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Let’s explore meiosis in the fungus Sordaria fimicola. The goal of this Demonstration is to measure the distance between a particular Sordaria gene and the centromere of the chromosome it’s on by using crossover data. This is the first step of gene mapping! The result of crossovers can be determined just by observing Sordaria offspring under the microscope!
Sordaria is haploid for most of its life cycle, but becomes diploid when two different strains fuse. The diploid zygote immediately undergoes meiosis, and then mitosis. The result is eight haploid cells called ascospores, located in sacs called asci. When ascospores mature, the asci rupture, releasing them to develop into new haploid fungi.
Consider the fusion between a normal Sordaria strain with black ascospores and a mutant strain with tan spores. The spore color gene is located near the end of one of Sordaria’s chromosomes. By following spore color throughout meiosis, we can also follow the segregation pattern of this chromosome.
For simplicity, we’ll color-code the two versions of the chromosome black and tan, respectively. The arrangement of the ascospores in the asci directly reflects whether or not crossing over has occurred. If no crossovers occur, meiosis I creates two cells, each containing one type of spore color gene. Meiosis II and mitosis result in four black ascospores next to four tan ascospores. This is called a 4-to-4 arrangement.
When crossing over occurs, meiosis I results in two cells, each containing both types of spore-color genes. Meiosis II and mitosis result in eight spores arranged in one of four ways: two in the 2-to-2-to-2-to-2 arrangement, and two in the 2-to-4-to-2 arrangement.
Crossover frequencies between the spore-color gene and the centromere are directly proportional to the distance between them. We’ll study crossover frequencies later t in the course.
For now, think in terms of probabilities: the greater the distance between a gene and the centromere, the more likely it is that a crossover event will occur between them.
We measure crossover frequencies in terms of the map unit, which is equal to the crossover percentage. For example, if a crossover event occurs between two genes 20% of the time, we’d say that they are 20 map units apart.
In diploids like Sordaria, only half of the spores in each ascus are the result of a crossover event. Thus, the percentage of crossovers is divided by 2 to calculate the map distance. Now let’s observe the arrangements of ascospores from a cross between black and tan strains of Sordaria and use them to estimate the percentage of crossovers that occur between the spore-color gene and the centromere.
We find 69 asci are in the 4-to-4 arrangement, and 81 are in either the 2-to-2-to-2-to-2 or 2-to-4-to-2 arrangement. That means a crossover occurred in 81 out of 150, or 54%, of the total asci. To calculate the map distance, we divide the percentage of crossovers by 2, which results in a distance of 27 map units.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education