Unit 5: Drosophila Development

Question 1

Why is Drosophila melanogaster used as a model organism?

Answer 1

• Short generation time (~10 days)
• Well-mapped genome
• External, translucent embryos make development easy to observe

Question 1

What is a Syncytial Blastoderm?

Answer 1

A stage where nuclei divide without cellular division, forming a shared cytoplasm with many nuclei (first ~13 nuclear divisions in Drosophila).

Question 2

What happens during Cellularization?

Answer 2

Membranes form around nuclei, turning the syncytium into individual cells → forms the cellular blastoderm

Question 3

What do the germ layers in Drosophila give rise to?

Answer 3

• Ectoderm → epidermis, nervous system
• Mesoderm → muscles, organs
• Endoderm → gut lining

Question 4

What genes establish the Anterior-Posterior (A-P) axis in Drosophila?

Answer 4

• Bicoid (bcd): anterior; activates head genes
• Nanos (nos): posterior; inhibits hunchback mRNA

Question 5

How is the Dorsal-Ventral (D-V) axis controlled?

Answer 5

• Toll pathway: Dorsal protein enters ventral nuclei to activate ventral genes
• Pipe protein: Expressed only ventrally; starts signaling cascade

Question 6

What are the 5 major gene classes in Drosophila segmentation (in order)?

Answer 6

1. Maternal effect – set up A-P and D-V axes (e.g., bicoid, nanos)
2. Gap genes – broad regions like head, thorax, abdomen (e.g., hunchback, kruppel)
3. Pair-rule genes – divide embryo into 7 stripes (e.g., even-skipped, fushi tarazu)
4. Segment polarity genes – A/P pattern of each segment (e.g., engrailed, wingless)
5. Homeotic (Hox) genes – define segment identity (e.g., Antennapedia, Ultrabithorax)

Question 7

What triggers the start of Drosophila development?

Answer 7

Fertilization: sperm entry completes meiosis and initiates nuclear division.

Question 8

What happens during the syncytial blastoderm stage?

Answer 8

13 rapid nuclear divisions occur in a shared cytoplasm

Question 9

What is cellularization in development?

Answer 9

Formation of membranes around nuclei, producing the cellular blastoderm

Question 10

What occurs during gastrulation in Drosophila?

Answer 10

The mesoderm invaginates and the three germ layers are established

Question 11

What patterns the embryo during segmentation?

Answer 11

A gene cascade from maternal → gap → pair-rule → segment polarity genes.

Question 12

What is organogenesis in Drosophila development?

Answer 12

Development of tissues and organs from patterned segments.

Question 13

What is a homeotic transformation?

Answer 13

A mutation causing one segment to develop like another (e.g., legs instead of antennae)

Question 14

What do Hox genes do?

Answer 14

Control segment identity, are conserved across species, and are arranged in clusters showing colinearity (DNA order matches A-P expression pattern)