The segmentation of the Drosophila is a well-studied one. But it is not representative of the phylum arthropod in general. Genetic screening was performed to identify the genes involved in the development of cuticle and this resulted in the discovery of a set of genes that were very important for the segmentation in the embryo of Drosophila. To segment the embryo of the Drosophila properly, the maternally provided transcripts were designated to define the anterior-posterior axis, thereby, giving rise to the protein gradient. This gradient results in the generation of expression for the gap genes that can form the boundaries between the segments. The gradients generated from the expression of gap genes later defines the expression pattern to exhibit pair-rule genes. The pair-rule genes are like transcription factors that are expressed in stripes along the length of the embryo. The segment polarity of the genes is regulated by the expression of these transcription factors. The polarity of every segment is defined at this stage. The identity and the boundaries of each segment are defined later on. In the arthropods, the nervous system, body wall, muscles, body cavity, and kidneys are segmented. However, in the sister taxon known as onychophoran, the body is not segmented this much.
Segments and Para segments
The morphological segmentation in the arthropods is developed due to a fundamental unit known as the “Para segment”. During embryogenesis, the boundaries of the parasegment are established by the segment polarity genes like the wingless and engrailed that are expressed in the form of continuous stripes along the anterior-posterior axis. Para segment boundaries are present on the anterior corner of every engrailed domain and it is present along with the appendages. On the other hand, the segment boundaries are present in the posterior corner of every engrailed domain and also in between each appendage. The role of the Para segments was to help in the development of the nervous system and also the associated structures to it. Morphologically, it helps in the protection of these structures and generates exoskeleton movement in between them. Within the segmental unit, in particular positions, each segment-polarity gene is expressed and the overall organization is conserved across the species of Panarthropod.
Sequential segmentation and segment addition zone
In the majority of the arthropods, the segmentation happens sequentially from top to bottom in addition to the extension of the segments till the axial end. They normally mention a few numbers of anterior segments present in the blastoderm. However, the maximum of the segments develops rhythmically from the “segment addition zone” after the transition of the blastoderm. The segment addition zone is mentioned as a growth zone in recent days as it does not explain anything about the cell proliferation and the localization of it in the posterior region of the embryo. The raw materials to form the new segments are usually derived from a combination of convergent extension and cell division. However, invertebrates, the contributions of these factors differ greatly between the species. The continuous cell division might contribute to the addition of segments in some species but the pattern of segments is not based on the cell division and cell cycle. Only certain crustaceansâ€™ segment and pattern their segments based on cell cycle.
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