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Drosophila embryogenesis, the process by which Drosophila 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.
Segmentation in biology is the division of some animal and plant body plans into a linear series of repetitive segments that may or may not be interconnected to each other. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups form segments by using a "growth zone" to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing free movement and development of certain body parts. It also allows for regeneration in specific individuals.
A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various specialized cell types within a tissue. More specifically, a morphogen is a signaling molecule that acts directly on cells to produce specific cellular responses depending on its local concentration.
The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output. Because arthropods have an open circulatory system, decapitated insects can still walk, groom, and mate — illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.
Compartments can be simply defined as separate, different, adjacent cell populations, which upon juxtaposition, create a lineage boundary. This boundary prevents cell movement from cells from different lineages across this barrier, restricting them to their compartment. Subdivisions are established by morphogen gradients and maintained by local cell-cell interactions, providing functional units with domains of different regulatory genes, which give rise to distinct fates. Compartment boundaries are found across species. In the hindbrain of vertebrate embryos, rhombomeres are compartments of common lineage outlined by expression of Hox genes. In invertebrates, the wing imaginal disc of Drosophila provides an excellent model for the study of compartments. Although other tissues, such as the abdomen, and even other imaginal discs are compartmentalized, much of our understanding of key concepts and molecular mechanisms involved in compartment boundaries has been derived from experimentation in the wing disc of the fruit fly.
Krüppel is a gap gene in Drosophila melanogaster, located on the 2R chromosome, which encodes a zinc finger C2H2 transcription factor. Gap genes work together to establish the anterior-posterior segment patterning of the insect through regulation of the transcription factor encoding pair rule genes. These genes in turn regulate segment polarity genes. Krüppel means "cripple" in German, named for the crippled appearance of mutant larvae, who have failed to develop proper thoracic and anterior segments in the abdominal region. Mutants can also have abdominal mirror duplications.
A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the normal body plan. Each gap gene, therefore, is necessary for the development of a section of the organism.
Decapentaplegic (Dpp) is a key morphogen involved in the development of the fruit fly Drosophila melanogaster and is the first validated secreted morphogen. It is known to be necessary for the correct patterning and development of the early Drosophila embryo and the fifteen imaginal discs, which are tissues that will become limbs and other organs and structures in the adult fly. It has also been suggested that Dpp plays a role in regulating the growth and size of tissues. Flies with mutations in decapentaplegic fail to form these structures correctly, hence the name. Dpp is the Drosophila homolog of the vertebrate bone morphogenetic proteins (BMPs), which are members of the TGF-β superfamily, a class of proteins that are often associated with their own specific signaling pathway. Studies of Dpp in Drosophila have led to greater understanding of the function and importance of their homologs in vertebrates like humans.
In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.
A segmentation gene is a gene involved in the early stages of pattern formation that define repeated units (metameres) in a segmented organism, usually the embryo. They are classified into 3 groups: gap genes, pair-rule genes, and segment polarity genes. The expression of gap genes result in the formation of gaps in the normal pattern of structure in the embryo. Expression of pair-rule genes subdivides the embryo into a series of stripes and sets the boundaries of the parasegments. Segment polarity genes define the anterior and posterior polarities within each embryonic parasegment.
A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair-rule gene expression. Pair-rule genes are defined by the effect of a mutation in that gene, which causes the loss of the normal developmental pattern in alternating segments.
engrailed is a homeodomain transcription factor involved in many aspects of multicellular development. First known for its role in arthropod embryological development, working in consort with the Hox genes, engrailed has been found to be important in other areas of development. It has been identified in many bilaterians, including the arthropods, vertebrates, echinoderms, molluscs, nematodes, brachiopods, and polychaetes. It acts as a "selector" gene, conferring a specific identity to defined areas of the body, and co-ordinating the expression of downstream genes.
Ganglion mother cells (GMCs) are cells involved in neurogenesis, in non-mammals, that divide only once to give rise to two neurons, or one neuron and one glial cell or two glial cells, and are present only in the central nervous system. They are also responsible for transcription factor expression. While each ganglion mother cell necessarily gives rise to two neurons, a neuroblast can asymmetrically divide multiple times. GMCs are the progeny of type I neuroblasts. Neuroblasts asymmetrically divide during embryogenesis to create GMCs. GMCs are only present in certain species and only during the embryonic and larval stages of life. Recent research has shown that there is an intermediate stage between a GMC and two neurons. The GMC forms two ganglion cells which then develop into neurons or glial cells. Embryonic neurogenesis has been extensively studied in Drosophila melanogaster embryos and larvae.
The Bithorax complex (BX-C) is one of two Drosophila melanogaster homeotic gene complexes, located on the right arm of chromosome 3. It is responsible for the differentiation of the posterior two-thirds of the fly by the regulation of three genes within the complex: Ultrabithorax (Ubx), abdominal A (abd-A), and Abdominal B (Abd-B).
Germ-band extension is a morphological process widely studied in Drosophila melanogaster in which the germ-band, which develops into the segmented trunk of the embryo, approximately doubles in length along the anterior-posterior axis while subsequently narrowing along the dorsal-ventral axis.
mir-279 is a short RNA molecule found in Drosophila melanogaster that belongs to a class of molecules known as microRNAs. microRNAs are ~22nt-long non-coding RNAs that post-transcriptionally regulate the expression of genes, often by binding to the 3' untranslated region of mRNA, targeting the transcript for degradation. miR-279 has diverse tissue-specific functions in the fly, influencing developmental processes related to neurogenesis and oogenesis, as well as behavioral processes related to circadian rhythms. The varied roles of mir-279, both in the developing and adult fly, highlight the utility of microRNAs in regulating unique biological processes.
Fasciclin 2 is a 95 kilodalton cell membrane glycoprotein in the immunoglobulin (Ig) – related superfamily of cell adhesion molecules (CAMs). It was first identified in the developing grasshopper embryo, seen dynamically expressed on a subset of fasciculating axons in the central nervous system (CNS), functioning as a neuronal recognition molecule in the regulation of selective axon fasciculation. Subsequently, fasII was cloned and has mainly been studied in the fruit fly. Its extracellular structure consists of two Fibronectin type III domains and five Ig-like C2 domains, having structural homology to the neural cell adhesion molecule (NCAM) found in vertebrates. Alternative splicing of fasII gives rise to its expression in three major isoforms, including a membrane-associated form that is attached to the outer leaflet of the plasma membrane via a glycophosphatidylinositol linkage and two integral transmembrane forms. The larger transmembrane form has an amino acid motif contained in its cytoplasmic domain that is rich in proline, glutamic acid, serine and threonine residues. The fasciclin 1 (Fas1) and fasciclin 3 (Fas3) genes in Drosophila also code for cell adhesion proteins in the nervous system but do not show any structural or functional similarities with NCAM.
Homeotic protein bicoid is encoded by the bcd maternal effect gene in Drosophilia. Homeotic protein bicoid concentration gradient patterns the anterior-posterior (A-P) axis during Drosophila embryogenesis. Bicoid was the first protein demonstrated to act as a morphogen. Although bicoid is important for the development of Drosophila and other higher dipterans, it is absent from most other insects, where its role is accomplished by other genes.
Gooseberry (gsb) is a segment polarity gene located on chromosome 2 of the Drosophila genome. Gooseberry is known for its interactions with key embryonic signaling pathways Wingless and Hedgehog. The gene also has clinal significance, being linked to diseases such as Waardenburg Syndrome and rhabdomyosarcoma.
Hunchback is a maternal effect and zygotic gene expressed in the embryos of the fruit fly Drosophila melanogaster. In maternal effect genes, the RNA or protein from the mother’s gene is deposited into the oocyte or embryo before the embryo can express its own zygotic genes.
References
- ↑ "Interactive Fly, Drosophila". Archived from the original on 2015-10-03. Retrieved 2015-10-02.
- ↑ Sanders, Mark Frederick; Bowman, John L. (2019). Genetic analysis : an integrated approach (Third ed.). New York: Pearson Education, Inc. ISBN 978-0134605173.
- ↑ "Polarity". Biology Articles, Tutorials & Dictionary Online. 2019-10-07. Retrieved 2023-05-26.
- ↑ Perrimon, N., & Mahowald, A. P. (1987). Multiple functions of segment polarity genes in Drosophila. Developmental biology, 119(2), 587-600.
- 1 2 "Segment polarity in Drosophila: Cell-cell signaling and the origins of patterning". Archived from the original on 2015-10-04. Retrieved 2015-10-02.
- ↑ "Multiple Functions of Segment Polarity Genes in Drosophila" (PDF). Archived from the original (PDF) on 2015-10-03. Retrieved 2015-10-02.
- ↑ Brower DL. Engrailed gene expression in Drosophila imaginal discs. The EMBO Journal. 1986;5(10):2649-2656.
- ↑ He, H., & Noll, M. (2013). Differential and redundant functions of gooseberry and gooseberry neuro in the central nervous system and segmentation of the Drosophila embryo. Developmental biology, 382(1), 209-223.
- ↑ Bopp, D., Burri, M., Baumgartner, S., Frigerio, G., & Noll, M. (1986). Conservation of a large protein domain in the segmentation gene paired and in functionally related genes of Drosophila. Cell, 47(6), 1033-1040.
- ↑ Patel, N. H., Schafer, B., Goodman, C. S., & Holmgren, R. (1989). The role of segment polarity genes during Drosophila neurogenesis. Genes & development, 3(6), 890-904.
- ↑ Bhat, K. M. (1999). Segment polarity genes in neuroblast formation and identity specification during Drosophila neurogenesis. Bioessays, 21(6), 472-485.