This week’s papers from Boston labs
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Big dinosaurs had bird-size genomes
Bird genomes are about half the size of the human genome, and until now, researchers commonly believed that birds unloaded excess genetic baggage during the evolution of flight. But new work by Harvard researchers suggests that this paring down occurred at least a hundred million years earlier than was previously thought, and that it was already apparent in birds’ dinosaur ancestors.
A model of a Tyrannosaurus rex outside of Boston’s Museum of Science (Credit: Paul Keleher)
The results put small genomes on the growing list of bird-like characteristics, including feathers and nesting behavior, that apparently arose first in dinosaurs.
And rather than co-evolving with flight, the research suggests, a small genome and the associated metabolic efficiency may have promoted the emergence of flight.
It’s impossible to directly measure the size of the genomes of long-extinct dinosaurs, so Chris Organ, Scott Edwards, and colleagues took an indirect approach. In studies of 26 existing animals, they showed that bone cell size correlates with genome size. To estimate the cell size of extinct animals, they measured the size of empty pockets left in fossilized bone where cells used to be. From those measurements, they estimated the genome size of 31 extinct species of dinosaurs and birds.
The researchers found that dinosaurs known as Theropods, which are in the bird lineage and include Tyrannosaurus rex and Velociraptor, had cell and thus genome sizes comparable to those of modern birds.
Another group of dinosaurs likely had larger genomes. This family includes Stegosaurus and Triceratops, dinosaurs that more closely resemble modern reptiles.
The work appears in this week’s Nature. Pat McCaffrey
Water on Mars came from within
How vast deposits of salt ended up on Mars’s surface is a long-standing mystery, but MIT researchers appear to have solved it. Their new computer model suggests that the salts came from water seeping up from stores of groundwater and then evaporating from the planet’s surface.
A wide range of evidence suggests that Mars was a warm and wet planet more than three billion years ago. But some of that evidence, such as the salt deposits, raises further questions about what form the water took—oceans or streams or underground reservoirs.
In particular, researchers have puzzled over the Meridiani Planum, a large, flat plain with salt deposits 200 to 800 meters thick. In order for an evaporating sea to leave behind that much salt, it would have to have been more than two kilometers deep—but the plain doesn’t lie in a basin where water could accumulate.
Mars’s Meridiani Planum, as seen by NASA’s Mars exploration rover Opportunity in April 2006 (Credit: NASA)
Instead, the new research suggests, the salts came from water percolating up from the ground. In Nature this week, Jeffrey Andrews-Hanna and Maria Zuber of MIT, along with Roger Phillips of Washington University in St. Louis, describe their new model, which shows how water could flow through porous rocks in Mars’s crust.
That water could have accumulated high concentrations of salts as it traveled through thousands of miles of rock before leaking out onto the plain. Small amounts of water seeping up over many years could thus leave behind as much salt as the evaporation of a huge sea. Mason Inman
Pausing menopause makes mice healthier
Keeping the ovaries of older female mice functioning longer reduces the animals’ risk of age-related health problems, according to research reported this week.
In humans, menopause sets in when a woman’s finite supply of egg cells runs out. Ovary cells die, estrogen levels plummet, and health problems like heart disease and bone loss emerge.
Mice don’t undergo menopause like humans, but they do lose ovary function with age. When researchers Jonathan Tilly and Gloria Perez of Massachusetts General Hospital, along with colleagues from the University of Toronto, prevented that loss by inactivating a gene involved in cell death called Bax, the mice remained fertile into old age.
The mice seemed younger in other ways, too. They were leaner and more active than normal mice their age and showed minimal hair loss, wrinkles, bone loss, deafness, and cataracts. They also showed less anxiety and scored better in a battery of tests of mental abilities.
Surprisingly, the mice did not live longer. And neither did they suffer a higher incidence of cancer, a feared side effect of prolonging ovarian function.
Some of the observed health benefits, like higher bone density, depended on the presence of functioning ovaries, disappearing when the ovaries were surgically removed. Others, like decreased fat accumulation, seemed instead to result from Bax inactivation in key tissues.
The health benefits were not linked to estrogen levels, however, which were the same in both the control mice and those with the disabled gene.
With hormone replacement therapy for women now out of favor, the researchers concluded that finding ways to prolong ovarian function might some day help improve the quality of life of elderly women.
The report was published in the Proceedings of the National Academy of Sciences. Pat McCaffrey