Genes VII

The development of segments in Drosophila occurs by the actions of segmentation genes that delineate successively smaller regions of the embryo. Asymmetry in the distribution of maternal gene products is established by interactions between the oocyte and surrounding cells. This leads to the expression of the gap genes, in 4 broad regions of the embryo. The gap genes in turn control the pair-rule genes, each of which is distributed in 7 stripes; and the pair-rule genes define the pattern of expression of the segment polarity genes, which delineate individual compartments. At each stage of expression, the relevant genes are controlled both by the products of genes that were expressed at the previous stage, and by interactions among themselves. The segmentation genes act upon the homeotic genes, which determine the identities of the individual compartments.

Each of the 4 maternal systems consists of a cascade which generates a locally distributed or locally active morphogen. The morphogen either is a transcription factor or causes the activation of a transcription factor. The transcription factor is the last component in each pathway.

The major anterior-posterior axis is determined by two systems: the anterior system establishes a gradient of bicoid from the anterior pole; and the posterior system produces nanos protein in the posterior half of the egg. These systems function to define a gradient of hunchback protein from the anterior end, with broad bands of knirps and giant in the posterior half. The terminal system acts to produce localized events at both termini. The dorsal-ventral system produces a gradient of nuclear localization of dorsal protein on the ventral side, which represses expression of dpp and zen; this leads to the ventral activation of twist and snail, and the dorsal-side activation of dpp and zen.

The early embryo consists of a syncytium, in which nuclei are exposed to common cytoplasm. It is this feature that allows all 4 maternal systems to control the function of a nucleus according to the coordinates of its position on the anterior-posterior and dorsal-ventral axes. At cellular blastoderm, zygotic RNAs are transcribed, and the developing embryo becomes dependent upon its own genes. Cells form at the blastoderm stage, after which successive interactions involve a cascade of transcriptional regulators.

Three gap genes are zinc-finger proteins, and one is a basic zipper protein. Their concentrations control expression of the pair-rule genes, which are also transcription factors. In particular, the expression of eve and ftz controls the boundaries of compartments, functioning in every other segment. The segment polarity genes represent the first step in the developmental cascade that involves functions other than transcription factors. Interactions between the segmentation gene products define unique combinations of gene expression for each segment.

Homeotic genes impose the program that determines the unique differentiation of each segment. The complex loci ANT-C and BX-C each contain a cluster of functions, whose spatial expression on the anterior-posterior axis reflects genetic position in the cluster. Each cluster contains one exceedingly large transcription unit as well as other, shorter units. Many of the transcription units (including the largest genes, Ubx and Antp) have patterns of alternative splicing, but no significance has been attributed to this yet. Proceeding from left to right in each cluster, genes are expressed in more posterior tissues. The genes are expressed in overlapping patterns in such a way that addition of a function confers new features of posterior identity; thus loss of a function results in a homeotic transformation from posterior to more anterior phenotype. The genes are controlled in a complex manner by a series of regulatory sites that extend over large regions; mutations in these sites are cis-acting, and may cause either loss-of-function or gain-of-function. The cis-acting mutations tend to act on successive segments of the fly, by controlling expression of the homeotic proteins.

The genes of the ANT-C and BX-C loci, and many segmentation genes (including the maternal gene bicoid and most of the pair-rule genes) contain a conserved motif, the homeobox. Homeoboxes are also found in genes of other eukaryotes, including worms, frogs, and mammals. In each case, these genes are expressed during early embryogenesis. In mammals, the Hox genes (which specify homeodomains in the Antennapedia class) are organized in clusters. There are 4 Hox clusters in both man and mouse. These clusters can be aligned with the ANT-C/BX-C clusters in such a way as to recognize homologies between genes at corresponding positions. Proceeding towards the right in a Hox cluster, a gene is expressed more towards the posterior of the embryo. The Hox genes have roles in conferring identity on segments of the brain and skeleton (and other tissues). The analogous clusters represent regulators of embryogenesis in mammals and flies. Hox clusters may be a characteristic of all animals.

Drosophila genes containing homeoboxes form an intricate regulatory network, in which one gene may activate or repress another. The relationship between the sequence of the homeodomain, the DNA target it recognizes, and the regulatory consequences, remains to be fully elucidated. Specificity in target choice appears to reside largely in the homeodomain; we have yet to explain the abilities of a particular homeoprotein to activate or to repress gene transcription at its various targets. The general principle is that segmentation and homeotic genes act in a transcriptional cascade, in which a series of hierarchical interactions between the regulatory proteins is succeeded by the activation of structural genes coding for body parts.