Kristin has taught college Biology courses and has her doctorate in Biology.
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.
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.
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.
Genes in Fly Development
If we take a step back to look at all the genes involved in fly development, we can start to appreciate this cascade of regulatory genes and gene products that control development. The maternal-effect genes are in essence the master regulators of development that are established before a fly egg is even fertilized. These genes set up the anterior-to-posterior axis of the egg. Next, the segmentation genes control the boundaries of individual segments, while the homeotic genes we've discussed help mold the final determination of these segments.
Together, these genes create a regulatory gene cascade whose products include transcription factors and translational control factors as well as signaling molecules. The important things to remember about these genes are that they are expressed in a precise order, control development of specific regions of the fly embryo and that some control expression of each other.
Errors in this process will result in a chain reaction. For example, an error in early development can affect the expression of genes that are expressed later in development. It's a domino reaction. If one domino is removed from the chain, the falling dominoes might not be able to complete from start to finish. At the very least, the amazing reaction that was supposed to happen might not be as perfect, with a few glitches along the way.
Let's review. Remember that the genes controlling fly development are expressed in a precise order. Homeotic genes fit into this gene regulatory cascade of fly development by following the expression of maternal-effect genes and segmentation genes including gap genes, pair-rule genes and segment polarity genes. Homeotic genes control the final determination of individual segments into structures in a Drosophila embryo. An example of a fly with a homeotic gene mutation would be a fly that has segments that developed into legs instead of antennae. HOX genes are one large group of homeotic genes that have been identified in fly development that are also involved in the development of higher organisms.
After this lesson, you'll be able to:
- Identify the order of gene expression in fly development
- Explain the expression of homeotic genes in flies
- Provide an example of a fly with a homeotic gene mutation
- Define HOX genes
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