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Ch. 18 - Developmental Genetics
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 18 - Developmental GeneticsProblem 17
Chapter 18, Problem 17

In contrast to Drosophila, some insects (e.g., centipedes) have legs on almost every segment posterior to the head. Based on your knowledge of Drosophila, propose a genetic explanation for this phenotype, and describe the expected expression patterns of genes of the Antennapedia and bithorax complexes.

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Understand the role of Hox genes: Hox genes are a group of related genes that control the body plan of an embryo along the anterior-posterior axis. In Drosophila, the Antennapedia and bithorax complexes are subsets of Hox genes responsible for segment identity.
Analyze the Antennapedia complex: The Antennapedia complex in Drosophila specifies the identity of anterior segments, such as the head and thorax. Mutations or misexpression in these genes can lead to legs forming in inappropriate locations, such as on the head.
Examine the bithorax complex: The bithorax complex specifies the identity of posterior segments, such as the abdomen. In Drosophila, these genes suppress the development of legs in abdominal segments. A loss of function or altered regulation of these genes could result in legs forming on posterior segments.
Propose a genetic explanation for centipedes: In centipedes, the phenotype of legs on almost every segment posterior to the head could be explained by differences in the regulation or expression of Hox genes. Specifically, the bithorax complex may not suppress leg development in posterior segments, or the Antennapedia complex may promote leg development across more segments.
Describe expected expression patterns: In centipedes, the Antennapedia complex genes might be expressed in a broader range of segments, promoting leg development. The bithorax complex genes might either be absent or expressed differently, failing to suppress leg formation in posterior segments.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Segmental Development in Insects

Insects exhibit a segmented body plan, where each segment can develop distinct structures. In Drosophila, segments are specified by the expression of particular genes, which determine the identity and characteristics of each segment. This segmentation is crucial for understanding how different insects, like centipedes, can have varying numbers of legs based on their segmental development.
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Antennapedia Complex

The Antennapedia complex is a group of homeotic genes in Drosophila that control the development of segment identity, particularly in the thorax and head regions. Mutations in these genes can lead to the transformation of one body part into another, such as legs developing where antennae should be. Understanding this complex helps explain how variations in leg development can occur in different insect species.
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Bithorax Complex

The Bithorax complex is another set of homeotic genes that regulate the development of the posterior segments in Drosophila. It plays a critical role in determining the identity of the third thoracic segment and the abdominal segments. The expression patterns of these genes can elucidate how certain insects, like centipedes, develop additional legs on multiple segments, contrasting with the more limited leg development seen in Drosophila.
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Related Practice
Textbook Question
What phenotypes do you expect in flies homozygous for loss-of-function mutations in the following genes: Krüppel, odd-skipped, hedgehog, and Ultrabithorax?
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Textbook Question

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?

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Textbook Question

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 the expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa. Based on your prediction, can you predict the phenotype of the fushi tarazu and engrailed double mutant?

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Textbook Question
The bristles that develop from the epidermis in Drosophila are evenly spaced, so that two bristles never occur immediately adjacent to each other. How might this pattern be established during development?
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Textbook Question

You are traveling in the Netherlands and overhear a tulip breeder describe a puzzling event. Tulips normally have two outer whorls of brightly colored petal-like organs, a third whorl of stamens, and an inner (fourth) whorl of carpels. However, the breeder found a recessive mutant in his field in which the outer two whorls were green and sepal-like, whereas the third and fourth whorls both contained carpels. What can you speculate about the nature of the gene that was mutated?

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Textbook Question

A powerful approach to identifying genes of a developmental pathway is to screen for mutations that suppress or enhance the phenotype of interest. This approach was undertaken to elucidate the genetic pathway controlling C. elegans vulval development. A lin-3 loss-of-function mutant with a vulva-less phenotype was mutagenized. Based on your knowledge of the genetic pathway, what types of mutations will suppress the vulva-less phenotype?

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