Thursday, October 15, 2009

HOW DO HIGH DUCKS GET ENOUGH OXYGEN?


In a paper published in the November issue of The American Naturalist, evolutionary biologist Kevin McCracken and his colleagues at the University of Alaska Fairbanks offer insight into how ducks living in the high Andes might get enough oxygen.

Blood doping or boosting the number of oxygen-carrying hemoglobin in red blood cells is one way humans have tried to acclimate to high elevation and enhance athletic performance. But there are risks involved. There is an upper limit to the number of blood cells that can circulate in the blood, and packing in extra increases the load on the heart and the circulatory system. Species native to high elevation regions don't typically have noticeably higher red blood cell counts.

A potentially better way to cope with low atmospheric pressure at high-altitude is to enhance the ability of hemoglobin to bind oxygen.

Following a trip to Argentina in 2001, McCracken began to ponder whether the differences he observed between populations of ducks that lived in the high Andes, above 3,000 meters (ca. 10,000 feet), were also evident in their hemoglobin genes.

The high plateau of Andes is a cold, dry, oxygen-depleted region. McCracken predicted that the ducks that resided here year-round likely experienced strong natural selection and that their hemoglobin genes would differ from closely related populations in the lowlands.

Each year from 2001 to 2005, McCracken or members of his research team returned to South America to sample five pairs of dabbling duck populations in both the lowlands and the highlands and compare their hemoglobin genes using DNA sequencing. The ducks were collected from the far north of Peru, through Bolivia and Argentina, south to the Strait of Magellan and the Falkland Islands, a 6,000 kilometer (3,700 mile) transect which spans approximately a 5,000 meter (16,400 feet) change in elevation.

In the highland duck populations, McCracken and his colleagues found, on average, three amino acid substitutions (as few as one but never more than five) in the two genes that code for the major hemoglobin protein. The same substitutions were either absent or very rare in lowland populations of the same species. The hemoglobins also differed from other genes in that they showed a much stronger pattern of genetic differentiation.

That few genetic changes might have resulted in adaptation to high altitude is not surprising, McCracken says, as one or two substitutions can have large measureable effects on a protein like hemoglobin. The selection pressure, or intensity with which an environment weeds out unfit organisms, is strong and well defined in this case and such changes could come about relatively rapidly over few generations.

"We still haven't answered the question of whether these mutations increase hemoglobin-oxygen affinity," McCracken said. "An additional series of studies will be required to determine how each substitution that was observed influences blood-oxygen and hemoglobin-oxygen affinity."

University of Alaska Fairbanks

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