Science & Environment

Genetic clue to how limbs evolved from fins

Zebrafish
Zebrafish have the genes necessary to form digits but they are not switched on

Fish have the genetic machinery necessary to make fingers, but it is not switched on, a study suggests.

The research in Plos Biology journal sheds light on how fish evolved into the earliest land animals millions of years ago.

For fish to make the transition to land, an existing DNA architecture had to be "hijacked" in order to make digits, the researchers said.

In order to study this, they took genes from fish and inserted them into mice.

It was already known that the genes for limbs are found in fish but how they evolved to form digits remained unclear.

To unravel the genetics, the authors used the zebrafish as a model. But other scientists said that zebrafish were not a useful species for studying limb evolution.

Lead author Joost Woltering from the University of Geneva, Switzerland, said that he was interested in the "longstanding evolutionary question - how did limbs actually develop out of ancestral fish fins?"

In order to answer this, Dr Woltering and his colleagues looked at the genetics of fin and limb developments in zebra fish and mice.

He was particularly interested in the division of the hand and arm (or digits), which does not exist in fish fins and "is considered one of the major morphological innovations during the fin-to-limb transition".

'Architect' genes

Tetrapods, the first four-legged creatures to walk the Earth, evolved from water to land over 380 million years ago in an era known as the Devonian period, often referred to as "the age of fish".

Fish and land animals both have clusters of genes called HoxA and HoxD and both are known to be essential in fin and limb development.

These Hox genes are sometimes referred to as "architect genes" as they are involved in making many of the physical structures animals possess.

Tiktaalik roseae, artwork
The now extinct Tiktaalik is thought to be a key transition between fish and tetrapods

However, when these Hox genes from fish were placed into mouse embryos, the genes that result in the arm were switched on but not the genes responsible for the hand or the digits.

This suggests that the genetic information needed to make tetrapod limbs was already present in fish before the tetrapods evolved.

"During embryogenesis it is key that developmental genes are switched on at exactly the right time and right place to ensure the development of a complete, coherent good functioning adult organism," Dr Woltering told BBC News.

Modernised genes

"The most surprising result is we found [DNA in fish] which is almost identical to the higher order DNA structure that we found in the mouse."

Another important conclusion of the study is that fish fins are not equivalent to the tetrapod hand and digits. Instead, the evolution of digits in land animals involved the repurposing of existing genetic infrastructure.

One of the co-authors of the study, Prof Denis Duboule, also from the University of Geneva, said: "Altogether, this suggests that our digits evolved during the fin-to-limb transition by modernisation of an already existing regulatory mechanism."

Computer artwork of an Ichthyostega around 350 million years ago during the Late Devonian period
Ichthyostega (artwork) was an early tetrapod which lived about 350 million years ago

Other researchers in the field say that the study contains some flaws.

Jennifer Clack, from the Cambridge University Museum of Zoology, who was not involved with the study, said using the zebrafish as a model for the experiments was a bad choice.

"We know that this animal, and by inference its relatives... lack some of the developmental stages that make digits in tetrapods," she explained.

Prof Clack added that other finned fish such as Polydon [paddlefish] "do have that mechanism, operating in a similar way to that in tetrapods, to make a complex fin skeleton".

This suggests, she said, that the zebrafish at some point lost the ability to make digits.

This view was echoed by Per Ahlberg from the University of Uppsala, Sweden. He said that the molecular analysis was of a very high quality but that the evolutionary conclusions were flawed.

"This entire inference is based on the assumption that the zebrafish fin skeleton is reasonably representative of the ancestral condition for tetrapods, and it just isn't," he explained.

"Essentially, modern-day sturgeon, gar and bowfin (living primitive ray-finned fishes) have fin skeletons that are reasonably close to the shared ancestral condition for mouse and zebrafish."

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