In this lesson, you'll learn how those pesky fruit flies develop from a fertilized egg through the help of specific genes. Maternal-effect, segmentation and homeotic genes help to establish proper pattern formation and development in Drosophila.
Fruit Fly Development
Although you have probably noticed a swarm of fruit flies around your garbage can when you have waited too long to take out the trash, you've probably not noticed their complex body structure or appreciated how an adult fly can be such an incredible example of animal development. Nobody blames you for that; flies are not exactly cute and cuddly. However, in this lesson, we'll start to learn how tiny fly larvae develop into adult fruit flies.
You might ask yourself why we care, as long as these fruit flies are not in our kitchen, right? However, scientists and geneticists have learned a lot about human development and genetics from studying the development of this model organism. In fact, many of the genes involved in fruit fly development are conserved through human development.
Development of the common fruit fly, or Drosophila melanogaster, begins with an egg that becomes fertilized. This fertilized egg, or embryo, hatches into a larva. This larva feeds on the rotting fruit in your trashcan. And then, this larva develops into a pupa. This pupa eventually becomes what you know as the adult fly through metamorphosis.
It is during the early stages of fly embryonic development that the body plan of a fly is arranged into what will become the adult fly. Pattern formation is the development of a body according to a specific and planned spatial arrangement. Pattern formation in fruit flies has been well studied by geneticists and developmental biologists. This patterning is tightly controlled by gene expression and begins even before an egg is fertilized. The right genes must be turned on and turned off at the correct times in order for a fly embryo to develop into an adult fly. Mistakes in this process can lead to some funny looking flies, like having legs where their antennas should be.
The formation of the head-to-tail body axis in flies is well-understood. Scientists have spent years uncovering the genes that are responsible for determining the patterning of a fly embryo along the anterior-posterior axis. If we look at a fly embryo, the anterior is the part of the embryo that will develop into the head, while the posterior is the tail end of the embryo. If you were to draw an imaginary line through the embryo to connect these two ends, this would be the anterior to posterior axis.
Genes Controlling Pattern Formation
Scientists have uncovered three classifications of genes that are involved in proper pattern formation of the anterior to posterior axis. The first set of genes is called the maternal-effect genes. The second set of genes is called the segmentation genes, and the third set of genes is known as the homeotic genes.
We will go into specific examples of each of these genes and how they function in other lessons. In this lesson, let's appreciate that these genes control fly development in a specific order. First, maternal-effect genes encode for proteins that establish a concentration gradient across an embryo. In fact, products of these genes are in eggs even before fertilization occurs. This helps correctly set up the anterior to posterior axis early in embryonic development.
Proteins from these maternal-effect genes control the proper expression of the next group of genes, the segmentation genes. Notice these ridged segments on a Drosophila embryo. Segmentation genes do exactly what they sound like; they establish the segmented body plan of the embryo. There are several types of segmentation genes, and they are expressed differently to create these segments. The products from segmentation genes are necessary for the proper expression of this last group, the homeotic genes. These homeotic genes control the development of these segments into specific body parts.
In future lessons, we'll delve into exactly how this works and what happens when it doesn't. Mistakes in the expression of these genes can literally lead to a segment forming the wrong structure. In other words, mistakes can lead to legs coming out of a fly's face! You might laugh, but these flies are not laughing. Finding flies with these odd mutations are exactly what helped geneticists discover the roles of these pattern formation genes!
Let's review. In this lesson, we have learned one well-studied aspect of fruit fly development is pattern formation, or the development of a body plan according to a specific and spatial arrangement. Specifically, we know what groups of genes are responsible for patterning the anterior to posterior axis of fruit fly embryos. This process begins before the eggs are fertilized with the maternal-effect genes and follows with the proper expression of the segmentation genes, as well as the homeotic genes through normal fly development. In future lessons, we'll learn examples of each of these groups and how they establish a normal body plan in Drosophila.
After viewing this video lesson, you should be able to:
- Explain why the Drosophila melanogaster is important scientifically
- Describe pattern formation as it applies to Drosophila
- List the three classifications of genes involved in pattern formation