Arctic mismatch threat?

Facing a massive loss of habitat brought on by rapidly melting ice near the North Pole, arctic mammals may start looking across species lines for potential mates. As a result, researchers worry that the unique adaptations to arctic conditions that have brought us iconic species such as polar bears, harp seals, and walruses will be lost in the genetic shuffle.

Harp seal
Image: Wikimedia commons, Matthieu Godbout

"There's a potential for widespread hybridization that has not been previously recognized," said geneticist linkurl:Andrew Whiteley;http://eco.umass.edu/people/faculty/whiteley-andrew/ of the University of Massachusetts. Central to the problem is the loss of Arctic sea ice, which has structured the way many species have been evolving for thousands of years. "We're expecting the sea ice in the summer months to be essentially gone in the Arctic within the next few decades," said linkurl:Brendan Kelly,;http://www.afsc.noaa.gov/nmml/personnel/nmmlprofile.php?name=Brendan.Kelly a marine mammologist at the National Marine Mammal Laboratory in Juneau, Alaska. This will mean that mammal populations that have long been separated by thick permanent ice sheets will come into contact during these warmer months, possibly resulting in hybrid offspring of the previously distinct populations. Kelly, Whiteley, and marine biologist linkurl:David Tallmon;http://www.sfos.uaf.edu/directory/faculty/tallmon/ of the University of Alaska Southeast outlined in a brief Comment in __Nature__ the possible threats hybridization poses to polar diversity, particularly to marine mammals. Although it's well known that hybridization over large evolutionary time scales has been the source of novel species, "what we're concerned with is when hybridization occurs very quickly due to human-related causes," Whiteley said. For some dwindling arctic species, hybridization may put an additional stress on their survival. Of primary concern are species with alarmingly low numbers, such as the North Pacific right whale, with as few as 200 individuals left in the wild. According to the authors, ice-free summers in the Arctic will likely bring the endangered right whales into close proximity with the more numerous bowhead whales, which are currently separated by thick ice sheets. Widespread cross-breeding between the two species could result in the North Pacific right whales essentially being "bred out of existence," the authors wrote, as unique sets of genes that have allowed these species to thrive in arctic conditions could be lost in the genetic shuffle. "The basic problem with hybridization is a potential loss of local adaptations," said linkurl:Fred Allendorf,;http://www.cas.umt.edu/casweb/for_faculty/FacultyDetails.cfm?id=873 a conservation geneticist at the University of Montana, who did not contribute to the paper. "You end up homogenizing" all the populations in the area.

Narwhal skull
Image: Wikimedia commons

Two decades ago, for example, researchers linkurl:discovered;http://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.1993.tb00454.x/abstract?systemMessage=Due+to+essential+maintenance%2C+access+to+the+Online+Library+will+be+disrupted+on+Sunday%2C+19th+Dec+%2710+between+10%3A00-12%3A00+GMT. the skull of what seemed to be a cross between a narwhal and a beluga whale. Several teeth were growing at odd angles, resembling what may in fact have been stunted narwhal tusks. Because that tusks are essential for narwhals during mating, Whiteley said this may be an example of when hybridization puts a particular organism at a disadvantage, a phenomenon known as outbreeding depression. On rare occasions, researches have been able to witness firsthand the behavioral effects of hybridization. Studying a pair of polar-grizzly bear hybrids born in a German zoo back in 2004, German scientists linkurl:found;http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B8JHX-4X8BV5M-3&_user=10&_coverDate=12%2F31%2F2009&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1580376736&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=1fe9d0149a59daa982159dac206d16d9&searchtype=a that even though the bears retained the instincts of hardy seal predators characteristic of polar bears, they were not as adept at swimming as their purely polar counterparts. In these cases, where the hybrid is not as fit as either parent species, Kelly said, cross-breeding becomes a conservation issue. Although several polar-grizzly bears have been sighted in the wild in recent years, whether and to what extent hybridization will contribute to a further decline in the polar bear populations (which are currently estimated at around 25,000) and other arctic species is unknown. "In the long term, if there is no sea ice, it's going to be very hard for a polar bear to remain a polar bear," said linkurl:Kit Kovacs,;http://npweb.npolar.no/person/kit a marine biologist at the Norwegian Polar Institute, who was also not involved in the paper. However, she added that it's not yet clear whether the sighted arctic hybrids are a result of the changing climate. In the early 1990s, for example, before any significant changes to the Arctic ice mass were recorded, Kovacs spotted a mix between a hooded seal and the furry, earless harp seal -- a surprising cross between two species from different genera. The __Nature__ article lists 34 examples of cross-breeding between species, sub-species, genera, and among discrete populations of a same species in arctic marine mammals. The authors recommend comprehensive genetic monitoring of arctic mammals to keep track of hybrids, as well as national and international policies to deal with the potential problems of hybridization. Kelly also suggested collaborating with Indigenous populations such as Eskimo hunters, who harvest many of the species in question. "If in the process of harvesting these animals they took tissue samples, which could be analyzed and monitored," he said, "[we could] see if and when hybridization is occurring and to what degree." B. Kelly, et al., "The Arctic melting pot," Nature, 468: 891, 2010.
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