Though it's common knowledge that ancient lobe-finned fish grew rudimentary limbs and became the first animals to walk on land, it hasn't been known exactly what genetic changes occurred to allow the emergence of those limbs. After all, modern fins and hands are very dissimilar structures. A new study examined the genome on a fish called a spotted gar, and found out that a suite of genes responsible for wrist and digit formation called HoxA and HoxD are the likely drivers of this incredible morphological metamorphosis. Senior author of the paper, published in the Proceedings of the National Academy of Sciences, is Neil Shubin of the University of Chicago.
"Fossils show that the wrist and digits clearly have an aquatic origin," Shubin said in a press release. "But fins and limbs have different purposes. They have evolved in different directions since they diverged. We wanted to explore, and better understand, their connections by adding genetic and molecular data to what we already know from the fossil record."
This is not the first time scientists have tried to compare the fins of modern fish to mammalian wrists and hands. However, it could have been that the previous comparisons were using the wrong kind of fish. Thin-finned fish known as teleosts did not have highly-conserved sequences of the Hox genes, making them poor candidates to make the comparison.
This difference turned out to be due to a split about 300 million years between the bony fish that would become the teleosts and the lobe-finned species that would eventually give rise to the tetrapods. The teleosts underwent a whole-genome duplication event that permitted incredible genetic diversity, allowing them to adapt to nearly every marine environment. However, they did lose Hox function because of that.
"[T]he genetic switches that control autopod-building genes were able to drift and shuffle, allowing them to change some of their function, as well as making them harder to identify in comparisons to other animals, such as mice," added lead author Andrew Gehrke.
On the other hand, not all modern fish species had ancestors that underwent the genome duplication. One notable example is the spotted gar; a freshwater fish found in the Great Lakes and some North American river systems. The team found that the spotted gar had a large amount of conserved sequences among their Hox genes that drive mammalian limb development.
Shubin's group extracted the Hox genes from the spotted gar and inserted them into mice. Surprisingly, the resulting limbs were quite similar to the mice's natural limb patterning. This gives a good amount of weight to the researchers' understanding of which primitive fish eventually gave rise to the tetrapods.
"Overall," the paper states, "our results provide regulatory support for an ancient origin of the 'late' phase of Hox expression that is responsible for building the autopod."