生物谷報(bào)道：21日的《美國國家科學(xué)院學(xué)報(bào)》刊載了一篇關(guān)于樹蛙的研究報(bào)道,，科學(xué)家發(fā)現(xiàn)這種黃色巴拿馬兩棲動(dòng)物是首個(gè)見證能水陸兩地產(chǎn)卵的脊椎動(dòng)物，見證了水生過渡到陸生的進(jìn)化軌跡,。
科學(xué)家目睹了此沙漏樹蛙（學(xué)名Dendropsophus ebraccatus）將卵產(chǎn)在懸掛在池塘上的植物上。在蝌蚪孵化之后,，它們只需簡單地落入水中就能行了,。為了查明這些青蛙是如何選擇將卵產(chǎn)在池塘里還是池塘上，研究人員調(diào)查了巴拿馬的3處池塘,。他們發(fā)現(xiàn)有樹蔭遮擋的2處池塘里的青蛙將卵產(chǎn)在池塘上,，而第三處池塘位于一處舊采石場(chǎng)中，沒有樹蔭遮擋,，這里大約四分之三的卵都產(chǎn)在水里,，另有大約四分之一的卵產(chǎn)在池塘上,，不過池塘上的卵因天氣熱和沒有樹蔭而導(dǎo)致死亡率高,。
為了弄清基因差異是否導(dǎo)致青蛙偏愛水中或陸地產(chǎn)卵,，科學(xué)家在開闊地帶和森林中修建了微型池塘，結(jié)果發(fā)現(xiàn)是環(huán)境而不是基因在起關(guān)鍵作用,。也就是說,，有樹蔭遮擋的池塘里的青蛙往往將卵產(chǎn)在池塘上，而沒有樹蔭遮擋的池塘里的青蛙往往將卵產(chǎn)在水中,。
雖然人類不是由青蛙進(jìn)化來的,，也不會(huì)進(jìn)化回到其它的動(dòng)物，但我們的遠(yuǎn)古祖先可能進(jìn)化出陸地繁殖方法以逃避水生捕食者或應(yīng)付環(huán)境的變化,，就像這些青蛙一樣,，以此來提高生存機(jī)會(huì)。
科學(xué)家發(fā)現(xiàn)這些青蛙卵并不是最適合水生或陸生的,，因?yàn)楫a(chǎn)在水中的卵面臨獲得足夠氧氣的挑戰(zhàn),，而產(chǎn)在陸地上的卵面臨被干死等危險(xiǎn)。托程表示盡管它們不最適合水生或陸生,，但它們都做到,。這是人們之前沒有看到過的情景。此發(fā)現(xiàn)讓科學(xué)家首次有機(jī)會(huì)能測(cè)試有關(guān)進(jìn)化到陸地繁殖的不同理論的正確性,。
科學(xué)家認(rèn)為這可能不是惟一一種能水陸兩地產(chǎn)卵的動(dòng)物,，他們計(jì)劃調(diào)查相近的青蛙種類，看是否也能水陸兩地產(chǎn)卵,。而且,，這可能不局限于兩棲動(dòng)物，因?yàn)榭茖W(xué)家已知一些魚將卵產(chǎn)在陸地上,，托程表示其中一些魚類可能也能水陸兩地產(chǎn)卵,。（生物谷www.bioon.com）
生物谷推薦原始出處：
PNAS,，Published online on May 21, 2008,10.1073/pnas.0711579105
Reproductive mode plasticity: Aquatic and terrestrial oviposition in a treefrog
Justin Charles Touchon* and Karen Michelle Warkentin
Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215
Edited by David B. Wake, University of California, Berkeley, CA, and approved March 31, 2008 (received for review December 8, 2007)
Abstract
Diversification of reproductive mode is a major theme in animal evolution. Vertebrate reproduction began in water, and terrestrial eggs evolved multiple times in fishes and amphibians and in the amniote ancestor. Because oxygen uptake from water conflicts with water retention in air, egg adaptations to one environment typically preclude development in the other. Few animals have variable reproductive modes, and no vertebrates are known to lay eggs both in water and on land. We report phenotypic plasticity of reproduction with aquatic and terrestrial egg deposition by a frog. The treefrog Dendropsophus ebraccatus, known to lay eggs terrestrially, also lays eggs in water, both at the surface and fully submerged, and chooses its reproductive mode based on the shade above a pond. Under unshaded conditions, in a disturbed habitat and in experimental mesocosms, these frogs lay most of their egg masses aquatically. The same pairs also can lay eggs terrestrially, on vegetation over water, even during a single night. Eggs can survive in both aquatic and terrestrial environments, and variable mortality risks in each may make oviposition plasticity adaptive. Phylogenetically, D. ebraccatus branches from the basal node in a clade of terrestrially breeding species, nested within a larger lineage of aquatic-breeding frogs. Reproductive plasticity in D. ebraccatus may represent a retained ancestral state intermediate in the evolution of terrestrial reproduction.
