Genetics of Axis Specification in Drosophila

Part 1 - Basics of Drosophila Embryogenesis

Gilbert - Chapter 9

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

Drosophila melanogaster

• Classic genetic model organism

• Thomas Hunt Morgan first used this organism - Early 1900’s

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

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

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?

Figure 23-6a Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 23-6b Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 23-6c Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 23-6d Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 23-6e Copyright © 2006 Pearson Prentice Hall, Inc.

• 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

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

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

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

Drosophila body plan

• 3 thoracic segments– Each different from each other

• 8 abdominal segments– Each different from each other

• Able to tell the difference in the larvae based on cuticle– Covering of the embryo

• Correspond to the adult segments

Axis Specification

• Controlled by a variety of genes

• Maternal effect genes

• Gap genes

• Pair-rule genes

• Segment polarity genes

• Homeotic selector genes

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

The wild-type body is segmented

and each segment has a unique identityand thus produces

distinctive structures