大米回来了,呵呵。
今天的作业。
大家加油哦!
文章标题我是用白色的字色,大家选择一下就能看到了。
Giant squid population is one big happy species
Elusive deep ocean dwellers have low genetic diversitydespite living around the globe
By Tina Hesman Saey
【Time1】
Giant squid are so mysterious — and so huge— that theyinspired the legend of the kraken, a Scandinavian sea monster. Now, a geneticanalysis adds to the creatures’ mystique.
DNA evidence suggests that all giant squid are part ofone global interbreeding species instead of the three that scientistspreviously thought existed, an international group of scientists reports March20 in the Proceedings of the Royal Society B. The finding indicates the giantsquid must migrate long distances to keep the breeding population well mixed.The
massive invertebrates also have some of the lowestgenetic diversity of any species, the researchers report. That finding and thesquid’s global interbreeding are nearly impossible for scientists to explain,says study coauthor M. Thomas Gilbert, a geneticist at the Natural HistoryMuseum of Denmark in Copenhagen. “We were very surprised by the results,”Gilbert says.
Most of what is known about giant squid comes fromstudying carcasses washed up on beaches, hauled in by fishermen or found in thebellies of sperm whales, the giant squid’s natural predator. Only last year didan expedition film a giant squid in its natural habitat. Researchers candescribe the physical characteristics of the animals, which can grow to beabout 18 meters long with parrotlike beaks, sucker-studded tentacles and eyesbigger than a person’s head. But not much is known about how the creatureslive.
Gilbert and his team got a glimpse by examiningmitochondrial DNA from 43 giant squid. Mitochondria are the energy factoriesinside cells. Scientists can trace a species’ life history through the DNAcarried in mitochondria.
Researchers had thought there were three species of giantsquid: one in the North Atlantic, a second in the southern oceans and a thirdsmaller species in Japanese waters, says Clyde Roper, a marine biologistemeritus at the Smithsonian Institution’s Museum of Natural History inWashington, D.C. But the new analysis puts squid worldwide into what wasthought to be the Atlantic Ocean-dwelling species, Architeuthis dux.
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【Time2】
The genetic homogeneity suggests that somehow squid musttravel long distances. A previous chemical analysis of the giant squid’s sharpbeaks indicated that adults don’t migrate far. That means the juveniles andlarvae must leave local waters and migrate around the world. Gilbert and hiscolleagues think “the young float around on the surface before they dive deepand become kingpins.” Surface currents could disperse the youngsters far andwide.
Roper calls the idea reasonable but notes that scientistshave spotted only few young squid in the ocean’s upper reaches.
Generally, low diversity comes with small populations,but researchers think the giant squid population is huge, perhaps in thehundreds of millions. They only seem rare because they live in the deep sea,Roper says. “We don’t go where they go very often, and they don’t go where we go.”
Gilbert and his colleagues suggest that giant squid usedto be rare but had a population explosion between 32,000 years and 730,000years ago. The date spread is large because scientists don’t know how longgiant squid take to reproduce or how frequently their DNA mutates.
Alternatively, the giant squid population might have beenlarge in the past, but rapidly diminished — what scientists call a populationbottleneck — and then grew again. “How can you bottleneck a species that livesglobally?” Gilbert asks. “It’s a massive mystery.”
The researchers have found no trace of historicalenvironmental conditions that might have caused a global bottleneck or rapidpopulation expansion. Whaling may have dramatically reduced the number of spermwhales and enabled a giant squid boom. But whaling is far too recent to accountfor population growth starting 32,000 or more years ago. “It’s sort of ahead-scratching thing,” Roper says of the population puzzle.
Gilbert says he hasn’t given up on the whaling hypothesis,but hopes that another solution may present itself. “What we’re really hopingis that somebody will read our paper and tell us, ‘You guys are crazy. This isthe real answer.’”
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asmanian devil disease reveals its secrets
Deadly cancer turns off genes, evading the immune system
By Tina Hesman Saey
【Time3】
A contagious cancer decimating Tasmanian devils makesitself invisible to the animals’ immune systems, which might otherwise fight itoff, a new study shows.
Devil facial tumor disease shuts down production ofproteins that normally decorate the surface of cells, telling the body whethera cell is its own or not. As a result, the devil’s immune system doesn’trecognize cancer cells from another devil as a potentially worrisome invader,Hannah Siddle, a marsupial geneticist at the University of Cambridge and aninternational group of collaborators report online March 11 in the Proceedingsof the National Academy of Sciences.
The finding could lead to a way to stop the deadlydisease. “It’s really the first hope that there could be a vaccine or immunetherapy,” says Elizabeth Murchison of the Wellcome Trust Sanger Institute andthe University of Cambridge. Murchison, who was not involved in the new study,discovered in 2009 that the tumor originated in cells insulating a singledevil’s nerve fibers. Since that initial case, which probably occurred in thelate 1980s or early 1990s, the disease has spread across eastern and centralparts of Tasmania, killing every devil it infects.
Tasmanian devils have such low levels of geneticdiversity that many researchers thought the animals’ immune systems couldn’tdistinguish their own cells from other devils’, and wouldn’t recognize a tumorcell as foreign. Yet the marsupials aren’t exactly identical, Siddle says.Other studies have demonstrated that the animals reject skin grafts from oneanother, suggesting that the tumor cells should get the boot, too. But thedevils don’t appear to mount any serious defense against the cancer.
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