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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

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Molecular Biology of the Cell. 4th edition.

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Drosophila and the Molecular Genetics of Pattern Formation: Genesis of the Body Plan

It is the fly Drosophila melanogaster (Figure 21-23), more than any other organism, that has transformed our understanding of how genes govern the patterning of the body. The anatomy of Drosophila is more complex than that of C. elegans, with more than 100 times as many cells, and it shows more obvious parallels with our own body structure. Surprisingly, the fly has fewer genes than the worm—about 14,000 as compared with 19,000. On the other hand, it has almost twice as much noncoding DNA per gene (about 10,000 nucleotides on average, as compared with about 5000). The molecular construction kit has fewer types of parts, but the assembly instructions—as specified by the regulatory sequences in the non-coding DNA—seem to be more voluminous.

Figure 21-23. Drosophila melanogaster.

Figure 21-23

Drosophila melanogaster. Dorsal view of a normal adult fly. (A) Photograph. (B) Labeled drawing. (Photograph courtesy of E.B. Lewis.)

Decades of genetic study, culminating in massive systematic genetic screens, have yielded a catalogue of the developmental control genes that define the spatial pattern of cell types and body structures of the fly, and molecular biology has given us the tools to watch these genes in action. By in situ hybridization using DNA or RNA probes on whole embryos, or by staining with labeled antibodies to reveal the distribution of specific proteins, one can observe directly how the internal states of the cells are defined by the sets of regulatory genes that they express at different times of development. Moreover, by analyzing animals that are a patchwork of mutant and nonmutant cells, one can discover how each gene operates as part of a system to specify the organization of the body.

Most of the genes controlling the pattern of the body in Drosophila turn out to have close counterparts in higher animals, including ourselves. In fact, many of the basic devices for defining the body plan and patterning individual organs and tissues are astonishingly similar. Thus, quite surprisingly, the fly has provided the key to understanding the molecular genetics of our own development.

Flies, like nematode worms, are ideal for genetic studies: cheap to breed, easy to mutagenize, and rapid in their reproductive cycle. But there is a more fundamental reason why they have been so important for developmental geneticists. As emphasized earlier, as a result of gene duplications, vertebrate genomes often contain two or three homologous genes corresponding to a single gene in the fly. A mutation that disrupts one of these genes very often fails to reveal the gene's core function, because the other homologs share this function and remain active. In the fly, with its more economical gene set, this phenomenon of genetic redundancy is less prevalent. The phenotype of a single mutation in the fly therefore more often directly uncovers the function of the mutant gene.

The Insect Body Is Constructed as a Series of Segmental Units

The timetable of Drosophila development, from egg to adult, is summarized in Figure 21-24. The period of embryonic development begins at fertilization and takes about a day, at the end of which the embryo hatches out of the egg shell to become a larva. The larva then passes through three stages, or instars, separated by molts in which it sheds its old coat of cuticle and lays down a larger one. At the end of the third instar it pupates. Inside the pupa, a radical remodeling of the body takes place—a process called metamorphosis. Eventually, about nine days after fertilization, an adult fly, or imago, emerges.

Figure 21-24. Synopsis of Drosophila development from egg to adult fly.

Figure 21-24

Synopsis of Drosophila development from egg to adult fly.

The fly consists of a head, with mouth, eyes, and antennae, followed by three thoracic segments (numbered T1 to T3), and eight or nine abdominal segments (numbered A1 to A9). Each segment, although different from the others, is built according to a similar plan. Segment T1, for example, carries a pair of legs, T2 carries a pair of legs plus a pair of wings, and T3 carries a pair of legs plus a pair of halteres—small knob-shaped balancers important in flight, evolved from the second pair of wings that more primitive insects possess. The quasi-repetitive segmentation develops in the early embryo during the first few hours after fertilization (Figure 21-25), but it is more obvious in the larva (Figure 21-26), where the segments look more similar than in the adult. In the embryo it can be seen that the rudiments of the head, or at least the future adult mouth parts, are likewise segmental. At the two ends of the animal, however, there are highly specialized terminal structures that are not segmentally derived.

Figure 21-25. The origins of the Drosophila body segments during embryonic development.

Figure 21-25

The origins of the Drosophila body segments during embryonic development. The embryos are seen in side view in drawings (A-C) and corresponding scanning electron micrographs (D-F). (A and D) At 2 hours the embryo is at the syncytial blastoderm stage (see (more...)

Figure 21-26. The segments of the Drosophila larva and their correspondence with regions of the blastoderm.

Figure 21-26

The segments of the Drosophila larva and their correspondence with regions of the blastoderm. The parts of the embryo that become organized into segments are shown in color. The two ends of the embryo, shaded gray, are not segmented and become tucked (more...)

The boundaries between segments are traditionally defined by visible anatomical markers; but in discussing gene expression patterns it is often convenient to draw a different set of segmental boundaries, defining a series of segmental units called parasegments, half a segment out of register with the traditional divisions (see Figure 21-26).

Drosophila Begins Its Development as a Syncytium

The egg of Drosophila is about 0.5 mm long and 0.15 mm in diameter, with a clearly defined polarity. Like the eggs of other insects, but unlike vertebrates, it begins its development in an unusual way: a series of nuclear divisions without cell division creates a syncytium. The early nuclear divisions are synchronous and extremely rapid, occurring about every 8 minutes. The first nine divisions generate a cloud of nuclei, most of which migrate from the middle of the egg toward the surface, where they form a monolayer called the syncytial blastoderm. After another four rounds of nuclear division, plasma membranes grow inward from the egg surface to enclose each nucleus, thereby converting the syncytial blastoderm into a cellular blastoderm consisting of about 6000 separate cells (Figure 21-27). About 15 of the nuclei populating the extreme posterior end of the egg are segregated into cells a few cycles earlier; these pole cells are the germ-line precursors (primordial germ cells) that will give rise to eggs or sperm.

Figure 21-27. Development of the Drosophila egg from fertilization to the cellular blastoderm stage.

Figure 21-27

Development of the Drosophila egg from fertilization to the cellular blastoderm stage. (A) Schematic drawings. (B) Surface view—an optical-section photograph of blastoderm nuclei undergoing mitosis at the transition from the syncytial to the cellular (more...)

Up to the cellular blastoderm stage, development depends largely—although not exclusively—on stocks of maternal mRNA and protein that accumulated in the egg before fertilization. The frantic rate of DNA replication and nuclear division evidently gives little opportunity for transcription. After cellularization, cell division continues in a more conventional way, asynchronously and at a slower rate, and the rate of transcription increases dramatically. Gastrulation begins a little while before cellularization is complete, when parts of the sheet of cells forming the exterior of the embryo start to tuck into the interior to form the gut, the musculature, and associated internal tissues. A little later and in another region of the embryo, a separate set of cells move from the surface epithelium into the interior to form the central nervous system. By marking and following the cells through these various movements, one can draw a fate map for the monolayer of cells on the surface of the blastoderm (Figure 21-28).

Figure 21-28. Fate map of a Drosophila embryo at the cellular blastoderm stage.

Figure 21-28

Fate map of a Drosophila embryo at the cellular blastoderm stage. The embryo is shown in side view and in cross section, displaying the relationship between the dorsoventral subdivision into future major tissue types and the anteroposterior pattern of (more...)

As gastrulation nears completion, a series of indentations and bulges appear in the surface of the embryo, marking the subdivision of the body into segments along its anteroposterior axis (see Figure 21-25). Soon a fully segmented larva emerges, ready to start eating and growing. Within the body of the larva, small groups of cells remain apparently undifferentiated, forming structures called imaginal discs. These will grow as the larva grows, and eventually they will give rise to most of the structures of the adult body, as we shall see later.

A head end and a tail end, a ventral (belly) side and a dorsal (back) side, a gut, a nervous system, a series of body segments—these are all features of the basic body plan that Drosophila shares with many other animals, including ourselves. We begin our account of the mechanisms of Drosophila development by considering how this body plan is set up.

Genetic Screens Define Groups of Genes Required for Specific Aspects of Early Patterning

By carrying out a series of genetic screens based on saturation mutagenesis (see Chapter 8), it has been possible to amass a collection of Drosophila mutants that appears to include changes in a large proportion of the genes affecting development. Independent mutations in the same gene can be distinguished from mutations in separate genes by a complementation test (see Panel 8-1, p. 527), leading to a catalog of genes classified according to their mutant phenotypes. In such a catalog, a group of genes with very similar mutant phenotypes will often code for a set of proteins that work together to perform a particular function.

Sometimes the developmental functions revealed by mutant phenotypes are those that one would expect; sometimes they are a surprise. A large-scale genetic screen focusing on early Drosophila development revealed that the key genes fall into a relatively small set of functional classes defined by their mutant phenotypes. Some—the egg-polarity genes (Figure 21-29)—are required to define the anteroposterior and dorsoventral axes of the embryo and mark out its two ends for special fates, by mechanisms involving interactions between the oocyte and surrounding cells in the ovary. Others, the gap genes, are required in specific broad regions along the anteroposterior axis of the early embryo to allow their proper development. A third category, the pair-rule genes, are required, more surprisingly, for development of alternate body segments. A fourth category, the segment polarity genes, are responsible for organizing the anteroposterior pattern of each individual segment.

Figure 21-29. The domains of the anterior, posterior, and terminal systems of egg-polarity genes.

Figure 21-29

The domains of the anterior, posterior, and terminal systems of egg-polarity genes. The upper diagrams show the fates of the different regions of the egg/early embryo and indicate (in white) the parts that fail to develop if the anterior, posterior, or (more...)

The discovery of these four systems of genes and the subsequent analysis of their functions (an enterprise that still continues) was a famous tour-de-force of developmental genetics. It has had a revolutionary impact on all of developmental biology by showing the way toward a systematic, comprehensive account of the genetic control of embryonic development. In this section, we shall summarize only briefly the conclusions relating to the earliest phases of Drosophila development, because these are insect-specific; we dwell at greater length on the parts of the process that illustrate more general principles.

Interactions of the Oocyte With Its Surroundings Define the Axes of the Embryo: the Role of the Egg-Polarity Genes

Surprisingly, the earliest steps of animal development are among the most variable, even within a phylum. A frog, a chicken, and a mammal, for example, even though they develop in similar ways later, make eggs that differ radically in size and structure, and they begin their development with different sequences of cell divisions and cell specialization events.

The style of early development that we have described for C. elegans is typical of many classes of animals. In contrast, the early development of Drosophila represents a rather extreme variant. The main axes of the future insect body are defined before fertilization by a complex exchange of signals between the unfertilized egg, or oocyte, and the follicle cells that surround it in the ovary (Figure 21-30). Then, in the syncytial phase following fertilization, an exceptional amount of patterning occurs in the array of rapidly dividing nuclei, before the first partitioning of the egg into separate cells. Here, there is no need for the usual forms of cell-cell communication involving transmembrane signaling; neighboring regions of the early Drosophila embryo can communicate by means of gene regulatory proteins and mRNA molecules that diffuse or are actively transported through the cytoplasm of the giant multinuclear cell.

Figure 21-30. A Drosophila oocyte in its follicle.

Figure 21-30

A Drosophila oocyte in its follicle. The oocyte is derived from a germ cell that divides four times to give a family of 16 cells that remain in communication with one another via cytoplasmic bridges (gray). One member of the family group becomes the oocyte, (more...)

In the stages before fertilization, the anteroposterior axis of the future embryo becomes defined by three systems of molecules that create landmarks in the oocyte (Figure 21-31). Following fertilization, each landmark serves as a beacon, providing a signal, in the form of a morphogen gradient, that organizes the developmental process in its neighborhood. Two of these signals are generated from localized deposits of specific mRNA molecules. The future anterior end of the embryo contains a high concentration of mRNA for a gene regulatory protein called Bicoid; this mRNA is translated to produce Bicoid protein, which diffuses away from its source to form a concentration gradient with its maximum at the anterior end of the egg. The future posterior end of the embryo contains a high concentration of mRNA for a regulator of translation called Nanos, which sets up a posterior gradient in the same way. The third signal is generated symmetrically at both ends of the egg, by local activation of a transmembrane tyrosine kinase receptor called Torso. The activated receptor exerts its effects over a shorter range, marking the sites of specialized terminal structures that will form at the head and tail ends of the future larva and also defining the rudiments of the future gut. The three sets of genes responsible for these localized determinants are referred to as the anterior, posterior, and terminal sets of egg-polarity genes.

Figure 21-31. The organization of the four egg-polarity gradient systems.

Figure 21-31

The organization of the four egg-polarity gradient systems. The receptors Toll and Torso are distributed all over the membrane; the coloring in the diagrams on the right indicates where they become activated by extracellular ligands.

A fourth landmark defines the dorsoventral axis (see Figure 21-31): a protein that is produced by follicle cells underneath the future ventral region of the embryo leads to localized activation of another transmembrane receptor, called Toll, in the oocyte membrane. The genes required for this function are called dorsoventral egg-polarity genes.

All the egg-polarity genes in these four classes are maternal-effect genes: it is the mother's genome, not the zygotic genome, that is critical. Thus, a fly whose chromosomes are mutant in both copies of the bicoid gene but who is born from a mother carrying one normal copy of bicoid develops perfectly normally, without any defects in the head pattern. However, if that daughter fly is a female no functional bicoid mRNA can be deposited into the anterior part of her own eggs, and all of these will develop into headless embryos regardless of the father's genotype.

Each of the four egg-polarity signals—provided by Bicoid, Nanos, Torso, and Toll—exerts its effect by regulating (directly or indirectly) the expression of genes in the nuclei of the blastoderm. The use of these particular molecules to organize the egg is not a general feature of early animal development—indeed, only Drosophila and closely related insects possess a bicoid gene. And Toll has been coopted here for dorsoventral patterning; its more ancient and universal function is in the innate immune response.

Nevertheless, the egg-polarity system shows some highly conserved features. For example, the localization of nanos mRNA at one end of the egg is linked to, and dependent on, the localization of germ-cell determinants at that site, just as it is in C. elegans. Later in development, as the zygotic genome comes into play under the influence of the egg-polarity system, more similarities with other animal species become apparent. We shall use the dorsoventral system to illustrate this point.

The Dorsoventral Signaling Genes Create a Gradient of a Nuclear Gene Regulatory Protein

Localized activation of the Toll receptor on the ventral side of the egg controls the distribution of Dorsal, a gene regulatory protein inside the egg. The Dorsal protein belongs to the same family as the NF-κB gene regulatory protein of vertebrates (discussed in Chapter 15). Its Toll-regulated activity, like that of NF-κB, depends on its translocation from the cytoplasm, where it is held in an inactive form, to the nucleus, where it regulates gene expression. In the newly laid egg, both the dorsal mRNA (detected by in situ hybridization) and the protein it encodes (detected with antibodies) are distributed uniformly in the cytoplasm. After the nuclei have migrated to the surface of the embryo to form the blastoderm, however, a remarkable redistribution of the Dorsal protein occurs: dorsally the protein remains in the cytoplasm, but ventrally it is concentrated in the nuclei, with a smooth gradient of nuclear localization between these two extremes (Figure 21-32). The signal transmitted by the Toll protein controls this redistribution of Dorsal through a signaling pathway that is essentially the same as the Toll-dependent pathway involved in innate immunity.

Figure 21-32. The concentration gradient of Dorsal protein in the nuclei of the blastoderm, as revealed by an antibody.

Figure 21-32

The concentration gradient of Dorsal protein in the nuclei of the blastoderm, as revealed by an antibody. Dorsally, the protein is present in the cytoplasm and absent from the nuclei; ventrally, it is depleted in the cytoplasm and concentrated in the (more...)

Once inside the nucleus, the Dorsal protein turns on or off the expression of different sets of genes depending on its concentration. The expression of each responding gene depends on its regulatory DNA—specifically, on the number and affinity of the binding sites that this DNA contains for Dorsal and other regulatory proteins. In this way, the regulatory DNA can be said to interpret the positional signal provided by the Dorsal protein gradient, so as to define a dorsoventral series of territories—distinctive bands of cells that run the length of the embryo (Figure 21-33A). Most ventrally—where the concentration of Dorsal protein is highest—it switches on, for example, the expression of a gene called twist that is specific for mesoderm (Figure 21-34). Most dorsally, where the concentration of Dorsal protein is lowest, the cells switch on decapentaplegic (dpp). And in an intermediate region, where the concentration of Dorsal protein is high enough to repress dpp but too low to activate twist, the cells switch on another set of genes, including one called short gastrulation (sog).

Figure 21-33. Morphogen gradients patterning the dorsoventral axis of the embryo.

Figure 21-33

Morphogen gradients patterning the dorsoventral axis of the embryo. (A) The gradient of Dorsal protein defines three broad territories of gene expression, marked here by the expression of three representative genes—dpp, sog, and twist. (B) Slightly (more...)

Figure 21-34. Origin of the mesoderm from cells expressing twist.

Figure 21-34

Origin of the mesoderm from cells expressing twist. Embryos were fixed at successive stages, cross-sectioned, and stained with an antibody against the Twist protein, a gene regulatory protein of the bHLH family. The cells that express Twist move into (more...)

Dpp and Sog Set Up a Secondary Morphogen Gradient to Refine the Pattern of the Dorsal Part of the Embryo

Products of the genes directly regulated by the Dorsal protein generate in turn more local signals that define finer subdivisions of the dorsoventral axis. These signals act after cellularization, and take the form of conventional extracellular signaling molecules. In particular, dpp codes for the secreted Dpp protein, which forms a local morphogen gradient in the dorsal part of the embryo. The gene sog, meanwhile, codes for another secreted protein that is produced in the neurogenic ectoderm and acts as an antagonist of Dpp. The opposing diffusion gradients of these two proteins create a steep gradient of Dpp activity. The highest Dpp activity levels, in combination with certain other factors, cause development of the most dorsal tissue of all—extraembryonic membrane; intermediate levels cause development of dorsal ectoderm; and very low levels allow development of neurogenic ectoderm (Figure 21-33B).

The Insect Dorsoventral Axis Corresponds to the Vertebrate Ventrodorsal Axis

Dpp is a member of the TGFβ superfamily of signaling molecules that is also important in vertebrates; Sog is a homolog of the vertebrate protein chordin. It is striking that a Dpp homolog, BMP4, and chordin work together in vertebrates in the same way as do Dpp and Sog in Drosophila. These two proteins control the dorsoventral pattern of the ectoderm, with high levels of chordin defining the region that is neurogenic and high levels of BMP4 activity defining the region that is not. This, combined with other molecular parallels, strongly suggests that this part of the body plan, has been conserved between insects and vertebrates. However, the axis is inverted, so that dorsal in the fly corresponds to ventral in the vertebrate (Figure 21-35). At some point in its evolutionary history, it seems, the ancestor of one of these classes of animals took to living life upside down.

Figure 21-35. The vertebrate body plan as a dorsoventral inversion of the insect body plan.

Figure 21-35

The vertebrate body plan as a dorsoventral inversion of the insect body plan. The mechanism of dorsoventral patterning in a vertebrate embryo is discussed in more detail later in this chapter. Note the correspondence with regard to the circulatory system (more...)

Three Classes of Segmentation Genes Refine the Anterior- Posterior Maternal Pattern and Subdivide the Embryo

After the initial gradients of Bicoid and Nanos are created to define the anteroposterior axis, the segmentation genes refine the pattern. Mutations in any one of the segmentation genes alter the number of segments or their basic internal organization without affecting the global polarity of the embryo. Segmentation genes are expressed by subsets of cells in the embryo, so their products are the first components that the embryo's own genome, rather than the maternal genome, contributes to embryonic development. They are therefore called zygotic-effect genes to distinguish them from the earlier maternal-effect genes.

The segmentation genes fall into three groups according to their mutant phenotypes and the stages at which they act (Figure 21-36). First come a set of at least six gap genes, whose products mark out coarse subdivisions of the embryo. Mutations in a gap gene eliminate one or more groups of adjacent segments, and mutations in different gap genes cause different but partially overlapping defects. In the mutant Krüppel, for example, the larva lacks eight segments, from T1 to A5 inclusive.

Figure 21-36. Examples of the phenotypes of mutations affecting the three types of segmentation genes.

Figure 21-36

Examples of the phenotypes of mutations affecting the three types of segmentation genes. In each case the areas shaded in green on the normal larva (left) are deleted in the mutant or are replaced by mirror-image duplicates of the unaffected regions. (more...)

The next segmentation genes to act are a set of eight pair-rule genes. Mutations in these cause a series of deletions affecting alternate segments, leaving the embryo with only half as many segments as usual. While all the pair-rule mutants display this two-segment periodicity, they differ in the precise positioning of the deletions relative to the segmental or parasegmental borders. The pair-rule mutant even-skipped (eve), for example, which is discussed in Chapter 9, lacks the whole of each odd-numbered parasegment, while the pair-rule mutant fushi tarazu (ftz) lacks the whole of each even-numbered parasegment, and the pair-rule mutant hairy lacks a series of regions that are of similar width but out of register with the parasegmental units.

Finally, there are at least 10 segment-polarity genes. Mutations in these genes produce larvae with a normal number of segments but with a part of each segment deleted and replaced by a mirror-image duplicate of all or part of the rest of the segment. In gooseberry mutants, for example, the posterior half of each segment (that is, the anterior half of each parasegment) is replaced by an approximate mirror image of the adjacent anterior half-segment (see Figure 21-36).

We see later that, in parallel with the segmentation process, a further set of genes, the homeotic selector genes, serve to define and preserve the differences between one segment and the next.

The phenotypes of the various segmentation mutants suggest that the segmentation genes form a coordinated system that subdivides the embryo progressively into smaller and smaller domains along the anteroposterior axis, distinguished by different patterns of gene expression. Molecular genetics has helped to reveal how this system works.

The Localized Expression of Segmentation Genes Is Regulated by a Hierarchy of Positional Signals

About three-quarters of the segmentation genes, including all of the gap genes and pair-rule genes, code for gene regulatory proteins. Their actions on one another and on other genes can therefore be observed by comparing gene expression in normal and mutant embryos. By using appropriate probes to detect the gene transcripts or their protein products, one can, in effect, take snapshots as genes switch on and off in changing patterns. Repeating the process with mutants that lack a particular segmentation gene, one can begin to dissect the logic of the entire gene control system.

The products of the egg-polarity genes provide the global positional signals in the early embryo. These cause particular gap genes to be expressed in particular regions. The products of the gap genes then provide a second tier of positional signals that act more locally to regulate finer details of patterning through the expression of yet other genes, including the pair-rule genes (Figure 21-37). The pair-rule genes in turn collaborate with one another and with the gap genes to set up a regular periodic pattern of expression of segment-polarity genes, and the segment-polarity genes collaborate with one another to define the internal pattern of each individual segment. The strategy, therefore, is one of sequential induction (see Figure 21-15). By the end of the process, the global gradients produced by the egg-polarity genes have triggered the creation of a fine-grained pattern through a hierarchy of sequential, progressively more local, positional controls. Because the global positional signals that start the process do not have to directly specify fine details, the individual cell nuclei do not have to be governed with extreme precision by small differences in the concentration of these signals. Instead, at each step in the sequence, new signals come into play, providing substantial localized differences of concentration to define new details. Sequential induction is thus a robust strategy. It works reliably to produce fly embryos that all have the same pattern, despite the essential imprecision of biological control systems, and despite variations in conditions such as the temperature at which the fly develops.

Figure 21-37. The regulatory hierarchy of egg-polarity, gap, segmentation, and homeotic selector genes.

Figure 21-37

The regulatory hierarchy of egg-polarity, gap, segmentation, and homeotic selector genes. The photographs show expression patterns of representative examples of genes in each category, revealed by staining with antibodies against the protein products. (more...)

The Modular Nature of Regulatory DNA Allows Genes to Have Multiple Independently Controlled Functions

The elaborate patterning process just described depends on the long stretches of noncoding DNA sequence that control the expression of each of the genes involved. These regulatory regions bind multiple copies of the gene regulatory proteins produced by the patterning genes expressed earlier. Like an input-output logic device, an individual gene is thus turned on and off according to the particular combination of proteins bound to its regulatory regions at each stage of development. In Chapter 7 we describe one particular segmentation gene—the pair-rule gene even-skipped (eve)—and discuss how the decision whether to transcribe the gene is made on the basis of all these inputs (see Figure 7-55). This example can be taken further to illustrate some important principles of developmental patterning.

Individual stripes of eve expression depend on separate regulatory modules in the eve regulatory DNA. Thus, one regulatory module is responsible for driving eve expression in stripes 1 + 5, another for stripe 2, another for stripes 3 + 7 , and yet another for stripes 4 + 6 (Figure 21-38). Each regulatory module defines a different set of requirements for gene expression according to the concentrations of the products of the egg-polarity and gap genes. In this way, the eve regulatory DNA serves to translate the complex nonrepetitive pattern of egg-polarity and gap proteins into the periodic pattern of expression of a pair-rule gene.

Figure 21-38. Modular organization of the regulatory DNA of the eve gene.

Figure 21-38

Modular organization of the regulatory DNA of the eve gene. In the experiment shown, cloned fragments of the regulatory DNA were linked to a LacZ reporter (a bacterial gene). Transgenic embryos containing these constructs were then stained by in situ (more...)

The modular organization of the eve regulatory DNA just described is typical of gene regulation in multicellular animals and plants, and it has profound implications. By stringing together sequences of modules that respond to different combinations of regulatory proteins, it is possible to generate almost any pattern of gene expression on the basis of almost any other. Modularity, moreover, allows the regulatory DNA to define patterns of gene expression that are not merely complex, but whose parts are independently adjustable. A change in one of the regulatory modules can alter one part of the expression pattern without affecting the rest, and without requiring changes in regulatory proteins that would have repercussions for the expression of other genes in the genome. As described in Chapter 7, it is such regulatory DNA that contains the key to the complex organization of multicellular plants and animals, and its properties make possible the independent adaptability of each part of an organism's body structure in the course of evolution.

Most of the segmentation genes also have important functions at other times and places in the development of Drosophila. The eve gene, for example, is expressed in subsets of neurons, in muscle precursor cells, and in various other sites, under the control of additional enhancers (see Figure 21-38). By addition of new modules to its regulatory DNA, any gene can be coopted during evolution for new purposes at new sites in the body, without detriment to its other functions.

Egg-Polarity, Gap, and Pair-Rule Genes Create a Transient Pattern That Is Remembered by Other Genes

Within the first few hours after fertilization, the gap genes and the pair-rule genes are activated one after another. Their mRNA products appear first in patterns that only approximate the final picture; then, within a short time—through a series of interactive adjustments—the fuzzy initial distribution of gene products resolves itself into a regular, crisply defined system of stripes (Figure 21-39). But this system itself is unstable and transient. As the embryo proceeds through gastrulation and beyond, the regular segmental pattern of gap and pair-rule gene products disintegrates. Their actions, however, have stamped a permanent set of labels—positional values—on the cells of the blastoderm. These positional labels are recorded in the persistent activation of certain of the segment-polarity genes and of the homeotic selector genes, which serve to maintain the segmental organization of the larva and adult. The segment-polarity gene engrailed provides a good example. Its RNA transcripts are seen in the cellular blastoderm in a series of 14 bands, each approximately one cell wide, corresponding to the anteriormost portions of the future parasegments (Figure 21-40).

Figure 21-39. The formation of ftz and eve stripes in the Drosophila blastoderm.

Figure 21-39

The formation of ftz and eve stripes in the Drosophila blastoderm. ftz and eve are both pair-rule genes. Their expression patterns (shown in brown for ftz and in gray for eve) are at first blurred but rapidly resolve into sharply defined stripes. (From (more...)

Figure 21-40. The pattern of expression of engrailed, a segment-polarity gene.

Figure 21-40

The pattern of expression of engrailed, a segment-polarity gene. The engrailed pattern is shown in a 5-hour embryo (at the extended germ-band stage), a 10-hour embryo, and an adult (whose wings have been removed in this preparation). The pattern is revealed (more...)

The segment-polarity genes are expressed in patterns that repeat from one parasegment to the next, and their bands of expression appear in a fixed relationship to the bands of expression of the pair-rule genes that help to induce them. However, the production of this pattern within each parasegment depends on interactions among the segment-polarity genes themselves. These interactions occur at stages when the blastoderm has already become fully partitioned into separate cells, so that cell-cell signaling of the usual sort has to come into play. A large subset of the segment-polarity genes code for components of two signal transduction pathways, the Wnt pathway and the Hedgehog pathway, including the secreted signal proteins Wingless (a Wnt family member) and Hedgehog. These are expressed in different bands of cells that serve as signaling centers within each parasegment, and they act to maintain and refine the expression of other segment-polarity genes. Moreover, although their initial expression is determined by the pair-rule genes, the two signaling proteins regulate one another's expression in a mutually supportive way, and they proceed to help trigger expression of genes such as engrailed in precisely the correct sites.

The engrailed expression pattern will persist throughout life, long after the signals that organized its production have disappeared (see Figure 21-40). This example illustrates not only the progressive subdivision of the embryo by means of more and more narrowly localized signals, but also the transition between the transient signaling events of early development and the later stable maintenance of developmental information.

Besides regulating the segment-polarity genes, the products of pair-rule genes collaborate with the products of gap genes to cause the precisely localized activation of a further set of spatial labels—the homeotic selector genes. It is the homeotic selector genes that permanently distinguish one parasegment from another. In the next section we examine these selector genes in detail and consider their role in cell memory.

Summary

The fly Drosophila has been the foremost model organism for study of the genetics of animal development. Like other insects, it begins its development with a series of nuclear divisions generating a syncytium, and a large amount of early patterning occurs in this single giant multinucleate cell. The pattern originates with asymmetry in the egg, organized both by localized deposits of mRNA inside the egg and by signals from the follicle cells around it. Positional information in the multinucleate embryo is supplied by four intracellular gradients that are set up by the products of four groups of maternal-effect genes called egg-polarity genes. These control four distinctions fundamental to the body plan of animals: dorsal versus ventral, endoderm versus mesoderm and ectoderm, germ cells versus somatic cells, and head versus rear.

The egg-polarity genes operate by setting up graded distributions of gene regulatory proteins in the egg and early embryo. The gradients along the anteroposterior axis initiate the orderly expression of gap genes, pair-rule genes, segment-polarity genes, and homeotic selector genes. These, through a hierarchy of interactions, become expressed in some regions of the embryo and not others, progressively subdividing the blastoderm into a regular series of repeating modular units called segments. The complex patterns of gene expression reflect the modular organization of the regulatory DNA, with separate enhancers of an individual gene responsible for separate parts of its expression pattern.

The segment-polarity genes come into play toward the end of the segmentation process, soon after the syncytium has become partitioned into separate cells, and they control the internal patterning of each segment through cell-cell signaling via the Wnt (Wingless) and Hedgehog pathways. This leads to persistent localized activation of genes such as engrailed, giving cells a remembered record of their anteroposterior address within the segment. Meanwhile, a new cell-cell signaling gradient is also set up along the dorsoventral axis, with the TGFβ family member Decapentaplegic (Dpp) and its antagonist, Short gastrulation, acting as the morphogens. This gradient helps to refine the assignment of different characters to cells at different dorsoventral levels. Homologous proteins are also known to control the patterning of the ventrodorsal axis in vertebrates.

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Copyright © 2002, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter; Copyright © 1983, 1989, 1994, Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson .
Bookshelf ID: NBK26906

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