Drosophila Embryogenesis Genes Broken Down!!
Idea creds @Delphinidin.
For an amorphous ball of cells to become a rather complex fruit fly, a lot of regulation is needed. In this article, we'll explore how embryogenesis occurs in the anterior-posterior axis (from head to butt) of Drosophila melanogaster, a common model organism for this topic.
There are three kinds of genes that are relevant to the USABO level: maternal effect genes, segmentation genes, and homeotic genes, also known as homeotic selector genes. Within the category of segmentation genes are gap genes, pair-rule genes, and segment polarity genes. Let's dive into the commonly tested genes in each category.
Maternal effect inheritance dictates how maternal effect genes are inherited. This mode of inheritance, commonly explained using the example of the direction of snail shell coiling, follows a similar mechanism in Drosophila development. (Before you read this explanation, watch this if you aren't familiar with the snail thing.) There are four genes under this category: bicoid, hunchback, nanos, and caudal. Bicoid and nanos exert their effects using both their mRNA and protein gradients in the egg, while hunchback and caudal only exert their effects through protein gradients. Bicoid and hunchback specify anterior structures, while caudal and nanos specify posterior structures. Below is a visual of this information:
Some random stuff about maternal effect genes - The concentration gradients of these molecules are orchestrated by attachment of mRNAs to cytoskeletal components (usually microtubules) and/or a dsRNA-binding protein, STAUFEN (STAU1). Also, the bicoid, hunchback, and caudal proteins function as transcription factors (they regulate which DNA is expressed to produce a certain effect). The bicoid protein is also a morphogen, which means it acts on other proteins (not at the DNA level) to regulate their activity and, again, produce a specific result. The nanos protein is a translational repressor, which means it blocks the mRNA-->protein transition for factors involved in anterior structure formation.
Next: segmentation genes. These are regulated by the maternal effect genes, the broadest level of organization. Let's go over each of the three categories.
First, gap genes. These include many genes, but the two relevant ones are hunchback and Kruppel: hunchback because it serves a second role in this context, and Kruppel because it reminded me of KRAB domains. Gap genes function to establish and outline the general segmentation pattern of the embryo.
Second, pair-rule genes include even-skipped (eve) which regulate the finest level of segment organization, the segment polarity genes (third kind of segmentation gene). Segment polarity genes, which regulate development within each segment, include engrailed and hedgehog. (Hedgehog is the Drosophila version of mammalian Sonic Hedgehog, which also functions in embryonic development.) Below is a visual of where eve and engrailed act:
Finally, we have homeotic genes, which determine the identity and development of each segment established by the segmentation genes. I don't have any notable examples for you. Something interesting about these is that their order on chromosome 3 matches the order of the segments they affect on the body! It's like chromosome 3 is a map of homeotic genes, which is neat.
SUMMARY!
Hope that helped. Here's some more stuff:
https://en.wikipedia.org/wiki/Drosophila_embryogenesis - wikipedia clutch
hedgehog stuffs! - https://en.wikipedia.org/wiki/Hedgehog_signaling_pathway#Role_2
