生物谷报道：21日的《美国国家科学院学报》刊载了一篇关于树蛙的研究报道，科学家发现这种黄色巴拿马两栖动物是首个见证能水陆两地产卵的脊椎动物，见证了水生过渡到陆生的进化轨迹。
科学家目睹了此沙漏树蛙（学名Dendropsophus ebraccatus）将卵产在悬挂在池塘上的植物上。在蝌蚪孵化之后，它们只需简单地落入水中就能行了。为了查明这些青蛙是如何选择将卵产在池塘里还是池塘上，研究人员调查了巴拿马的3处池塘。他们发现有树荫遮挡的2处池塘里的青蛙将卵产在池塘上，而第三处池塘位于一处旧采石场中，没有树荫遮挡，这里大约四分之三的卵都产在水里，另有大约四分之一的卵产在池塘上，不过池塘上的卵因天气热和没有树荫而导致死亡率高。
为了弄清基因差异是否导致青蛙偏爱水中或陆地产卵，科学家在开阔地带和森林中修建了微型池塘，结果发现是环境而不是基因在起关键作用。也就是说，有树荫遮挡的池塘里的青蛙往往将卵产在池塘上，而没有树荫遮挡的池塘里的青蛙往往将卵产在水中。
虽然人类不是由青蛙进化来的，也不会进化回到其它的动物，但我们的远古祖先可能进化出陆地繁殖方法以逃避水生捕食者或应付环境的变化，就像这些青蛙一样，以此来提高生存机会。
科学家发现这些青蛙卵并不是最适合水生或陆生的，因为产在水中的卵面临获得足够氧气的挑战，而产在陆地上的卵面临被干死等危险。托程表示尽管它们不最适合水生或陆生，但它们都做到。这是人们之前没有看到过的情景。此发现让科学家首次有机会能测试有关进化到陆地繁殖的不同理论的正确性。
科学家认为这可能不是惟一一种能水陆两地产卵的动物，他们计划调查相近的青蛙种类，看是否也能水陆两地产卵。而且，这可能不局限于两栖动物，因为科学家已知一些鱼将卵产在陆地上，托程表示其中一些鱼类可能也能水陆两地产卵。（生物谷www.bioon.com）
生物谷推荐原始出处：
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.
