Developmental biologist Professor Peter Currie from Monash University and colleagues report their findings online in the open access journal PLOS Biology.
"Sadly it's a fact of evolution that we are just modified fish," says Currie.
But, he says, the question is how did the tiny fins of fish develop into the powerful legs of land-dwelling tetrapods.
"The biggest gap in our knowledge is how we go from these puny little pelvic fins that seem to do almost nothing in fish to these huge hind limbs that support our weight on land," says Currie.
But he says while there is some evidence about how the skeleton evolved for weight-bearing limbs muscles, they are not preserved in the fossil record.
"There is absolutely no information about how the muscles that drive the locomotion ... came into being," says Currie
Stepping stone
To gain insights into the evolution of these muscles, Currie and colleagues analysed the development of pelvic fin muscle in two living species of 'primitive' cartilaginous fish and three living species of bony fish.
"The fish that we've been looking at are sitting evolutionarily at important branches of the tree of life," says Currie.
At one end of the spectrum is the bamboo shark and the so-called elephant shark, which is the most primitive fish with paired fins.
The researchers also looked at the zebra fish and our "nearest fishy cousin", the Australian lungfish, along with a North American paddlefish, which, in evolutionary terms, sits halfway between the sharks and the other bony fishes.
Currie and colleagues found that bony fish have a different mechanism of pelvic fin muscle formation than that found in the cartilaginous fish.
This mechanism is an important stepping stone for the evolution of land-dwelling animals, say the researchers.
"We had to go from this halfway house mode of making muscles in the pelvic fin to the mode that we use in order to develop proper hind-limb muscles," says Currie.
"Making more muscle and bigger muscle and positioning those muscles appropriately to make the big hind leg muscles was a very import step for the movement onto land. That's the major evolutionary thrust of the paper."
Fluourescent fish
One experiment carried out by Currie and team involved tracking the development of pelvic fin muscles in zebra fish by tagging the muscle cells with fluorescent jellyfish protein.The researchers genetically modified one set of 'donor' embryos with red fluorescent jellyfish genes and transplanted their muscle progenitor cells into 'recipient' embryos with green fluorescent jellyfish genes.
They then watched how the red donor cells grew in the recipient.
"This is a technique that's been used very successfully in other developmental systems but so far hasn't been applied to fish because they are very small embryos," says Currie.
Interestingly, he says, pectoral fins seems to have evolved more rapidly than the pelvic fins, with the mechanism of their development in fish being similar to that used in the legs of land dwelling animals. Source
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