As embryos, cavefish such as Astyanax mexicanus, collected in caves beneath northeastern Mexico, begin to form eyes. But, the young lens soon degenerates, so that the cornea, iris, pupil, and other optic tissues that depend on signals from the lens are absent or rudimentary, the Science paper notes. In adult cavefish, all that remains is a collapsed remnant of the eye, covered by a flap of skin.
To learn whether the lens plays a role in triggering eye growth, the research team compared the sightless cavefish with closely related, sighted fish of the same species, gathered from surface streams outside the caves. The lens precursor tissue from embryos of the surface-dwelling fish was surgically implanted into the eye cup of the developing cavefish, after its own degenerating lens tissue was removed. By implanting the lens only into one eye of each cavefish, researchers were able to compare the eye-forming capacity of the transplanted lens with the degenerating lens in the same animal.
Within eight days, Jeffery and Yamamoto began to see a larger eye on the transplanted side of the cavefish. After two months, cavefish had grown a large eye with a distinct pupil, cornea, and iris. In addition, the retina of the eye with the implanted lens showed rod photoreceptor cells, which are absent or rare in the degenerate cavefish eye. This shows that "the lens is able to stimulate the development of eye parts that have been lost during the past million or so years of cavefish evolution," Jeffery says.
When the experiment was reversed, so that surface-dwelling fish received the lens from a cavefish, the transplant failed to support eye growth. The researchers concluded, therefore, that "a change in lens signaling to other parts of the eye is a major cause of eye degeneration in cavefish." They caution, however, that other factors also may contribute to eye loss, and further
Contact: Ginger Pinholster
American Association for the Advancement of Science