thumb|Ventral view of repeating denticle bands on the cuticle of a 22-hour-old embryo. The head is on the left.
Drosophila embryogenesis, the process by which Drosophila (fruit fly) embryos form, is a favorite model system for genetics and developmental biology. The study of its embryogenesis unlocked the century-long puzzle of how development was controlled, creating the field of evolutionary developmental biology. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.
Life cycle
Drosophila display a holometabolous method of development, meaning that they have three distinct stages of their post-embryonic life cycle, each with a radically different body plan: larva, pupa and finally, adult. The machinery necessary for the function and smooth transition between these three phases develops during embryogenesis. During embryogenesis, the larval stage fly will develop and hatch at a stage of its life known as the first larval instar. Cells that will produce adult structures are put aside in imaginal discs. During the pupal stage, the larval body breaks down as the imaginal disks grow and produce the adult body. This process is called complete metamorphosis. About 24 hours after fertilization, an egg hatches into a larva, which undergoes three molts taking about 5.5 to 6 days, after which it is called a pupa. The pupa metamorphoses into an adult fly, which takes about 3.5 to 4.5 days. The entire growth process from egg to adult fly takes an estimated 10 to 12 days to complete at 25 °C.
The mother fly produces oocytes that already have anterior-posterior and dorsal-ventral axes defined by maternal activities.
thumb|300x300px|Early embryogenesis, showing the cycles of nuclei divisions in the syncytial blastoderm and the morphogenetic movements of gastrulation.
Embryogenesis in Drosophila is unique among model organisms in that cleavage occurs in a multinucleate syncytium (strictly a coenocyte). Early on, 256 nuclei migrate to the perimeter of the egg, creating the syncytial blastoderm. The germ line segregates from the somatic cells through the formation of pole cells at the posterior end of the embryo. After thirteen mitotic divisions and about 4 hours after fertilization, an estimated 6,000 nuclei have accumulated in the unseparated cytoplasm of the oocyte before they are encompassed by plasma membranes to form cells surrounding the yolk sac, producing a cellular blastoderm.
Like other triploblastic metazoa, gastrulation leads to the formation of three germ layers: the endoderm, mesoderm, and ectoderm. The mesoderm invaginates from the ventral furrow (VF), as does the ectoderm that will give rise to the midgut. The pole cells are internalized by a different route.
Germ band elongation involves many rearrangements of cells, and the appearance of distinct differences in the cells of the three germ bands and various regions of the embryo.
The posterior region (including the hindgut) expands and extends towards the anterior pole along the dorsal side of the embryo. At this time, segments of the embryo become visible, creating a striped arrangement along the anterior-posterior axis. The earliest signs of segmentation appear during this phase with the formation of parasegmental furrows. This is also when the tracheal pits form, the first signs of structures for breathing.
Germ band retraction returns the hindgut to the dorsal side of the posterior pole and coincides with overt segmentation. The remaining stages involve the internalization of the nervous system (ectoderm) and the formation of internal organs (mainly mesoderm).
Anterior-posterior axis patterning in Drosophila
thumb| The abdominal cuticular segments of the Drosophila embryo consist of repeating denticle bands separated by naked cuticle.
One of the best understood examples of pattern formation is the patterning along the future head to tail (antero-posterior) axis of the fruit fly Drosophila melanogaster. There are three fundamental types of genes that give way to the developmental structure of the fly: maternal effect genes, segmentation genes, and homeotic genes. The development of Drosophila is particularly well studied, and it is representative of a major class of animals, the insects or insecta. Other multicellular organisms sometimes use similar mechanisms for axis formation, although the relative importance of signal transfer between the earliest cells of many developing organisms is greater than in the example described here.
Maternal effect genes
frame|mRNA distributions
frame|right|Protein distributions
The building-blocks of anterior-posterior axis patterning in Drosophila are laid out during egg formation (oogenesis), well before the egg is fertilized and deposited. The maternal effect genes are responsible for the polarity of the egg and of the embryo. The developing egg (oocyte) is polarized by differentially localized mRNA molecules.
The genes that code for these mRNAs, called maternal effect genes, encode for proteins that get translated upon fertilization to establish concentration gradients that span the egg. Bicoid and Hunchback are the maternal effect genes that are most important for patterning of anterior parts (head and thorax) of the Drosophila embryo. Nanos and Caudal are maternal effect genes that are important in the formation of more posterior abdominal segments of the Drosophila embryo.
In embryos from bicoid mutant mothers, the head and thoracic structures are converted to the abdomen making the embryo with posterior structures on both ends, a lethal phenotype. Overall, a difference in the localization of the oocyte nucleus becomes a difference in the signaling state of the surrounding follicle cells which then signal to the resulting blastoderm nuclei.
Once in the nucleus, Dorsal activates different genes depending upon its nuclear concentration. This process sets up a gradient between the ventral and dorsal side of the blastoderm embryo with the repression or induction of Dorsal target genes being differentially regulated.
At the ventral end of the embryo, blastoderm nuclei exposed to high concentrations of dorsal protein induce the transcription of the transcription factors twist and snail while repressing zerknüllt and decapentaplegic. This results in the formation of the mesoderm.
In the lateral regions of the embryo, low nuclear concentrations of Dorsal lead to the expression of rhomboid which identifies future neuroectoderm. More dorsally, active Dpp signaling represses rhomboid thus confining it to the lateral blastoderm nuclei.
At the dorsal side of the embryo, blastoderm nuclei where there is little or no nuclear dorsal protein express zerknüllt, tolloid, and decapentaplegic (Dpp). This leads to the specification of non-neural ectoderm and later in the blastula stage to anmioserosa.
The ventral activity of the TGF-β family signaling protein Dpp is maintained by the expression of the secreted Dpp-antagonist Sog (short gastrulation) in the neuroectoderm. Sog binds to and prevents Dpp from diffusing to the ventral side of the embryo and through the cleavage of Sog by Tolloid also enables a sharpening of the Dpp gradient on the dorsal side. The DV axis of Drosophila is due to the interaction of two gradients – a ventral concentration of nuclear Dorsal and a dorsal concentration of Dpp activity.