Segmentation

Segmentation refers to the process by which repeated units of similar groups of cells, the metameres, are created. Segmentation proceeds nearly simultaneously with gastrulation. Current understanding of the process of segmentation comes from the genetic dissection of development in Drosophila by Ed Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus. These researchers used a large-scale mutant screen to uncover developmental defects in Drosophila. They found a complex genetic regulatory cascade that specifies the insect body plan (Fig. 2), which has since been shown to have many commonalties with molecular patterning in vertebrate embryos. The three were awarded the Nobel Prize for their efforts. However, even before the genetic dissection of segmentation, the elegant work of Klaus Sander had defined the basic mechanisms of embryo patterning that helped in the interpretation of the new genetic data. By analyzing the outcomes of embryonic manipulations of leafhopper embryos (Eucelis), Sander concluded that two morphogenetic gradients specify the pattern elements along the anteroposterior axis of

Anterior-posterior axis

Maternal coordinate gene

Gap gene

Pair-rule gene

Qflnp bicoid krüppel hairy

Segment polarity gene

Homeotic gene

Segment polarity gene

Homeotic gene

FIGURE 2 Simplified diagram of the segmentation gene cascade in Drosophila melanogaster and its relation to limb development. Diagrammatic representation of some of regulatory interactions between genes in the Drosophila segmentation cascade: a maternal coordinate gene, a representative gap gene, a pair-rule gene, the segment polarity gene, a homeotic gene, and the limb-patterning gene Distal-less. [Modified from Nagy, L. (1998). Am. Zool. 38(6).]

abdominal-A

Distal-less

FIGURE 2 Simplified diagram of the segmentation gene cascade in Drosophila melanogaster and its relation to limb development. Diagrammatic representation of some of regulatory interactions between genes in the Drosophila segmentation cascade: a maternal coordinate gene, a representative gap gene, a pair-rule gene, the segment polarity gene, a homeotic gene, and the limb-patterning gene Distal-less. [Modified from Nagy, L. (1998). Am. Zool. 38(6).]

the germ band: one gradient with a high point in the posterior pole and another with a high point at the anterior pole (Fig. 3). In 1976 Sander reviewed these experiments, and a large body of other experimental work on insect embryos, setting the stage for modern understanding of the molecular basis of insect development.

Bee Keeping

Bee Keeping

Make money with honey How to be a Beekeeper. Beekeeping can be a fascinating hobby or you can turn it into a lucrative business. The choice is yours. You need to know some basics to help you get started. The equipment needed to be a beekeeper. Where can you find the equipment you need? The best location for the hives. You can't just put bees in any spot. What needs to be considered when picking the location for your bees?

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