Mating Mystery: Hybrid Animals Hint at Desperation in Arctic

Janelle Weaver Yahoo News 15 Dec 10;



A team led by ecologist Brendan Kelly of the National Marine Mammal Laboratory counted 34 possible hybridizations between distinct populations or species of Arctic marine mammals, many of which are endangered or threatened.

"The greatest concern is species that are already imperiled," said Kelly, first author of the Nature comment. "Interbreeding might be the final straw."

Pizzlies and Narlugas

When hunters encountered a hybrid of a polar bear and a grizzly in 2006, Kelly's colleagues remarked that the incident was just a fluke. But as Kelly delved into the issue, he found more evidence of similar anomalies. In 2009, a cross between a bowhead and a right whale was spotted in the Bering Sea, between Alaska and Russia. And a museum specimen in Alaska attests to breeding between spotted seals (Phoca largha) and ribbon seals (Histriophoca fasciata), which belong to different genera, a scientific classification of organisms that is broader than the species level.

Evidence suggests at least five other types of hybrids that may arise from animals of distinct genera, Kelly's team reported. These include:

* Narwhal (Monodon monoceros) and beluga whale (Delphinapterus leucas)
* Ringed seal (Phoca hispida) and ribbon seal (Histriophoca fasciata)
* Bowhead whale (Balaena mysticetus) and right whale (Eubalaena spp.)
* Harp seal (Phoca groenandica) and hooded seal (Cystophora cristata)
* Harbour porpoise (Phocoena phocoena) and Dall's porpoise (Phocoenoides dalli)

Breedings between these marine mammals near the North Pole are likely to result in fertile offspring, because many of these animals have the same number of chromosomes, said comment co-author Andrew Whiteley, a conservation geneticist at the University of Massachusetts, Amherst.

Chromosomes that are unmatched in number cannot pair during meiosis, a special type of cell division necessary for sexual reproduction. During meiosis, chromosomes duplicate and the cell divides to form daughter cells, which split apart to form gametes, or mature sexual reproductive cells. But with unmatched chromosomes, the split into gametes would be uneven, creating sterility.

Over the short term the hybrid offspring from these Arctic animal matings will likely be strong and healthy, because unlike inbreeding, which magnifies deleterious genes, so-called outbreeding can mask these genes. Most of these genes are recessive, meaning the offspring need a pair to actually show the trait. Different species or different genera generally don't have the same bad recessive alleles, and so there's not a high chance of a pair turning up. (Alleles are different versions of the same gene.)


Kelly said that breeding between species usually isn't beneficial when it's caused by accelerated environmental change, because the hybrid animals don't have time to evolve survival traits. "This change is happening so rapidly that it doesn't bode well for adaptive responses."

For instance, a cross between a narwhal and a beluga whale spotted in Greenland lacked the narwhal's spiral tusk, which contributes to breeding success. The polar-grizzly hybrid bears in a German zoo showed behaviors associated with seal hunting, but not the strong swimming abilities of polar bears.

Animals already threatened with extinction could take a hit from hybridization. The breedings between the North Pacific right whale, whose numbers have fallen below 200, and the more numerous bowhead whale, could push the former to extinction. (Over time, the hybrids would begin to outnumber the sparse right whales.)

Climate Crisis

"This is one of the consequences of the rapid changes we're inducing in that environment and one more reason to consider whether we really want to continue warming the climate as rapidly as we are," Kelly told LiveScience.

The Arctic Ocean may lack summer ice by the end of the century, "removing a continent-sized barrier to interbreeding," the researchers wrote. As such, Kelly and his collaborators urge scientists to model the prevalence and outcomes of hybridization, genetically monitor at-risk populations, and generate a priority list.

And they're pushing policy-makers to incorporate hybrids into their management and protection plans. Currently, the Endangered Species Act doesn't protect hybrid animals, Kelly said. "It's just not something that has been on people's radar screen, and we think it should be."

Inter-species mating could doom polar bear: experts
Yahoo News 15 Dec 10;


"Rapidly melting Arctic sea ice imperils species through interbreeding as well as through habitat loss," they said in a commentary appearing in the British science journal Nature.


In 2006, they said, scientists were startled to discover a "pizzly," or a hybrid of a grizzly bear and a polar bear, and in 2010, another bear shot dead by a hunter also was found to have mixed DNA.

Global warming has hit the Arctic region two or three times harder than other parts of the planet, redesigning the environment in which dozens of terrestrial and marine mammals live.

In particular, the shrinking of the Arctic ice cap -- set to disappear in summer by century's end without a deep cut in greenhouse gas emissions -- has pushed polar bears outside their normal hunting grounds.

The fierce predators use the edge of the ice cap as a staging area to stalk seals, their preferred food.

How far Arctic species have intermingled is unclear, although some important examples abound, according to the article, lead-authored by Brendan Kelly of the US National Oceanic and Atmospheric Administration (NOAA).

Last year what appeared to be cross between a bowhead and a right whale was photographed in the Bering Sea between Alaska and Russia.

There are fewer than 200 North Pacific right whales left, and the far more numerous bowhead could, through interbreeding, quickly push this remnant population to extinction, the researchers warned.

Different species of Arctic porpoises and seals are also known to have produced offspring with a mixed bag of chromosomes.

Hybridization is not necessarily a bad thing in itself, and has been a key driver of evolution, Kelly said.

But when it is caused by human activities, the phenomenon tends to occur over a short period, which leads to a damaging drop in genetic diversity.

When mallard ducks were introduced to New Zealand in the late 19th century, for example, they mated with native grey ducks. Today, there are few, if any, pure grey ducks left.

In the case of "pizzlies," the mixed heritage poses a survival risk: while showing the polar bear's instinct for hunting seals, one such hybrid has the morphology of a grizzly, which is poorly adapted to swimming.

Kelly's team recommended culling hybrid species when possible, as has been done for the offspring of red wolves and coyotes in the United States.

They also pointed out that sharply reducing the amount of carbon dioxide (CO2) pumped into the atmosphere will help slow the disappearance of the Arctic ice cap.

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