Stages of Development and Mitotic Domains

Bownes stages

Gastrulation and morphogenetic movements

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Introduction to Bownes Stages

Fly development proceeds through a complex series of stages and processes, some successive, others simultaneous. The unfolding of any new life is a wonder to behold, and even more compelling when understood at the biochemical or genetic level. The stages in fly development [Images] are identified by the events taking place and the time after fertilization at which they occur. Below is a list of developmental activities: the morphogenic processes that occur from fertilization to larval hatch, with the corresponding Bownes stage number and time frame for each event (Campos-Ortega, 1985).

Gastrulation [Images], the invagination of the blastula creating the mesodermal and endodermal germ layers in vertebrates, is complex. In the fly, it is overwhelming. There is not just a single site for cell invagination, but taken together, one finds approximately ten morphogenetic movements, three of which can be considered gastrulation proper, and seven more that should be understood in order to understand Drosophila development as a whole.

Of the three gastrulation events one is involved in mesoderm formation (ventral furrow invagination), and two others involve endoderm formation (both anterior and posterior midgut invagination). Seven other events [Images] resembling gastrulation are listed below:

• 1) formation of the cephalic furrow
• 2) formation of dorsal transverse folds
• 3) germ band extention
• 4) germ band retraction
• 5) segmentation
• 6) dorsal closure
• 7) head involution

Before considering these ten tissue movements in some detail, it should be made clear that there are other programs of cell movement, including trachea formation, imaginal disc development and segregation of neuroblasts from the neuroectoderm. The initial structuring or ground work for most of these events can be traced back to the four maternal systems which establish polarity in the egg and, as a consequence, in the zygote. Thus these events are related to patterns of segmentation built early in development. Ventral furrow formation and dorsal closure have their origins in the dorsoventral system; the other eight events originate with the anterior group, the posterior group and the terminal group of maternal genes, all of which are responsible for anterior-posterior polarity.

Gastrulation begins three hours after fertilization. By this time there have been 13 mitotic cycles. Prior to the 10th cycle, the dividing nuclei lie in the interior of the egg, but move out toward the surface, going through four more division cycles at the periphery until cellularization occurs (the surrounding of the nuclei by a plasma membrane) (Foe, 1993). Immediately after cellularization, a process taking less than an hour from start to finish, the ventral furrow, which marks the beginning of gastrulation, begins to form.

How do the invaginating cells know which way to go? Invagination is driven by cell shape changes. Cells in the midventral region undergo an apical constriction. A part of the cell on the embryo surface constricts, driving its cytoplasm basally (that is, toward the interior of the embryo), taking on a shape like a wedge of pie with the narrow end pointing apically (toward the exterior of the embryo. As a consequence of this change in cell shape, adjacent cells are pulled toward the inturning, developing furrow. Actin is the protein driving these cell shape changes, the same protein involved in muscle tightening, and a constituent of the cytoskeleton. The process of invagination is driven by the dorsal group and multiple proteins that function after fertilization: Regulating cytoskeletal changes that drive gastrulation are Concertina, Folded gastrulation, DRhoGEF2, and Rho1. concertina and folded gastrulation code for a G alpha-like protein and a secreted polypeptide, respectively (See G protein salpha60A, Evolutionary homologs section for information about Concertina). The role of Rho1 and DRhoGEF2 are dealt with in the Biological Overview of Rho1. Both snail and twist, regulated by Dorsal, are required for successful completion of gastrulation.

The terminal group and its principle genes torso and tailless, ensure that the invaginating ventral furrow [Images] does not extend too far to the anterior or posterior. The closing of the ventral furrow creates the ventral midline, a future site of neurogenesis marked by the presence of cells transcribing single minded. On either side of the ventral midline is the neurogenic ectoderm, tissue that will give rise to the ventral cord, otherwise known as the central nervous system.

Anterior and posterior midgut invagination, the beginning of endoderm formation, is controlled by terminal genes, in particular by tailless, huckebein and forkhead. It begins soon after the start of ventral invagination, and is also a consequence of changes in cell shape requiring Rho1 and DRhoGEF2.

The cephalic furrow [Images], first of the seven additional gastrulation-like events listed above, begins at the same time as the ventral furrow begins to form. It forms a partial necklace of inturning tissue, driven by the same changes in cell shape and movement described above. The precise position of the cephalic furrow corresponds to the anterior most even-skipped stripe. Furrow formation is abnormal in even-skipped mutants. This furrow demarcates head from thorax in the developing fly (Costa, 1993).

The thick ventral portion of the embryo by now consists of a one cell thick outer layer of columnar cells and an invaginated inner layer of irregularly shaped cells, several cells deep. Taken together this layered, invaginated ventral area of the embryo is referred to as the germ band (Sonnenblick, 1950). It gives rise to the germ layers (ectoderm and mesoderm), (not to be confused with germ line stem cells that give rise to egg and sperm). As the posterior midgut primordium begins to invaginate, germ band extention also begins. In approximately 105 minutes the germ band [Images] will have doubled its length and halved its width. This elongation process pushes the posterior midgut invagination closed and compresses even further the previously flattened dorsal tissue of the embryo (the amnioserosa), even as the germ band thrusts the amnioserosa to the side. In the process, cells shift their positions relative to one another, intercalating like traffic forced to narrow its flow from multiple lanes into just one. The increase in the number of cells occupying the germ band area, caused by germ band extention, may be required in order to establish more detailed segmental fields (Costa, 1993). Germ band extension is coordinated by serotonin acting through a serpentine receptor, Serotonin receptor 2 (Colas, 1999).

While germ band extention is accompanied by cellular interdigitation, germ band retraction is accompanied by the transition from a parasegmental to segmental division of the embryo. During germ band shortening, the amnioserosa spreads out from its compressed state (see above) to cover the whole of the dorsal surface. In the process of segmentation, deep ventral-lateral grooves form, corresponding to the incipient segmental boundaries. The interior aspects of these segmental boundaries are the sites for future muscle attachment. Genes involved in germ band retraction include the Epidermal growth factor receptor, pebbled and serpent.

During segmentation, the segregation of imaginal discs takes place. Imaginal discs are sacs of cells that give rise to adult structures during pupal metamorphosis. What drives the segmentation? Two of the maternal systems to establish anterior/posterior polarity are central to this segmentation process. The anterior/posterior axis of the fly is structured by both the gradient of Bicoid and the posterior system that gives rise to Nanos mRNA localization. The proper anterior/posterior axis determines the location of gap gene transcription. Gap genes structure the striped pattern of pair rule genes. The striped pattern leads to the segmental array of segment polarity gene transcription. Here is a developmental hierarchy, each chain in the link of events made possible by what has preceded it, and making possible what has yet to come. Its end, if successful, is the even pattern of segmentation vital to animal structure (Martinas Arias, 1993).

Dorsal closure [Images] takes place progressively, during germ band retraction. It takes about two hours, beginning 11 hours after the start of development. Dorsal closure is the process whereby the stretched amnioserosa is covered by epidermal cells that will ultimately fuse at the dorsal midline. With its part in development played, the amnioserosa is then absorbed by the yolk. Genes involved in dorsal closure include rho, hemipterous and basket.

The most rapid period of head involution occurs at the same time as dorsal closure. The larvae of Diptera are acephalic (headless). In the process of head involution, the anterior ectoderm moves to the interior, beginning with stomodeal invagination. Beginning at 6 hours into development, the process will take another 5 hours. The most active period of involution is of short duration, a burst beginning at 10.5 hours and lasting for half an hour (Poulson, 1950).

REFERENCES

Campos-Ortega, J. A. and Hartenstein, V. (1985). The embryonic development of Drosophila melanogaster. Springer-Verlag: Berlin

Colas, J., Launay, J. and Maroteaux, L. (1999a). Maternal and zygotic control of serotonin biosynthesis are both necessary for Drosophila germband extension. Mech. Dev. 87(1-2): 67-76

Costa, M., Sweeton, D. and Wieschaus, E. (1993). Gastrulation in Drosophila: Cellular mechanisms of morphogenetic movements, pp 425-465. In: The development of Drosophila melanogaster. Bate, M. and Hartenstein, V. (editors). Cold Spring Harbor Laboratories: Long Island NY

Foe, V. E. (1998). Mitotic domains reveal early commitment of cells in Drosophila embryos. Development 107(1): 1-22.

Foe, V.E., Odell, G.M. and Edgar, B.A. (1993). Mitosis and Morphogenesis in the Drosophila Embryo: Point and Counterpoint pp 149-300. In: The development of Drosophila melanogaster. Bate, M. and Hartenstein, V. (editors). Cold Spring Harbor Laboratories: Long Island NY

Martinez Arias, A. (1993). Development and patterning of the larval epidermis of Drosophila. pp 517-608. In: The development of Drosophila melanogaster. Bate, M. and Hartenstein, V. (editors). Cold Spring Harbor Laboratories: Long Island NY

Poulson, D.F. (1950). Histogenesis, organogenesis and differentiation in the embryo of Drosophila melanogaster. Mergen. pp 168-270 In: Biology of Drosophila (M. Demerec, editor). Wiley and Son: NY

Sonnenblick, B.P. (1950). The early embryology of Drosophila melanogaster, pp 62-167. In: Demerec, M. (editor). Biology of Drosophila. Wiley and Son: New York

date revised: 21 December 99 

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