Chapter SummaryThe model organism from which we get most of our information on the mechanisms of development is the fruit fly, Drosophila melanogaster. Due to the power of genetics, the fruit fly has become the major model organism of modern developmental biology. Eric Wieschaus and Christianne Nusslein-Volhard were awarded the Noble Prize in 1995 for their use of genetic screens to identify defects in early development of Drosophila, and we owe much of our knowledge today of early animal development to these experiments.
Early development of Drosophila begins with formation of an oocyte during oogenesis [fig. 3.2]. In Drosophila, an oocyte develops inside the egg chamber encased by follicle cells [fig. 3.5]. The oocyte makes very little of its own material; most of the material is made by the nurse cells [fig. 3.3]. The oocyte is connected to nurse cells through cytoplasmic bridges called ring canals [fig. 3.4]. Through ring canals, nurse cells pump maternal components, such as mRNA, proteins, ribosomes, and mitochondria, into the maturing oocyte
After fertilization, the first mitotic divisions of Drosophila occur without any cytokinesis [fig. 3.6]. This is called syngamy, and the result of these rapid nuclear divisions is a syncytcium, a cell with many nuclei in a common cytoplasm. The nuclei migrate to the surface of the embryos to form a syncytial blastoderm. The first uninucleated cells to bud off of the syncytium are the pole cells [fig. 3.7], which form at the posterior of the embryo and will form primordial germ cells. The cellular blastoderm forms simultaneously, as the nuclei are enclosed by invaginations of the membrane during cellularization [fig. 3.8].
By the time cellularization has taken place, the cells of the Drosophila embryo are determined, and their developmental potential has been restricted in a particular way [fig. 3.9]. Transplantation experiments have defined a fate map of the Drosophila embryo and indicate the organization of the cellular blastoderm into tissue types and germ layers [fig. 3.10].
After cellularization, the embryo proceeds through a series of morphogenetic movements called gastrulation [fig. 3.11]. Cells from the ventral surface invaginate, or move to the interior of the embryo, during gastrulation. This invagination creates the ventral furrow and is important for the formation of the mesoderm. Then the presumptive endoderm will begin to invaginate at the anterior and posterior ends of the ventral furrow.
During the process of germ band extension, the germ band in between these sites of endodermal invagination undergoes convergent extension in which the germ band elongates and loops back on itself within the chorion. During germ band extension, neuroblasts ingress [fig. 3.12], detaching from their epithelial neighbors and migrating inward, and eventually form nerve cells and glial cells of the nervous system. During germ band retraction, grooves begin to define the 14 parasegments [fig. 3.13], each having a different pattern of denticles on the ventral surface. Some segments contain imaginal discs [fig. 3.14] that will unfold during metamorphosis to form adult fly structures such as antennae, legs, and wings.
All embryonic patterning is predetermined by the spatial organization of maternal components in the egg, called localized determinants [fig. 3.17]. Embryological experiments involving transplantation of cytoplasm had suggested the presence of morphogens, molecules producing a pattern in the embryo using concentration gradients. Many morphogens have since been discovered using a genetic screen for maternal effect mutations in Drosophila. Different morphogens control the patterning of different regions of the embryo. There are anterior morphogens, such as bicoid and hunchback, and posterior morphogens, such as nanos, giant, and knirps. The morphogen that patterns the extreme termini of the embryo is torso. The patterning of the dorsal ventral axis is controlled by the morphogen dorsal [fig. 3.19].
Spatial patterning of localized determinants in the egg is initiated by the prepatterning of the surrounding follicle cells. During early oogenesis, reciprocal signaling between the follicle cells and the oocyte is important for both posterior patterning and dorso-ventral patterning [fig. 3.20].
Further ReadingStudies of developmental genes in Drosophila have been paramount to our modern understanding of developmental pathways. The Interactive Fly, a Web site dedicated to Drosophila genes and their roles in development, is an excellent up-to-date resource for all aspects of fly development. The site also offers an evolutionary and comparative perspective on genes and processes during development. http://sdb.bio.purdue.edu/fly/aimain/1aahome.htm
Another essential Drosophila resource is Fly Base, a database of the Drosophila genome available at http://flybase.bio.indiana.edu/. Here you can search the entire genome and find information on mutant stocks, Drosophila anatomy, references, and people in the Drosophila research community.
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