Question

The mutations called bobbed in Drosophila result from variable reductions (deletions) in the number of amplified genes coding for rRNA. Researchers trying to maintain bobbed stocks have often documented their tendency to revert to wild type in successive generations. Propose a mechanism based on meiotic recombination which could account for this reversion phenomenon. Why would wild-type flies become more prevalent in Drosophila cultures?

Short answer

Answer: Meiotic recombination can lead to the reversion of bobbed mutations in Drosophila by exchanging genetic material between a chromosome carrying the bobbed mutation and a normal chromosome with intact rRNA genes. During crossing over, the deleted region in the bobbed chromosome may be replaced with an intact rRNA gene region from the normal chromosome, effectively restoring the full complement of rRNA genes in the previously mutated chromosome. As wild-type flies have better growth, development, and reproductive fitness, they become more prevalent in subsequent generations due to their higher survival and reproductive success.

Step-by-step solution

Understanding Bobbed Mutations

Bobbed mutations in Drosophila occur due to deletions in the genes coding for rRNA. These deletions lead to reduced rRNA genes, which can have a pronounced effect on the organism's growth, development, and reproductive fitness. The variable reductions refer to the different sizes of deletion in amplified rRNA genes.

Mechanism of Meiotic Recombination

In sexually reproducing organisms, meiotic recombination is a process that randomly shuffles genetic material between homologous chromosomes during the formation of gametes (eggs and sperm). This process results in increased genetic diversity among offspring. During meiotic recombination, DNA is exchanged between homologous chromosomes during a process called crossing over.

Proposed Mechanism for Reversion of Bobbed Mutations

One possible mechanism that could explain the reversion of bobbed mutations to wild-type is through meiotic recombination. During this process, a bobbed chromosome carrying the deletion could pair up with a normal chromosome carrying the full complement of rRNA genes. During crossing over, genetic material with the intact rRNA genes could be swapped from the normal chromosome and replace the deleted region in the bobbed chromosome. This process would restore the full complement of rRNA genes in the chromosome with the previously bobbed mutation. Thus, through meiotic recombination, the bobbed mutation could revert to wild-type.

Why Wild-Type Flies Become More Prevalent

Wild-type flies carry the full complement of rRNA genes, and as a result, they display better growth, development, and reproductive fitness than those with bobbed mutations. As the bobbed mutations revert to wild type through meiotic recombination, flies carrying the wild-type alleles will be more prevalent in the population. This is because the flies with intact rRNA genes are more successful in terms of survival and reproduction, leading to an increase in the proportion of wild-type flies in the Drosophila culture.

Key concepts

Bobbed Mutations in Drosophila

Bobbed mutations in the fruit fly, Drosophila melanogaster, are characterized by a shorter and stubbier appearance of the bristles (setae) on the flies, hence the term 'bobbed'. These mutations occur due to deletions in the genes coding for ribosomal RNA (rRNA). Ribosomes, the cell's protein factories, are vital for growth and development, and they require rRNA to function properly. The deletions, which can vary in size, lead to a reduced number of rRNA genes.

The genetic mechanism behind these mutations suggests a vulnerability to further changes, meaning that they can be prone to reversion. During meiotic recombination, an important form of genetic reshuffling, changes occur that can result in the bobbed phenotype reverting to the wild-type appearance. Understanding bobbed mutations not only elucidates aspects of fly morphology but also offers insights into the larger picture of genetic stability and variability.

rRNA Gene Deletions

Ribosomal RNA gene deletions are at the crux of bobbed mutations. The genes coding for rRNA are repeated several times in the genome, and their amplification is essential for robust ribosome function. When deletions occur, the complement of rRNA genes is reduced, impacting the efficiency of the ribosomes and, consequently, protein synthesis. This reduction can manifest as developmental and physiological defects in Drosophila, since ribosomes play a critical role in translating genes into the proteins that carry out most cellular functions.

It's important to note that these rRNA gene deletions are not just a matter for academic study. They represent a real-world problem when maintaining laboratory fly stocks, as the deletions can lead to weakened strains that are less viable over time. The deletions bring to light the delicate balance of gene dosage and the way it affects cellular health and organismal vitality.

Genetic Diversity

Genetic diversity is the foundation of a population's ability to adapt to changing environments and survive the forces of natural selection. In the context of bobbed mutations in Drosophila, genetic diversity allows for a range of phenotypes—from those with severe rRNA deletions to those that are wild-type. Meiotic recombination is a key driver of genetic diversity; during the creation of gametes, it mixes parental chromosomes, producing unique combinations of genes.

Increased genetic diversity due to meiotic recombination can lead to increased resilience against environmental stresses and diseases. In breeding Drosophila stocks, genetic diversity might help the population rebound from detrimental mutations, like the bobbed mutation, as some individuals will inherit fewer or no deletions in their rRNA genes.

Crossing Over

Crossing over is a fundamental aspect of meiotic recombination. It is the physical exchange of DNA between homologous chromosomes, which occurs during the prophase I stage of meiosis. During crossing over, corresponding segments of DNA are swapped, which can create new combinations of alleles within a chromosome. This can be an important mechanism for the reversion of the bobbed phenotype.

When a bobbed chromosome carrying a deletion in the rRNA genes pairs up with a normal chromosome during meiosis, crossing over can result in the normal intact rRNA genes swapping onto the bobbed chromosome. This genetic exchange could potentially restore the full set of rRNA genes, which explains how the bobbed mutation might revert to wild-type in successive generations.

Wild-type Allele Prevalence

The prevalence of wild-type alleles is critical to understand why certain traits, like the bobbed mutation in Drosophila, may become less common over time. Wild-type alleles are those that are commonly found in the natural population and code for the effective functioning of rRNA genes. They provide the organism with the best chance of survival and reproduction.

Flies with a full complement of rRNA genes (wild-type) generally show better overall fitness than their bobbed counterparts. Through the process of meiotic recombination, wild-type alleles can become more prevalent, as they are typically favored by natural selection. As a result, in Drosophila cultures, the individuals with the greatest fitness—those with wild-type alleles—tend to dominate, leading to an increase in the frequency of these alleles over time.