Suppose we want to maintain a large number of homozygous lethal chromosomes, exposed to chemical mutagens, for a mutation screening experiment in a small orb web building spider species. One of these lines has a dominant mutant named Disordered (D) that results in “messy” web building.
How can we modify the Disordered line to use as a genetic tool to maintain the mutant chromosome stocks?
To modify the Disordered (D) line in order to use it as a genetic tool to maintain the mutant chromosome stocks, you can employ the process of balancers and translocations.
First, let's understand the concept of balancers. Balancer chromosomes are special genetic constructs that contain multiple inversions, ensuring that they do not undergo recombination with their corresponding homologous chromosomes, thus preventing genetic shuffling. Balancers are typically homozygous lethal, which means that individuals carrying two copies of the balancer chromosome will not survive.
To begin, you would need to create a balancer chromosome with the Disordered (D) mutation. This would involve crossing a Disordered (D) individual with a suitable balancer stock, such as a balancer line commonly used in Drosophila genetics.
The resulting offspring will be heterozygous for the Disordered (D) mutation and the balancer chromosome. These heterozygous individuals can be identified by observing the "messy" web building phenotype.
Next, you will need to induce homologous recombination between the normal chromosome and the balancer chromosome to create a translocation. Translocations are rearrangements of genetic material between non-homologous chromosomes.
To achieve this, you can expose the heterozygous individuals carrying the Disordered (D) mutation and the balancer chromosome to chemical mutagens such as ethyl methanesulfonate (EMS). EMS treatment increases the likelihood of chromosomal breakage and rearrangements.
After EMS treatment, you can screen the offspring for those with chromosomal rearrangements. This can be done by examining their web-building behavior to identify those displaying the Disordered (D) phenotype. These individuals are likely to be carrying a translocation that includes the Disordered (D) gene.
By selecting and breeding these individuals carrying the translocation, you can establish a line of organisms that maintain the Disordered (D) mutation. Because the balancer chromosome also carries multiple inversions, preventing recombination with normal chromosomes, it can be used to maintain the homozygous Disordered (D) stocks without fitness loss.
It's important to note that this process requires careful monitoring and selection to ensure the desired genetic makeup. Additionally, establishing and maintaining large stocks of homozygous lethal chromosomes can be challenging, so it's recommended to consult with experts in the field for specific guidance and techniques applicable to your spider species.