SEGMENTATION IN DROSOPHILA

With emphasis on Parasegments

and Anatomical Segments

By Shreya Ahuja

MscII BiotechnologyFergusson College, Pune

RIGHT GENES BEING

EXPRESSED AT THE RIGHT

TIME

Maternal Effect Genes

Segmentation Genes

Homeotic Genes

Control development of Segments

• Genes that control development in Drosophila are very similar to those that control development in vertebrates

• Early patterning occurs in the syncytial blastoderm and it becomes multicellular at the beginning of segmentation

• Concentration gradients of proteins (transcription factors) can diffuse, enter nuclei & provide positional information

• The process of cell fate commitment in Drosophila has two steps: Specification and Determination

 

Drosophila Development

WHAT HAVE WE LEARNT SO FAR

All arthropods are segmented. The body of Drosophila melanogaster is built from 14 segments.

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. 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.

SEGMENTS OF A DROSOPHILA

SEGMENTATION GENES – Formation of ‘Molecular Blueprint’

Gap Genes

Pair Rule

GenesSegme

nt Polarit

y GenesSegmentation is a stepwise exercise that divides the embryo up into ever smaller units. Its like for cutting a cake into a large

number of equal slices, you would first cut it into large chunks then progressively cut each chunk into smaller slices.

MUTANTS OF SEGMENTATION GENES

Gap genes Krüppel (Kr) Pair-rule genes Secondary

fushi tarazu (ftz)

knirps (kni) odd-paired (opa)hunchback (hb) odd-skipped (slp)giant (gt) sloppy-paired

(slp)tailless (tll) paired (prd)huckendein (hkb)buttonhead (btd) Segment

polarity genesengrailed (en)

empty spiracles (ems)

wingless (wg)

hedgehog (hh)Pair-rule genes Primary

hairy (h) fused (fu)

even-skipped (eve)

armadillo (arm)

runt (run) patched (ptc)gooseberry (gsb)

PAIR RULE GENES First Indication of Segmentation

The transcription patterns of these genes are striking in that they divide the embryo into the areas that are the precursors of the segmental body plan

One vertical band of nuclei (the cells are just beginning to form) expresses a pair-rule gene, then another band of nuclei does not express it, and then another band of nuclei expresses it again. The result is a “zebra stripe” pattern

How are some nuclei of the Drosophila embryo told to transcribe a particular gene while their neighbors are told not to transcribe it? The answer appears to come from the distribution of the protein products of the gap genes

Three genes are known to be the primary pair-rule genes — hairy, even-skipped, and runt—are essential for the formation of the periodic pattern, and they are directly controlled by the gap gene proteins. The enhancers of the primary pair-rule genes are recognized by gap gene proteins, and it is thought that the different concentrations of gap gene proteins determine whether a pair-rule gene is transcribed or not

eve ftz

PERIODIC EXPRESSION OF PAIR RULE GENE even-skipped

EXPRESSION OF SEGMENT POLARITY GENES Once cells form, interactions take place between the cells which are

mediated by the segment polarity genes

Through this cell-to-cell signaling, cell fates are established within each parasegment.

Encoded proteins are constituents of the Wingless and Hedgehog signal transduction pathways

One row of cells in each parasegment is permitted to express the Hedgehog protein, while the other expresses the Wingless protein

Activation of Engrailed gene - cells express Hedgehog protein

In turn, engrailed gene is activated when cells have high levels of the Even-skipped or Fushi tarazu transcription factors

Engrailed transcription marks the anterior boundary of each parasegment

The wingless gene is activated – presence of Sloppy-paired protein. Marks posterior boundary

EXPRESSION OF Engrailed GENE

REFERENCES• http://

study.com/academy/lesson/segmentation-genes-in-drosophila-development-pair-rule-segment-polarity-gap-genes.html

• http://www.ncbi.nlm.nih.gov/books/NBK10039/

• http://www.ncbi.nlm.nih.gov/books/NBK26906/

• http://www.mun.ca/biology/desmid/brian/BIOL3530/DB_02/DBNDros.html

• http://www.dnaftb.org/37/animation.html