Washington DC - Frogs with multiple limbs and other disfigurements have been reported more and more often over the last several years, feeding worries that the ecologically sensitive amphibians are bellwethers of some larger environmental catastrophe. But now two studies in the 30 April issue of Science provide the strongest evidence yet that at least some of the frogs' bizarre deformities are the direct result not of pesticides or ozone loss, but of a parasite that naturally infects the animals.
Scientists first began noticing a global decline in amphibian populations about 15 years ago, but public concern really took off in the mid-1990s, when a group of schoolchildren in Minnesota discovered a strikingly high rate of limb deformities in local frogs. Speculation as to the cause has included possible DNA damage to frog embryos (as a result of too much UV light passing through the thinning ozone layer) and pollution from pesticide run-off. A strong candidate for the latter has been the powerful chemicals known as retinoids. In amphibians-which regenerate lost limbs-retinoids can scramble the genetic information at the site of the new limb bud, resulting in what looks like multiple legs.
But in one of the Science reports, Stanley Sessions and colleagues provide firm evidence in support of another candidate, which Sessions first put forth in 1987: small parasitic flatworms called Riberoria trematodes. These creatures burrow into the hindquarters of tadpoles where they physically rearrange the limb bud cells and thereby interfere with limb development.
"It's about as close to using an egg beater on the limb bud cells as you can get," said Sessions.
Sessions and his colleagues drew upon earlier laboratory studies showing that
burrowing parasites cause a different type of limb deformation than the chemical
effects of retinoids. Retinoids turn a limb bud topsy-turvy, the most common
result being that cells that would normally dev
Contact: Gabriel Paal
American Association for the Advancement of Science