Building the perfect fruit fly is no simple task. In this lesson, we’ll learn how segmentation genes control the development of repeating regions of a fly’s body. These include the gap genes, pair-rule genes, and segment polarity genes.
It’s true that your body shape has a lot to do with what you eat and how you exercise, but we all know that a lot of what you look like also comes from what is in your genes. If we take it down to the basics, we’d say you have a good head on your shoulders simply because your genes told your body to put it there.In fact, your body plan started developing when you were just an embryo.
The fruit fly is no exception. In this lesson, we’ll continue with how the body plan of a fly is developed when it is just an embryo. The location of its head, as well as the rest of its body plan, relies on the right genes being expressed at the right time for the proper pattern formation.We’ve learned how there are three main classes of genes that control pattern formation in Drosophila, or fruit flies.
As a reminder, these classes include the maternal-effect genes, segmentation genes and homeotic genes. In this lesson, we’ll focus on the role of the segmentation genes in development of the anterior-to-posterior axis of fruit fly embryos. You’ll also remember that transcripts from maternal-effect genes are deposited in unfertilized fly eggs by the mother fly and that the products from these genes influence the expression of segmentation genes.The term ‘segmentation gene‘ is a classification given to a broad class of genes that are further subdivided into three smaller classes of genes. Within the segmentation gene group, there are gap genes, pair-rule genes and segment polarity genes. They control development in this order.
These segmentation genes control the development of segments, or small repeated regions of a body that make up specific structures. If you were to look at a fly embryo during development, you’d quickly notice these segments along their bodies. Overall, segmentation genes will set up the boundaries of each of these segments. The next group of genes, the homeotic genes, will then direct what structures each of these segments develop into, such as wings or legs.
The gap genes are expressed early in development in broad regions. The products from these genes generally define the different parts of the embryo that will later develop into segments.
While maternal-effect genes have already defined the anterior and posterior end of the embryo, these gap genes define the location of the head, thorax and abdominal region.These gap genes encode transcription factors that control the expression of other genes, including the pair-rule genes (the next group of segmentation genes). There are several different gap genes, each expressed in a specific region. If there is a mutation in one of them so that it does not produce a functional protein, then the segments in that region will not properly develop.
This develops a ‘gap’ in the body plan of the embryo, and it’s how the genes got their name.
The pair-rule genes are expressed later in development to define the edges of individual segments. They are expressed in alternating bands to create these edges. You can identify these genes by this nifty striped pattern along an embryo. Here, you can see an example of what the expression of two different pair-rule genes would look like.
The two different colors represent the expression of these genes in an alternating pattern. The expression of these genes is controlled not only by gap genes but also by each other.They get their name from the fact that a mutation in a pair-rule gene will result in a mutation in alternating segments. This suggests that segments come in pairs.
For example, suppose each segment of a fly has these hairy bristles on them. In adult flies with some pair-rule gene mutations, this would mean every other segment could be missing some of these hairy bristles. This creates an alternating segment pattern of the phenotype.
Now that’s an interesting hairdo.
Segment Polarity Genes
You’ve probably guessed that the pair-rule genes are needed for the proper expression of the last set of segmentation genes, the segment polarity genes. These genes are expressed even later in development to create polarity within individual segments.
The bands of expression of these genes are much tighter than pair-rule genes. In effect, they establish the anterior-to-posterior direction of each of these segments.Unlike a pair-rule gene mutation, a mutation in a segment polarity gene might cause a phenotype, or change in physical appearance, in each segment rather than alternating segments. To put this in perspective, let’s cartoon the segments of a Drosophila embryo as having two different ends, an anterior and posterior side to the segment. If there were a mutation in a segment polarity gene, then the phenotype would be a segment that looks the same on each side – like a mirror image.
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