Our hands and feet are marvels of nature. But how the precursors of these limbs developed from the fins of fish has been a matter of dispute. Researchers have now gained new insights based on the embryonic development of the Australian lungfish. It shows that the genetic blueprint for the hands and feet of these primeval fish was already there – but incomplete. Because the genes that control the formation of the fingers and toes are not yet active in them. According to the scientists, this finding suggests that this was also the case with the last predecessors of the terrestrial vertebrates.
About 400 million years ago, evolution made a decisive leap: vertebrates conquered the land of the primeval earth for the first time. The ancestors of today’s lungfish and coelacanth developed strong fins with which they could move even on firm ground. From them, over time, the first tetrapods developed – the tribal line that includes all amphibians, reptiles, birds and mammals. Linked to this was the transformation of fish fins into the typical tetrapod limbs – articulated hands and feet with flexible, functional fingers and toes. But there are two conflicting theories about how this change will take place. According to one, the fingers and toes were created through new forms of the so-called Hox genes, the genes that control the formation of the limbs from undifferentiated cells in the embryo. According to a second theory, these genes already existed in the ancestors of the tetrapods. In these, they controlled the formation of fins and then expanded them at the transition to the hands and feet of the terrestrial vertebrates.
An eye on the architectural genes of hand training
“In order to decide which of these alternative theories about the formation of the limbs is correct, you have to check them against the sarcopterygii,” explain Joost Woltering from the University of Konstanz and his colleagues. The ancestors of the first land-goers once belonged to this group of fish, the last surviving representatives of which are the lungfish and coelacanth. “This verification is difficult, however, because the coelaceans inhabit the inaccessible deep ocean, while the African and Australian lungfish have greatly reduced fins afterwards,” the researchers say. The anatomy of the lungfish fins alone does not tell whether these animals already possessed the genes for the hands and feet of the tetrapods. Woltering and his team have therefore chosen a different approach: They concentrated on the embryonic development of the Australian lungfish (Neoceratodus forsteri).
For their study, the scientists analyzed which genes are active and when during the formation of the fish fins. They paid particular attention to the Hox genes, which orchestrate limb formation. To determine the gene activity, Woltering and his colleagues took tissue samples from fish embryos at various stages, in which they determined the gene activity via the presence of the respective gene-specific messenger RNA. It turned out that the lung fish already has the gene that is crucial for the formation of the articulated hand in the tetrapods. This Hoxa13 gene is also active in the fish larvae when they develop their fins. “Amazingly, we saw that the gene that specifies the hand in limbs – Hoxa13 – is activated in a similar skeletal region in lung fish fins,” explains Woltering.
Gene activity stops at half
According to this, the most important “architect” gene for hand training is already present in these fish. “This discovery clearly shows that a primitive hand was already present in the ancestors of the terrestrial vertebrates,” says Woltering. But the analyzes also revealed crucial differences. According to this, a gene that is important for the formation of fingers and toes, Hoxd13, is apparently activated differently in the fins of the lung fish. In terrestrial vertebrates, this gene is first activated in the developing little finger, then the activity spreads in the direction of the thumb over the entire future hand. This process coordinates the correct training of all five fingers. In the lung fish, however, the researchers observed that this gene was initially similarly active, but this activity did not spread over the entire fish fin – it virtually stopped halfway. In fish, some genes also remain active that are normally switched off in the fingers and toes.
“All of this shows that while lungfish fins unexpectedly share a primitive hand with tetrapods, the fins of our ancestors still needed an evolutionary ‘finishing touch’ to form limbs,” says Woltering. “In this sense, it looks as if the hand came first and was only added to fingers later in the course of evolution.” In the future, the researchers plan to do further analysis of the development of fins and limbs in order to fully understand what causes the expansion of this domain and thus makes our limbs so different from fish fins. Lung fish, but also more modern fish species such as cichlids, should be used here, as their embryos can be more easily examined genetically using modern techniques.
Source: Joost Woltering (University of Konstanz) et al., Science Advances, doi: 10.1126 / sciadv.abc3510