Le cas CRISPR, mutations « ready-made » et évolution lamarckienne d’un système immunitaire adaptatif

Abstract

La génétique a depuis longtemps établi que les mutations apparaissent indépendamment de leur valeur adaptative. Cependant, il a été identifié récemment un mécanisme d’évolution d’une des composantes du système immunitaire adaptatif CRISPR-Cas qui est volontiers qualifié de lamarckien. Ce système repose sur l’intégration de fragments du génome d’agents infectieux dans le génome de l’hôte qui lui confère une meilleure valeur adaptative. Il existe donc une diversité de mécanismes évolutifs et, si la plupart relèvent de processus strictement darwiniens, une minorité implique des mécanismes de mutation plus ou moins biaisés, donc plus ou moins lamarckiens. Pour être extrêmement rares, ces processus n’en demeurent pas moins importants pour comprendre les diverses modalités de l’évolution.

Since genetics has shown that mutation predates selection, biology has developed within the Darwinian paradigm framework. However, a mechanism that produces favorable mutations preferentially in response to adaptive constraints has been recently identified. This mechanism, the CRISPR-Cas adaptive immunity system, is considered as a bona fide example of Lamarckian evolution, even if it only reflects loosely Lamarck’s ideas. This unusual evolutionary process is made possible by two prokaryotic properties: i) somatic and germinal cells are not distinct sets of cells; ii) Archae and Bacteria very frequently integrate DNA fragments from the environment, and they therefore have access to a source of “ready-made” useful genetic information. The CRISPR-Cas is a defense system against viruses and plasmids that is based on the integration of genomic fragments of these infectious agents into the host genome, and that protects the host against subsequent infections. Therefore, this mechanism does produce advantageous mutations by integrating DNA from the environment and allowing its transmission to descendants. In conclusion, most of the time evolution relies on purely Darwinian processes, i.e. mutations occurring at random, but in a small minority of cases the occurrence of mutations is more or less biased, and is therefore more or less Lamarckian. Although they are rare, such processes are nevertheless important to our understanding of the plurality of modes of evolution.