An evo-devo look at limb develpment

The expression of fish Hox genes in a mouse embryo.
Photo Credit: Denis Duboule, UNIGE.

The transition from water to land is one of the most fascinating enigmas of evolution. Especially since both fish terrestrial animals have groups of architects genes, Hoxa and Hoxd , necessary for the formation of fins, as well as that of members during embryonic development. Editor Denis Duboule, a professor at the University of Geneva and EPFL, a team of scientists studied the parallel structure and behavior of these genes in the mouse embryo and in the zebrafish. In both species, the researchers found a similar three-dimensional organization of DNA genes architects observed. They have concluded that the main mechanism used to shape members of tetrapods was already in fish. They then inserted genes architects Hox fish in transgenic mouse embryos and saw that they were active only in the arms of the mice, but not in his fingers, showing that the DNA of fish does not have the elements Genetic essential to the formation of the fingers. Published in the journal PLoS Biology , these results highlight the fact that the digital part of the members of terrestrial animals is the result of a development from a pre-existing infrastructure ancestral DNA, although this represents an evolutionary novelty in tetrapods.

During embryonic development animal genes Hox or “architect genes” are responsible for the organization of the structures of the body. Fish and mammals have groups of genes Hoxa and Hoxd , both of which are necessary for the formation of fins and limbs. The team of Denis Duboule, a professor at the University of Geneva (University of Geneva) and the Ecole Polytechnique Fédérale de Lausanne (EPFL), has recently shown that during development of mammals, Hoxd genes depend on structure “bimodal” three-dimensional DNA to direct the development of members. This genetic structure provides a subdivision of the member in an arm and a hand. Fish them at the fins, do not show this subdivision. ancestral control strategy … “To determine the genetic origin of this subdivision in arm and leg during evolution, we compared the genetic processes at work during the development of fins and limbs, in embryos of zebrafish and mouse, “says first author of the study, Joost Woltering, researcher at the Department of Genetics and Evolution, Faculty of Science the University of Geneva.

Scientists were surprised to find a three-dimensional bimodal architecture similar DNA in the region of Hoxd genes in fish. These results indicate that the regulatory mechanism used to train members of tetrapods is probably predates the divergence between fish and tetrapods. “We expected the contrary that it is precisely this bimodal DNA conformation that makes all the difference in the production of members, compared to the fins,” says Joost Woltering. … that just needs to be modernized fingers would they therefore homologous rays, these bony structures located at the end of the fish fins? To answer this question, geneticists have inserted into mouse embryos genomic regions that regulate gene expression Hox in fish fins. “Surprisingly, the regulatory regions of the fish triggered gene expression Hox mainly in the arm, not in the fingers, explains Duboule.

Overall, this suggests that, during the transition between fins and limbs, the appearance of our fingers resulting from the modernization of an existing control mechanism. ” “What probably happened is comparable to a renewal process, as is done in engineering structures to equip machines with obsolete technologies. In this case, it is a DNA primitive architecture which has a new “technology” to make fingers and toes, “says Joost Woltering. The fin rays are not homologous fingers

The researchers conclude that although the fish have a tool kit gene regulation Hox to produce fingers, this potential is not used as it is in tetrapods. Therefore, they believe that the fin rays are not homologous fingers tetrapods, although they depend in part on a shared control strategy. Geneticists now intend to find out exactly what has changed between DNA elements in fish and tetrapods. “Now we know a lot of genetic switches in mice, which direct the expression of genes Hox fingers. It is important to find out exactly how these processes work today to understand what showed fingers and promoted the colonization of land, “says Denis Duboule. For if our first terrestrial ancestor fours came out of the sea there are some 350 million years, it is sufficient to observe a lungfish, our closest living relative among fish, crawling on all four fins pointed to imagine likely the first stages of evolution on land.

Joost M. Woltering, Daan Noordermeer, Marion Leleu, Denis Duboule. Conservation and Divergence of Regulatory Strategies at Hox Loci and the Origin of Tetrapod Digits. PLoS Biology, 2014; 12 (1): e1001773 DOI: 10.1371/journal.pbio.1001773

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