Wednesday, July 28, 2010

Jellyfish Eyes Solve Optical Origin Mystery

Eyes are one of evolution’s marvels, described by Darwin as “an organ of extreme perfection.” But whether the animal kingdom’s kaleidoscope of eyes evolved from a common structure, or separately in dozens of forms, is a nagging evolutionary question.

Now a study of optical genes in jellyfish, which are descended from creatures that swam Earth’s ancient seas, long before vertebrates and invertebrates took their separate paths, suggests a common optical origin.

“Eyes have evolved in parallel many times, but they all go back to one prototype,” said University of Basel cell biologist Walter Gehring.

In a study published July 27 in the Proceedings of the National Academy of Sciences, researchers led by Gehring describe genes isolated from Cladonema radiatum, a jellyfish with highly elaborate eyes. The genes belong to a family called Pax.

In earlier research, Gehring found that a gene called Pax-6 is a “master regulator” of optical development, controlling eye formation in creatures as simple as fruit flies and as complex as mice and men. That suggested a common origin — but Pax-6 couldn’t be found in jellyfish, leaving open the possibility that eyes evolved independently in higher animals.

In the jellyfish study, Gehring’s team found several other Pax genes. When they transplanted the genes into fruit flies, the flies formed extra eyes. It’s not Pax-6 that appears universal, but rather the whole Pax family.

“We’re convinced that the eye evolved in one phylum,” said Gehring. “All the higher animals have Pax-6. The jellyfish have Pax-a or Pax-b.”

The next question is where Pax genes and their resulting structures came from. According to Gehring, they could have arrived in jellyfish through symbiosis with dinoflagellates — a family of single-celled marine plankton, some with human-like eye structures inside their single cell.

Jellyfish absorbed dinoflagellates, speculates Gehring, after dinoflagellates absorbed Pax genes from red algae, which had absorbed light-sensitive cyanobacteria. Gehring describes this as his “wild Russian doll hypothesis.” His team is now searching jellyfish genomes for dinoflagellate genes.

“Evolution is very conservative. It uses the things that function well,” said Gehring.

Images: 1) Eyes formed in fruit flies after the insertion of Pax genes from jellyfish./PNAS.
2) A Cladonema jellyfish; arrow points to an eye structure./PNAS.


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