11.13.1 Pattern Formation in Drosophila

Question 1

Pattern Formation in Drosophila

Answer 1

• Pattern formation questions in development have to do with how the body axes and the placement of body parts are determined. Cytoplasmic determinants are molecules present in the cytoplasm of a cell that determine cell fate. Inductive signals are a form of cell-cell communication where chemical signals from one cell determine the fate of a neighboring cell.
• Lewis was the first researcher to notice mutations in Drosophila that affected the correct placement of body parts and therefore had to be mutants in the development process.
• Nüsslein-Volhard and Weischaus studied the estimated 12,000 genes in Drosophila and determined whether they were involved with the organism’s development both directly or through maternal effects.

Question 2

note

Answer 2

• Drosophila sp. has been widely used as a model organism in genetics since the beginning of the 20th century. It has also become important for studies in development.
• The life cycle of Drosophila begins when the egg cell is fertilized (1). The cell begins dividing without cytokinesis and the cell becomes multinucleate (2). Cells then partition and continue to divide (3,4). Segmentation begins and the insect enters the larval stage (5,6). The larva then enters the pupal stage during which it will develop into the adult fly (7).
• Edward Lewis studied developmental mutations in organisms and linked them to specific genes. One such gene, the antennapedia gene, when mutated, causes legs to form where antennae would normally be.
• When he performed test crosses with antennapedia mutants, he found that the mutation was inherited as a recessive trait.
• Nüsslein-Volhard and Weischaus wanted to determine which of the 12,000 Drosophila genes controlled development. Since most developmental mutations are lethal, they had to develop a protocol to identify them. They also needed to identify the genes for maternal cytoplasmic factors that affected the offspring of the mother, but not her own characteristics.
• They began by first identifying non-maternal developmental mutants. They exposed male flies to mutagens and crossed them with normal female flies. The resulting offspring potentially carried at least one developmental mutant. The F1 generation was interbred and the resulting larvae were examined. Dead larvae (those with two mutated alleles) were observed for developmental abnormalities. If present, the line carrying the mutation was maintained and the mutant
  gene was mapped.

Question 3

note 2

Answer 3

• The mother’s genes code for products, which directly affect embryonic growth. Maternal effect genes code for
  cytoplasmic factors that are present in the egg cell. In order to identify and map maternal effect genes, a mother that is homozygous for the mutant gene (–) must be obtained. This is not lethal for her because the gene codes for cytoplasmic factors that will be in her eggs. All offspring will die if the mother is homozygous recessive. Once maternal effect mutants were found, the line carrying the mutation was maintained and the mutant gene was mapped.
• In this way, Nusslein-Volhard and Weischaus were able to identify over one hundred genes that are involved in fruit fly development.

Question 4

What was one of the challenges that Nüsslein-Volhard and Weischaus faced?

Answer 4

• Developmental mutations are often lethal.

Question 5

The gene responsible for an antennapedia mutant

Answer 5

• is a recessive mutation

Question 6

Edward Lewis is responsible for

Answer 6

• linking genes to development

Question 7

During fruit fly development the fertilized egg cell

Answer 7

• undergoes several rounds of mitosis without cytokinesis

Question 8

A fly that is homozygous for a maternal effect mutation

Answer 8

• cannot be a mother of viable offspring.

Question 9

Nüsslein-Volhard and Weischaus identified individuals with mutant maternal-effect genes:

Answer 9

• when the mothers possessing them produced only offspring that died.