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Flukey frogs

发布时间:2019-03-08 08:10:14来源:未知点击:

By Kurt Kleiner THE saga of North America’s deformed frogs continues. Scientists who blame parasites, rather than environmental pollutants, have new evidence to support their case. Thousands of deformed frogs have been found in the US and Canada since 1995, when schoolchildren in Minnesota first noticed them on field trips. The frogs often have missing, extra or misshapen limbs. A team led by Pieter Johnson of Stanford University in California has now shown that Pacific tree frogs (Hyla regilla) exposed to parasitic flukes of the Ribeiroia family develop limb malformations (Science, vol 284, p 802). Johnson sampled frogs from 35 ponds and found that only the four that contained an aquatic snail, Planorbella tenuis, also had deformed frogs. This snail is an intermediate host for the flukes. In laboratory experiments, Johnson put tadpoles in water with varying numbers of flukes. The heavier the load of parasites, the more likely the tadpoles were to develop deformed limbs. In an accompanying paper in the same issue of Science (p 800), Stanley Sessions of Hartwick College in Oneonta, New York, argues that the flukes physically disrupt the development of the frogs’ legs, causing deformities. Interfering with limb development surgically produces similar abnormalities, he says, while treatment with hormones known to affect development—such as retinoic acid—produces subtly different changes. However, James La Clair of the Scripps Research Institute in La Jolla, California, who believes pollutants are to blame in the majority of cases of deformed frogs (This Week, 3 April, p 26), says physical disruption of the developing frog tends to produce multiple limbs. Most of the freak frogs found in the wild tend to have missing or deformed limbs,