Organizing the Embryo: Segmentation
- 3 segments make up the head with its antennae and mouth parts.
- 3 segments make up the thorax. Each thoracic segment has a pair of legs (insects are the six-legged creatures). In Drosophila (and other flies), the middle thoracic segment carries a single pair of wings; the hind segment a pair of halteres.
- 8 abdominal segments.
What signals guide segment formation?
The process begins with the gradients of messenger RNA (mRNA) that the mother deposited in her egg before it was fertilized.
|For the details, see Embryonic Development: Getting Started|
Shortly after fertilization, these are translated into their proteins with
- a gradient of bicoid diminishing from anterior to posterior and
- a gradient of nanos diminishing from posterior to anterior.
- Bicoid protein is a transcription factor. It binds to the promoter of a gene called hunchback (hb), turning it ON (red arrow).
Link to a general discussion of eukaryotic promoters and the transcription factors that bind to them.
- Nanos protein binds to hunchback mRNAs, inhibiting their translation (blue bar).
- These effects combine to produce a high level of hunchback protein at the anterior of the embryo; with a sharp cut-off toward the posterior.
- The hunchback protein is also a transcription factor (as we shall see).
- These concentration gradients regulate the turning on and off of other genes in sharply-defined regions of the embryo.
- These establish the various segments of the body.
An Example: eve stripe 2
The gene even-skipped (eve) is expressed in 7 bands or stripes corresponding to 7 of Drosophila's 14 segments (skipping the even-numbered ones). The photo (courtesy of Peter A. Lawrence and Blackwell Scientific Publications) shows the 7 stripes of eve activation.
At first the gene is expressed in fairly broad zones, but in time its expression becomes restricted to ever-narrower stripes. The mechanism by which this occurs is known for the second stripe.
- Binding of bicoid and hunchback proteins stimulates transcription of eve.
- Binding of giant and Krüppel represses transcription.
Drosophila development (and probably that of animals in general) passes through three rather different (although often overlapping) phases:
- establishing the main axes (dorsal-ventral; anterior-posterior; left-right). This is done by gradients of mRNAs and proteins encoded by the mother's genes and placed in the egg by her.
See Embryonic Development: Getting Started
- establishing the main body parts such as the notochord and central nervous system in vertebrates.
see Organizing the Embryo: The Central Nervous System
- filling in the details; that is, building the various organs of the animal. (Our example will include the wings, legs, and eyes of Drosophila.)
see Embryonic Development: Putting on the finishing touches
25 April 2014