Lecture 7: Drosophila Development

Question 1

What maternal determinants establish polarity across the A - P axis?

Answer 1

Bicoid and Nanos. The coordinated activity will result in a A –> P gradient of hunchback.

Question 2

How many segments does Drosophila have? List 3 traits of segments.

Answer 2

There are 14 segment (simple, repeating units) in Drosophila. Each segment has: (1) unique identity and an A –>P polarity.

Question 3

How does the embryo go from having maternal gradients to patterning the body in 14 segments?

Answer 3

Maternal determinants will activate a transcription factor cascade. Zygotic gene expression results from progressive division of the embryo
1. Occurs in broad zones
2. 14 Parasegments

Question 4

What are parasegments?

Answer 4

Units of embryonic gene expression and out of register with segments.

Question 5

How can you genetically screen for zygotic patterning mutations that disrupt segmentation?

Answer 5

Random mutagenesis can allow for mutations. Analysis of genes + expression can indicate how sequential expression of patterning genes controls segmentation.

Question 6

What are zygotic patterning genes?

Answer 6

Genes of the embryo that are essential for patterning along the maternally established AP and DV axis. Recessive homozygous mutants will express a patterning defect.

Question 7

What are the four classes of zygotic genes involved in segmentation along the AP axis?

Answer 7

GAP, PAIR-RULE, SEGMENT POLARITY, SEGMENT IDENTITY.

Question 8

What do mutations in GAP, PAIR-RULE, or SEGMENT POLARITY genes affect?

Answer 8

The number or polarity of segments.

Question 9

What do mutations in SEGMENT IDENTITY genes affect?

Answer 9

They affect the identity of segments.

Question 10

What are gap gene mutants?

Answer 10

Mutant embryos that are missing one or more continuous segments. The segments that are present are normal.

Question 11

What is an example of a gap gene?

Answer 11

Hunchback is the first gap gene to be expressed in Drosophila embryo.

Question 12

What two concentrations regulate the transcription of gap genes?

Answer 12

Bicoid and Hunchback will activate or repress different threshold concentrations.

Question 13

What is required for the expression of Kruppel?

Answer 13

Hunchback needs to be expressed in moderate concentrations for Kruppel to be expressed. (concentration-dependent manner)
1) At the anterior end, Hunchback is high = Kruppel repressed
2) At the posterior end, Hunchback is low = Kruppel X activated

Question 14

T or F: Gap Genes are expressed in different regions along the AP axis?

Answer 14

True. Bicoid and Hunchback proteins are responsible for gap gene expression. Gap genes encode for transcription factors. This broadly divides the embryo into a few zones along the AP axis (e.g Kruppel)

Question 15

What are pair-rule gene mutants?

Answer 15

Mutant embryos are missing alternative segments. The segments that are present are normal. In each pair rule mutant, 7 segments are present and 7 segments are missing.

Question 16

What are pair-rule gene expression controlled by?

Answer 16

Bicoid/Hunchback, Gap Gene TF. Each stripe is independently controlled by enhancer sequences in the regulatory regions of pair rule genes.

Question 17

Explain the Stripe 2 regulatory region of even-skipped (eve) pair rule gene.

Answer 17

Bicoid and Hunchback (when present above threshold levels) will activate transcription
Kruppel and Giant (gap proteins - when present above threshold levels) will repress transcription.

Question 18

What is a segment polarity gene mutant?

Answer 18

Mutant embryos will have the correct number of segments, but each segment is missing a fraction (A or P) and the remainder of the segment is duplicated in a mirror image.

Question 19

Where are segment polarity genes expressed?

Answer 19

At the boundary of every pair rule gene stripe. This gene expression defines anterior and posterior boundaries within each para-segment.

Question 20

What do most segment polarity genes encode for? Is engrailed the same?

Answer 20

Most segment polarity genes encode for cell-cell signaling proteins (as the embryo goes from syncytial –> cellular blastoderm, cell-cell communication becomes important). Engrailed is a TF that is a segment polarity gene, but does not encode for cell-cell signaling proteins.

Question 21

What do the boundary cells form?

Answer 21

Signaling centers that pattern each segment along the AP axis.

Question 22

What are segment identity mutants?

Answer 22

Mutant embryos who have the correct number and polarity of segments, but has the identity of one or more segments transformed to that of other segment(s).

Question 23

What determines segment identity?

Answer 23

Homeotic, HOX, or selector genes.

Question 24

What are HOX genes?

Answer 24

Hox genes encode transcription factors that play a central role in determining segment identity along the anterior-posterior axis.
They contain a conserved homeobox DNA sequence, which encodes a homeodomain —a DNA-binding domain that allows these transcription factors to bind DNA and regulate gene expression.
Expression is required throughout development for establishment and maintenance of segment identity.

Question 25

T or F: Are Hox genes evolutionarily conserved?

Answer 25

True. Hox genes are conserved in all bilaterians.

Question 26

T or F: Are Hox genes organized in gene complexes?

Answer 26

True.

Question 27

T or F: Do Hox gene clusters show colinearity?

Answer 27

True. Hox gene clusters can show spatial colinearity (the order of gene expression from head to tail can directly correspond to the order of genes along the chromosome). In mammals, Hox gene clusters can show temporal colinearity (the timing of gene expression corresponds to the order of genes along the chromosome)

Question 28

T or F: Are hox genes expressed in staggered domains along AP axis?

Answer 28

True. The domains of Hox gene expression define positional identity along the AP axis. Anterior = sharp, Posterior = not sharp (feature is conserved in segmented animals)

Question 29

Mutations in hox genes will cause what?

Answer 29

Mutations in hox genes will lead to homeotic transformations as the hox gene function for specifying segment identity is evolutionarily conserved.

Question 30

Once segment identity is established, it is maintained and propagated through metamorphosis.

Answer 30

The adult body plan develops according to the embryonic body plan. Polycomb and Trithorax are epigenetic regulators. Polycomb represses homeotic gene expressions where homeotic gene expresison hasn't been activated. Thrithorax activates homeotic gene expressions where homeotic gene s have been expressed.