The early development of flies is governed by two essentially different mechanisms: anterior and posterior. The difference in organizing the two regions of the embryo most likely reflects the evolutionary history of Drosophila, a higher insect. The anterior system is thought to be more advanced. In primitive insects such as the cricket or grasshopper the segment pattern in the anterior part of the body is laid out more or less simultaneously (all at once), as in Drosophila, but the posterior part of the body is formed sequentially, presumably by cellular outgrowth from a posteriorly located growth zone.
The initial body pattern generated during the early development of most insects, especially more primitive species, consists of a series of anterior body segments (head and perhaps thoracic segments) followed by a posterior terminus. The remaining thoracic and abdominal segments then arise in succession from a proliferating budding zone positioned posterior to the head/thorax.
A possible explanation for the seemingly arbitrary nature of the signalling strategies used is that they reflect chance mechanisms for creating morphological change during evolution. The general idea is that short germ band insects start with basically a head and then extend. The principal difference between flies and grasshoppers, is that
Drosophila has a rapid establishment of body plan, in comparison with primitive insects. These differences between Drosophila and grasshopper Schistocerca americana are discernible at the molecular level (Sommer & Tautz, 1993) in the mechanisms of initiation of pattern of the segment polarity gene engrailed. Segment formation in Drosophila involves the pair-rule genes network which define double segmental periodicities and which have been considered to represent a special adaptation to the long germ-band type development (Patel et al., 1992).
Namely, in both types of insects, engrailed is expressed in a pattern of regularly spaced stripes, one stripe per segment. However, in Drosophila, engrailed is thought to be directly regulated by the primary pair-rule gene even-skipped, while in grasshopper eve homologue does not serve a pair-rule function in early development (Patel et al., 1992). In Drosophila, the eve is required for expression of all 14 engrailed stripes. Early eve expression acting as a bell-shaped concentration-dependent morphogenetic field, biased by combinatorial interactions with other primary pair-rule genes, subdivides each of its domains into multiple functional subregions.
In more advanced insects, like beetle Tribolium, intermediate segmentation control system seems to be present (Sommer & Tautz, 1993; Wolff et al., 1995). The gap gene hunchback (hb) plays a central role in determining the anterior-posterior pattern in the Drosophila embryo. The HB-product morphogenetic gradient may have the same functions in early Drosophila and Tribolium development, despite the different types of embryogenesis in these two species (long versus short germ development). In Drosophila HB gradient activates Kruppel which play a key role in determination of thoracic and abdominal segments. Most importantly Tribolium hb could be involved in Tribolium Kruppel regulation in a similar manner that in Drosophila, namely by repressing Kruppel at high concentrations and activating it at lower concentrations (Schulz and Tautz, 1994).
In Drosophila, the third and the fourth primary pair-rule gene hairy stripes lie within the region of the Kruppel expression domain. Analysis of Kruppel expression indicates that the same is true for Tribolium. The first expression of Kruppel preceeds the expression of hairy in the same region. Initially single domain of the Tribolium hairy expression then splits into two. It is known that hairy stripes three and four form at first a single domain in Drosophila as well, which later splits into two stripes.
The striking parallels in the gene segmentation mechanisms between Drosophila and Tribolium suggest that the patterning mechanism through a transient pair-role pre-pattern is a fundamental component of the segmentation process in insects. However, not all pair-rule genes are involved in the pair-rule prepattern in case of more ancient insect embryogenesis.
Rules of Nets Complexification
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