China’s critically endangered Yangtze River porpoise is a distinct species, meaning it cannot interbreed with other porpoise types to pass on its DNA, a major analysis of the creature’s genome revealed on Tuesday.
The finless, dolphin-like creature, which sports a permanent, almost human grin on its snub-nosed face, is the world’s only freshwater porpoise.
However, there are only about 1,000 individuals left in the wild — a number shrinking by 14 percent per year — and conservationists warn the critter is poised to follow the long-snouted Yangtze River dolphin, or baiji, into extinction.
Photo: AFP
照片：法新社
For the latest study, intended to spur conservation efforts, an international research team analyzed the genome of the Yangtze River porpoise and compared it to 48 other finless porpoises from different regions.
The exercise revealed that the animal known as “river pig” in China was a “distinct” species and “genetically isolated from other porpoise populations,” the experts wrote in the journal Nature Communications.
Previously, finless porpoises were classified as a single species with three sub-species, of which the freshwater Yantze River group was one. The new data showed the three main groups had, in fact, “not shared gene flow for thousands of years,” the study said. Also, each group shows “unique, individualized signatures of genetic adaptation to different environments.”
In nature, cross-species mating results in sterile offspring, if any at all. No such obstacles exist for mating between members of two different sub-species. Finless porpoise is one of the porpoise species, which form part of the marine mammal family known as cetaceans, which also includes whales and dolphins.
Though they look somewhat alike, dolphins have longer noses, pointier teeth and longer, leaner bodies than porpoises, with a curvier, backward-pointing dorsal fin, according to America’s National Ocean Service.
All finless porpoises, said the researchers, originated from an ocean-dwelling ancestor. The Yangtze River group split from its seawater cousins some 5,000-40,000 years ago and “rapidly adapted to their new environment,” said Rasmus Nielson, one of the authors. Rasmus Nielsen is a professor of the Department of Integrative Biology at University of California, Berkeley.
The team found evidence for changes to genes regulating kidney function as well as the blood water-salt balance. The Yangtze River porpoise had a “unique, individualised” signature of genetic adaptation for living in freshwater, said the team.
“The authors hope that the genetic data, illustrating the distinctive genetic makeup of the Yangtze population, will spur ongoing efforts to prevent habitat destruction,” added the research press release on Nature Communications.
The Yangtze River porpoise is listed as endangered on the International Union for the Conservation of Nature (IUCN)’s “Red List.” The biggest threats are pollution from riverside industry, boat strikes, and getting caught in gillnets or other fishing gear. In 2015, China relocated a number of the creatures, which are rarer than pandas, to reserves in a species conservation bid.
(AFP)
中國極度瀕臨絕種的揚子江江豚是一個獨特的物種，也就是說，它們無法和其它種類的鼠海豚進行異種繁殖，將DNA傳遞給後代。這項該生物基因體的重大分析於週二被揭露。
這種長得像海豚，卻沒有鰭的生物，塌鼻子的臉上總掛著一抹幾乎像人類一般的露齒笑，是世界上唯一的淡水型鼠海豚。
只是，今日它們野生的個體僅存約一千隻──而這個數字正以每年百分之十四的速率逐步遞減──保育人士因此發出警示，指出這種動物很可能會追隨長吻揚子江豚（又稱白鱀）的命運步向絕種。
為了這份最新的調查，一個跨國研究團隊分析了揚子江江豚的基因體，再將它和棲息在不同地區的另外四十八隻不具背鰭的江豚進行比較，目的在於激發保育行動。
在學術期刊《自然通訊》上，這些專家寫道，實驗結果揭露出這種在中國俗稱為「江豬」的動物，其實是「獨特的」物種，而且「和其他江豚族群在遺傳上相互隔離。」
先前，不具背鰭的鼠海豚都被分類為單一物種，其下再分為三個亞種，其中的一種就是淡水的揚子江族群。這份研究指出，新的數據顯示這三個主要族群其實「好幾千年以來都不曾有過共同的基因交流」。而且，每一個族群都呈現出「為適應不同環境而發展出獨一無二、個別化的遺傳適應標誌。」
在自然界中，跨物種交配就算能夠產生後代，也會喪失繁殖能力。不過，如果是兩個不同亞種的生物交配，就不會有這種障礙。不具背鰭的江豚屬於鼠海豚的一種，構成海洋哺乳類這個被稱為「鯨下目」的大家族，其中也包含鯨魚和海豚。
雖然鼠海豚和海豚看起來有點相似，但根據美國國家海洋局的資料，海豚鼻子比較長，牙齒比較尖，身形也比較長而精瘦，帶著一個更富曲線感、尖端向後指的背鰭。
研究人員表示，這些無背鰭的江豚種類全部都可追溯到同一個住在海裡的遠祖。揚子江裡的族群，大約在五千到四萬年前之間和它們的海中遠親分家，然後「迅速地適應它們的新環境」，研究作者之一拉斯摩斯‧尼爾森這麼說。拉斯摩斯‧尼爾森是加州大學柏克萊分校的整合生物學系教授。
研究團隊發現證據，顯示出調節腎臟功能，以及控制血液中水和鹽分平衡的基因曾發生變異。他們說，揚子江江豚留有「獨一無二、個別化」的遺傳適應標誌，讓它們能夠在淡水中生活。
《自然通訊》期刊上發布的研究新聞稿補充表示：「這些遺傳數據闡示出這個揚子江的生物族群具備獨一無二的基因組成，作者群希望能夠藉此激勵人們持續付出努力，阻止棲息地遭到破壞。」
揚子江江豚在國際自然保育聯盟的「紅色名錄」上被列為瀕臨絕種。最大的威脅來自河流沿岸的工業汙染、船隻誤傷，以及被刺網或其他捕魚器具所捕獲。二○一五年，在一次物種保育行動中，中國將若干隻這類比熊貓還要稀少的生物重新安置於保護區內。
（台北時報章厚明譯）
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