 800 feet. Dense vegetation has covered the ridge.
Images: Left: USGS/photo by W.O. Field. Center: USGS/photo by W.O. Field. Right: USGS/photo by Bruce Molnia.
Posted: 09 Dec 2010 09:43 AM PST
In a weird feat of biotechnological virtuosity, scientists have engineered mice with genes from two dads, and none from a mom.
This was done by engineering females with eggs containing only chromosomes from a father. Mating added genes from a second father.
"These findings have novel implications for mammalian reproduction and assisted-reproductive technology," wrote researchers led by University of Texas geneticist Richard Behringer in a study published December 8 in Biology of Reproduction.
The findings are the latest in a string of strange reproduction tricks, including human embryos with DNA from three parents and cloned monkeys with genes from two mothers.
Those techniques, both of which are still far from practical or legal human use — and may never be used — are focused on preventing the transmission of inheritable genetic disorders.
Likewise, the double-dad technique could "conceivably provide a way to bypass the inheritance of mitochondrial disease theoretically," though logistical and ethical challenges make it extremely unlikely. It could, however, be used in breeding prize livestock or rare animals.
The researchers started with a line of induced pluripotent cells — cells that have been genetically reprogrammed into a stem cell-like state — from a male mouse, or Dad No. 1.
In about 1 percent of these cells, natural mistakes in cell division eliminated the Y-chromosome. (Male cells have two strands of chromosomes, X and Y, while female cells have two X-chromosomes.)
The DNA was then removed from these X-only cells, and injected into immature embryos, which were transplanted into a surrogate mother.
The resulting offspring had only X-chromosomes: two from the mother, and one from Dad No. 1. Among female offspring, some of their eggs contained only Dad No. 1's chromosomes.
When they mated, those eggs met the sperm of Dad No. 2. The resulting offspring had two dads.
A major impediment to any use of this technique, even in animals, is the tendency of reprogrammed cells to become cancerous.
Impediments aside, it may be possible to use reprogramming "to generate sperm from a female donor," wrote the researchers, "and produce viable male and female progeny with two mothers."
Image: Arrow at left points to an engineered female with eggs containing DNA from a male, surrounded by her progeny; arrows at left point to a female and her mate, surrounded by their progeny. /Biology of Reproduction.
Citation: "Generation of Viable Male and Female Mice from Two Fathers." By Jian Min Deng, Kei Satoh, Hao Chang, Zhaoping Zhang, M. David Stewart, Hongran Wang, Austin J. Cooney and Richard R. Behringer. Biology of Reproduction, published online, December 8, 2010.
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