The evolution has never ceased to surprise by its unexpected detours. The most complex forms of our body often come from rearrangements of old mechanisms, reused for new functions. Among these puzzles that intrigue researchers, the origin of the fingers occupies a special place, because it conditions the way in which vertebrates have conquered the firm land.
Researchers from the University of Geneva, EPFL, Harvard and the College of France have discovered that certain DNA sequences, now responsible for the development of fingers among mammals, were originally used to form a much less noble orifice: the cesspool. This unique structure in many fish and reptiles allows the evacuation of waste but also reproduction. And it is precisely in this terminal region of the body that it all started.
The origin of the fingers traced to the ends of the digestive tract
To understand this unexpected filiation, the researchers compared mouse and zebrasses embryos, targeting the regulatory regions of certain key development genes, the Hoxd. These genes act as conductors of the organization of the body, especially in the training of members. By marking some of these sequences with fluorescent tracers, the team observed a surprising phenomenon. In mice, the areas light up in the fingers in formation, while in Zebras fish, it is the cells of the cloaca that are activated.
The removal of these sequences using CRISPR technology has confirmed their role. Deprived of this regulation, the mice developed abnormal fingers, and the fish saw their cesspool that are poorly formed. This observation suggests that the gene regulators necessary for the manufacture of the fingers existed long before the appearance of the members, but that they were used at another end. Relayed by Sciencealert, this discovery illustrates how the evolution proceeds by misappropriation of tools already available rather than by creation Ex nihilo.
According to Denis Duboule, a geneticist at the origin of the study, it is now necessary to consider that the structure which allows today to be ticked on a keyboard or to enter an object draws its logic of development from a function formerly dedicated to excretion and reproduction. The fingers and the cessation, as the terminal parts of the body, would share the same morphogenetic origin.
Reuse rather than invent, the great strategy of the living
One of the major lessons in this work is the ability of the evolution to reuse existing bricks to build innovations. This principle, which scientists call cooptation, consists in assigning a new function to an old structure. In this specific case, the genes responsible for the formation of the fingers in tetrapods would have directly reused the genetic regulator landscape initially involved in the formation of the cloaca.
This landscape, called 5dom, acts as a control tower on the expression of several architects genes, including Hoxd13. In mice as in fish, these regulatory regions influence the same gene sub-family. But their field of action has slipped over time, from the posterior ends of the digestive tract to those of the members. The University of Geneva, which piloted the study, stresses that this strategy is not isolated. There are similar cases for the formation of genitals, also orchestrated by the same HOX regulators.
This evolutionary recycling does not testify to a hazardous DIY, but of a deep biological efficiency. He recalls that the genome is not a succession of frozen instructions, but a huge toolbox where certain parts can change use. Thanks to these diversions, evolutionary constraints become springboards, and a structure as complex as the human hand can arise from a program initially provided for a joint orifice with reptiles and fish.
