The pair-rule gene fushi tarazu is expressed in the seven even-numbered parasegments during Drosophila embryogenesis. In contrast, the segment polarity gene engrailed is expressed in the anterior part of each of the 14 parasegments. Since both genes are active at similar times and places during development, it is possible that the expression of one gene is required for the expression of the other. This can be tested by examining expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa. Given the hierarchy of gene action during Drosophila embryogenesis, what might you predict to be the result of these experiments?

Key Concepts

Pair-rule Genes

Pair-rule genes, such as fushi tarazu, are crucial in Drosophila embryogenesis as they define the segmented body plan by regulating the formation of parasegments. They are expressed in alternating segments and play a key role in establishing the basic organization of the embryo. Their expression is essential for the proper development of structures that will later form the segments of the adult fly.

Segment Polarity Genes

Segment polarity genes, like engrailed, are responsible for maintaining the boundaries and polarity within each segment of the developing embryo. They are expressed in specific regions of the parasegments and help to define the anterior-posterior axis of each segment. Their activity is critical for the correct positioning of segmental structures and for ensuring that segments develop with the appropriate identity.

Gene Interaction and Mutant Analysis

The interaction between genes during development can be studied through mutant analysis, where the expression of one gene is examined in the absence of another. This approach helps to determine whether the expression of one gene is dependent on the other. In the context of the question, testing fushi tarazu expression in an engrailed mutant background could reveal whether engrailed is necessary for fushi tarazu expression, indicating a regulatory relationship between these genes.