Spatial patterns of homeobox gene expression in the developing mammalian CNS.

1984 was a good year for those interested in the molecular genetics of development. In March and July, two groups 1'2 independently reported the discovery of the homeobox, a 180 base-pair DNA sequence found in Drosophila. The sequence is associated with a number of genes that are known to affect body pattern, and that are expressed in characteristic regions during the development of the fly. What was more surprising is that the homeobox turned out to be highly conserved in vertebratesa: it is remarkable, for example, that the homeo-domain associated with the Drosophila gene Antennapedia and the human gene HHo. c10 c~rf-fer by only three of the terminal 53 amino acids 4. An editorial review that accompanied the original papers called the homeobox 'A Rosetta stone...', betraying the hope that some fundamental principle about the control of gene expression during pattern formation was shortly to emerge. Since then, molecular biologists have looked hard for regional expression patterns of the homeobox genes in vertebrate embryos, while developmental biologists have started to cut sagittal or coronal sections of their embryos instead of transverse sections. Homeobox gene expression in the epidermis and nervous system of the fly In Drosophila, the genes involved in the establishment of body pattern are classified s into five distinct classes: (1) the selector (homeotic) genes, from which the homeobox takes its name, are those that specify segment identity-when mutated, they convert a whole epidermal segment, or a part of a segment, into another (for example, the gene Antennapedia, which in the mutant generates a leg instead of an antenna in the head); (2) the segment polarity genes, which affect the polarity of individual epidermal segments (for example, the gene engrailed, which is expressed in the posterior portion of each segment); (3) the pair-rule genes, which affect alternate metameres along the length of the animal (examples are the genes fushi tarazu, odd-skipped and even-skipped); (4) the gap genes, which are expressed in a group of contiguous epidermal segments, an example of which is the gene Kriippel, which in the mutant deletes a group of segments in the middle of the fly; and (5) the maternal effect genes, which affect development but are transcribed in the mother (an example is the gene bicoid, which when defective produces a fly with no head and two caudal ends). With the exception of the gap genes, there are homeobox genes in all these …