1. Genetics of Axis Specification in Drosophila Part 1 - Basics of Drosophila Embryogenesis
Gilbert - Chapter 9

2. Today’s Goals
Introduce Drosophila melanogaster as a developmental biology model organism
Describe key steps in Drosophila embryogenesis
Introduce a groundbreaking genetic screen that was used to better understand Drosophila development

3. Drosophila melanogaster
Classic genetic model organism
Thomas Hunt Morgan first used this organism - Early 1900’s

4. Drosophila melanogaster
Shares many of the characteristics of a good developmental biology model organisms Such as?
BUT - there are a number of disadvantages . . .
Embryos are REALLY tiny!
Embryos are quite complex
Larval stages, metamorphosis

5. Power of Genetics Polytene chromosomes
Ability to manipulate DNA, RNA, Protein made Drosophila a usable developmental bio model organism
Revolutionized the study of development, by providing a base to study the molecular genetics of development

8. Early Development of Drosophila
Egg is centrolecithal
After fertilization, series of superficial cleavages
Blastoderm is syncytial until 13th cleavage (256 nuclei!)
Nuclei begin dividing centrally, migrate toward the edges
Several nuclei migrate to posterior end, form cell membranes (pole cells)
Give rise to the adult gametes
What cells are like this in mammals?

10. Figure 23-6a Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 23-6a Early stages of embryonic development in Drosophila. (a) Fertilized egg with zygotic nucleus, about 30 minutes after fertilization. (b) Nuclear divisions occur about every 10 minutes, producing a multinucleate cell, the syncytial blastoderm. (c) After approximately nine divisions (512 nuclei), the nuclei migrate to the outer surface or cortex of the egg. (d) At the surface, four additional rounds of nuclear division occur. A small cluster of cells, the pole cells, form at the posterior pole about 2.5 hours after fertilization. These cells will form the germ cells of the adult. (e) About 3 hours after fertilization, the nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, creating the cellular blastoderm.
Figure 23-6a Copyright © 2006 Pearson Prentice Hall, Inc.

11. Figure 23-6b Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 23-6b Early stages of embryonic development in Drosophila. (a) Fertilized egg with zygotic nucleus, about 30 minutes after fertilization. (b) Nuclear divisions occur about every 10 minutes, producing a multinucleate cell, the syncytial blastoderm. (c) After approximately nine divisions (512 nuclei), the nuclei migrate to the outer surface or cortex of the egg. (d) At the surface, four additional rounds of nuclear division occur. A small cluster of cells, the pole cells, form at the posterior pole about 2.5 hours after fertilization. These cells will form the germ cells of the adult. (e) About 3 hours after fertilization, the nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, creating the cellular blastoderm.
Figure 23-6b Copyright © 2006 Pearson Prentice Hall, Inc.

12. Figure 23-6c Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 23-6c Early stages of embryonic development in Drosophila. (a) Fertilized egg with zygotic nucleus, about 30 minutes after fertilization. (b) Nuclear divisions occur about every 10 minutes, producing a multinucleate cell, the syncytial blastoderm. (c) After approximately nine divisions (512 nuclei), the nuclei migrate to the outer surface or cortex of the egg. (d) At the surface, four additional rounds of nuclear division occur. A small cluster of cells, the pole cells, form at the posterior pole about 2.5 hours after fertilization. These cells will form the germ cells of the adult. (e) About 3 hours after fertilization, the nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, creating the cellular blastoderm.
Figure 23-6c Copyright © 2006 Pearson Prentice Hall, Inc.

13. Figure 23-6d Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 23-6d Early stages of embryonic development in Drosophila. (a) Fertilized egg with zygotic nucleus, about 30 minutes after fertilization. (b) Nuclear divisions occur about every 10 minutes, producing a multinucleate cell, the syncytial blastoderm. (c) After approximately nine divisions (512 nuclei), the nuclei migrate to the outer surface or cortex of the egg. (d) At the surface, four additional rounds of nuclear division occur. A small cluster of cells, the pole cells, form at the posterior pole about 2.5 hours after fertilization. These cells will form the germ cells of the adult. (e) About 3 hours after fertilization, the nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, creating the cellular blastoderm.
Figure 23-6d Copyright © 2006 Pearson Prentice Hall, Inc.

14. Figure 23-6e Copyright © 2006 Pearson Prentice Hall, Inc.
Figure 23-6e Early stages of embryonic development in Drosophila. (a) Fertilized egg with zygotic nucleus, about 30 minutes after fertilization. (b) Nuclear divisions occur about every 10 minutes, producing a multinucleate cell, the syncytial blastoderm. (c) After approximately nine divisions (512 nuclei), the nuclei migrate to the outer surface or cortex of the egg. (d) At the surface, four additional rounds of nuclear division occur. A small cluster of cells, the pole cells, form at the posterior pole about 2.5 hours after fertilization. These cells will form the germ cells of the adult. (e) About 3 hours after fertilization, the nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, creating the cellular blastoderm.
Figure 23-6e Copyright © 2006 Pearson Prentice Hall, Inc.

15. Although nuclei share the same cytoplasm, the cytoplasm is not uniform in its makeup
Maternal molecules are distributed differently
Eventually cells will form plasma membranes and the embryo will consist of a cellular blastoderm
Mid-blastula transition occurs slowly, increasing transcription of zygotic genes

16. Gastrulation
At MBT, gastrulation begins, forming mesoderm, endoderm, ectoderm
Cells fold inward to form ventral furrow
Embryo bends to from cephalic furrow
Pole cells are internalized, endoderm invaginates
Ectoderm converges and extends along midline to form GERM BAND

18. Germ Band Wraps around the embryo
As it wraps around the dorsal surface, the A-P axis of the embryo is laid down
Body segments begin to form
At the end of germ band extension
Organs are beginning to form
Body segmentation is set-up
Groups of cells called imaginal discs are set aside, these cells will form adult structures

21. Drosophila Larvae During metamorphosis
3 “instar” larvae
Pupae
Adult
After gastrulaiton, 1st instar larvae is formed
Has head and tail end
Repeating segments along axis
Generally the same type of body plan as adult

22. Drosophila body plan 3 thoracic segments 8 abdominal segments
Each different from each other
8 abdominal segments
Able to tell the difference in the larvae based on cuticle
Covering of the embryo
Correspond to the adult segments

25. Axis Specification Controlled by a variety of genes
Maternal effect genes
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic selector genes

27. Genetic Screen for Genes involved in Drosophila Development
Nusslien-Volhard, Wieschaus
Fed mutagens to Drosophila
Then breed until mutation is homozygous recessive
Examined embryos for patterning defects
Used embryonic cuticles to do screens
Looked at pattern of denticles, shapes of segments

28. distinctive structures
The wild-type body
is segmented
and each segment
has a unique identity
and thus produces
distinctive structures