A change in just one gene may have jump-started the transition of ancient fish into animals that could move on land — by deleting a fin and replacing it with the rudiments of a limb, scientists reported Thursday in the journal Nature.
FOR THE RECORD:
An article in the June 24 LATExtra section about a gene that may have been involved in the evolutionary transition of a fin to a limb said that limbs first developed in vertebrates about 35 million years ago. It should have said about 350 million years ago.
Marie-Andree Akimenko and co-workers at the University of Ottawa in Canada identified a set of genes that was absent in tetrapods, the animals with four limbs that descended from fish, but present in all seven species of fish they investigated.
The scientists blocked the activity of these genes in zebrafish, which are commonly used in lab research. The result was stunted fin growth. Effects were apparent as early as two days after fertilization, at which point the structure that normally goes on to form the skeletal foundation of the fin was completely missing. This suggested that inactivating the genes provided a blank canvas for an entirely new structure.
The genes also appear to play a role in digit development. The fossil record shows that the earliest tetrapods had eight digits per appendage, which over time slowly decreased to the maximum of five seen today. The genetic program of the modified zebrafish looked like it was heading toward having many digits, like the early tetrapod ancestors.
The study is important, the scientists said, because it suggests how a pivotal step in the evolution of life may have occurred.
"I think it is really exciting that here we have this gene that obviously seems critical for fin development and it's lost in tetrapods," said Yale University biologist Scott Weatherbee, who studies limb development. "It really makes sense."
There are a few complications that still need to be smoothed out. For one thing, to affect the fin, the researchers needed to inactivate two similar genes in the zebrafish, an event that would be unlikely to occur naturally. The team suspects that the ancient fish ancestor had only one copy of this gene and that the second copy appeared later through a duplication in the modern fish lineage.
Second, if the loss of this gene was the first step toward limb development, the resulting fish probably would have had very short fins. Since such a fin would probably have put them at an evolutionary disadvantage; how they survived until further genetic modifications furnished more functional limbs remains an open question.
Still, the line between fin and limb becomes hazy in these early transitional species. Fish originally developed stumpy limb-like appendages for life in swamps, not on dry land. At the time, about 35 million years ago, most of the Earth was an arid wasteland, but plants were colonizing shallow waters, creating marshes rich with nourishment.
Finned fish, built to navigate open water, had trouble maneuvering in the crowded swamps. To provide an advantage, the developing limbs didn't have to be the long, lithe legs of a gazelle; they just needed to help steer around plants or push off the ground to pounce on passing prey.
"Limbs were a way of getting around in those swamps," said Ted Daeschler, associate curator of the Academy of Natural Sciences in Philadelphia. "Only later were they good for really becoming terrestrial."