Role of Homeotic Genes in Drosophila Development

Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Sometimes fruit flies have legs for antennae. It happens. When it does, it's usually the result of a mutation in a homeotic gene, such as a HOX gene. Homeotic genes are expressed late in fly development and control segment determination.

Homeotic Genes

My girls love playing with their Ms. Squash Head - you know, that plastic squash toy where you give the plastic squash a pair of eyes, a funky red nose and some big kissy lips. Sometimes, my oldest will put its plastic arms coming out of its head, and we laugh, because it looks silly to put arms where the Ms. Squash should obviously be sporting a ponytail.

Now, let's transition to a real-life situation that can cause an unfortunate Drosophila fruit fly to parallel a game of Ms. Squash. Scientists know a lot about animal development from research on flies. In fact, finding flies with feet coming out of their head where their antennae should be has helped scientists identify the genes that specify structural development. In this lesson, we'll learn about the group of genes that have this job.

You'll remember that there are three main categories of genes in early fly development that are expressed in a specific time and place to pattern the fruit fly body plan. The maternal-effect genes are expressed first. These are followed by the segmentation genes, which include the gap genes, pair-rule genes and segment polarity genes. All these genes are primer paint for the next set of gene products that will help put the final touches on a fly. Later in Drosophila development, the homeotic genes are expressed.

Along with the expression of other segmentation genes, homeotic genes control the final development of individual segments in a Drosophila embryo. While segmentation genes will set up the boundaries of the segments along the anterior-to-posterior axis of an embryo, it's these homeotic genes that will advance the final determination of these segments into specific structures likes antennae or legs.

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  • 0:07 Homeotic Genes
  • 1:35 HOX Genes
  • 2:33 Genes in Fly Development
  • 3:42 Summary
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HOX Genes

HOX genes represent a large number of identified homeotic genes. A homeotic mutant fly is a very strange sight to behold. For example, while a normal fly has the halteres seen below to stabilize its body, some homeotic mutants have wings where the halteres should be. This creates a four-winged fly. In us, that might be analogous to having legs coming out of our armpits. Another example of a homeotic mutant could have legs where the antennae should be. This is similar to when my girls put the wrong appendages on a Ms. Squash Head.

In some mutations, the halteres are replaced by wings.
image of fly showing halteres

There are multiple HOX genes, and they are found in an interesting order in the fly genome. They are actually located in the order in which they are expressed along the fly embryo. For example, as seen in the representative image of gene expression below, the first HOX gene at this gene locus is expressed on the anterior side of the fly, while the second HOX gene at this gene locus is expressed in the next portion of the fly. In addition, these developmental genes are conserved through higher organisms.

Representative image of gene expression
hox gene expression in fly

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