Theevo-devo gene toolkit is the small subset ofgenes in an organism'sgenome whose products control the organism'sembryonic development.Toolkit genes are central to the synthesis ofmolecular genetics,palaeontology,evolution and developmental biology in the science ofevolutionary developmental biology (evo-devo). Many of them are ancient and highlyconserved among animalphyla.
Toolkit genes are highlyconserved amongphyla, meaning that they are ancient, dating back to thelast common ancestor of bilaterian animals. For example, that ancestor had at least 7Pax genes fortranscription factors.[1]
Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. The majority of toolkit genes are components of signaling pathways and encode for the production of transcription factors,cell adhesion proteins, cell surfacereceptor proteins (and signallingligands that bind to them), and secretedmorphogens; all of these participate in defining the fate of undifferentiated cells, generating spatial and temporal patterns that, in turn, form thebody plan of the organism. Among the most important of the toolkit genes are those of theHox gene cluster, or complex. Hox genes, transcription factors containing the more broadly distributedhomeobox protein-binding DNA motif, function in patterning the body axis. Thus, by combinatorially specifying the identity of particular body regions, Hox genes determine wherelimbs and otherbody segments will grow in a developingembryo orlarva. Aparadigmatic toolkit gene isPax6/eyeless, which controlseye formation in all animals. It has been found to produce eyes in mice andDrosophila, even if mousePax6/eyeless was expressed inDrosophila.[2]
This means that a big part of the morphological evolution undergone by organisms is a product of variation in the genetic toolkit, either by the genes changing their expression pattern or acquiring new functions. A good example of the first is the enlargement of the beak in Darwin's large ground-finch (Geospiza magnirostris), in which the geneBMP is responsible for the larger beak of this bird, relative to the other finches.[3]
The loss of legs insnakes and othersquamates is another good example of genes changing their expression pattern. In this case the geneDistal-less is very under-expressed, or not expressed at all, in the regions where limbs would form in othertetrapods.[4]In 1994,Sean B. Carroll's team made the groundbreaking discovery that this same gene determines theeyespot pattern inbutterflywings, showing that toolkit genes can change their function.[5][6][7]
Toolkit genes, as well as being highly conserved, also tend to evolve the same functionconvergently orin parallel. Classic examples of this are the already mentionedDistal-less gene, which is responsible for appendage formation in both tetrapods and insects, or, at a finer scale, the generation of wing patterns in the butterfliesHeliconius erato andHeliconius melpomene. These butterflies areMüllerian mimics whose coloration pattern arose in different evolutionary events, but is controlled by the same genes.[8]This supportsMarc Kirschner andJohn C. Gerhart's theory ofFacilitated Variation, which states that morphological evolutionary novelty is generated by regulatory changes in various members of a large set of conserved mechanisms of development and physiology.[9]