Meiosis and Crossing Over in the Ascomycete Fungus ...



Overview

This laboratory investigates the process of nuclear division and gemete production by meiosis. The major differences between the processes and outcomes of mitosis and meiosis are identified. Genetic recombination in the form of crossing over is observed and crossover frequency and map distance will be quantified using the ascomycete fungus Sordaria firmicola.

Background

Meiosis involves two successive nuclear divisions that produce four haploid cells. Meiosis I is the reduction division. It is this first division that reduces the chromosome number from diploid to haploid and separates the homologous pairs. Meiosis II, the second division, separates the sister chromatids. The result is four haploid gametes.

Mitotic cell division produces new cells genetically identical to the parent cell. Meiosis increases genetic variation in the population. Each diploid cell undergoing meiosis can produce 2n different chromosomal combinations, where n is the haploid number. In humans the number is 223, which is more than eight million different combinations. Actually, the potential variation is even greater because, during meiosis I, each pair of chromosomes (homologous chromosomes) comes together in a process known as synapsis. Chromatids of homologous chromosomes may exchange parts in a process called crossing over. The relative distance between two genes on a given chromosome can be estimated by calculating the percentage of crossing over that takes place between them.

Sordaria, like other fungi and some stages of plant life cycles, exists as a haploid organism. Each of its cells has only one complete set of chromosomes. When hyphae of different strains of Sordaria meet as they grow from the fungal mycelium, cells from each strain fuse together to form diploid cells.These diploid cells then undergo meiosis followed by mitosis to form eight haploid cells called ascospores.The ascospores are contained within a special sac, the ascus (plural = asci). Each ascus is further protected by a surrounding structure called the perithecium. When ascospores are released into the environment, they may germinate and grow into new Sordaria individuals.

One of the obvious genetic characteristics in Sordaria is the color of the ascospores.The normal color, or wild type, for ascospore coloration is black, and is represented by the symbol +.The mutant ascospore coloration is tan, and is represented by the symbol tn. If hyphae from wild-type and mutant strains of Sordaria meet, asci containing ascospores of both colors, called hybrids, are produced.These hybrid asci are easy to identify by the presence of both tan and black ascospores. In addition, the location of the ascospores within the ascus indicates whether or not crossing over occurred during meiosis. Since the formation of ascospores occurs within an ascus, all of the daughter cells are packaged together. If crossing over did not occur, four black ascospores will be grouped together next to four tan ascospores within the hybrid ascus. If crossing over did occur, the arrangement of ascospores will be something different than the 4 tan/4 black arrangement.

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Geneticists have found that crossing over is statistically more likely to occur if the gene is relatively distant from the centromere, as is the case with the gene for ascospore color in Sordaria.This is also the case when genes for two separate characteristics, such as plant height and seed color, are at distant locations on the same chromosome. Genes located on the same chromosome are said to be linked genes. If no crossing over event occurs during meiosis, these linked genes will be passed together to offspring.The further these linked genes are from each other along the length of the chromosome, the more likely that one may be exchanged during a crossover event. Crossing over, then, leads to combinations of genes in an offspring that are different than in either of the parents.This is one way in which genetic recombination can occur. The offspring with the new characteristics is referred to as a recombinant organism.

The distance between two linked genes, or, as in the case of Sordaria, between the gene and the centromere, can be measured using map units. Map units are not measured on an absolute scale; rather, they are a relative measure of the distance between two points on a chromosome as indicated by the frequency of crossover events. For example, if you find that 34% of the organisms you are studying are recombinants, there are 34 map units between the two genes for those characteristics.

Procedure

Using the cards provided, identify the ascospore arrangement in a minimum of 50 hybrid asci and record these data in Table 1 of the Data and Analysis section.

DATA AND ANALYSIS

Table 1

|# Asci in Which Crossing Over Did NOT Occur |# Asci in Which Crossing Over Occurred |Total # Hybrid Asci |

| | | |

1. Calculation of frequency of crossing over events affecting ascospore color in Sordaria:

|# Asci in which Crossing | |Total # Hybrid Asci |( 100 = |% Asci Showing Crossover |

|Over Occurred |( | | | |

|____________ | |__________ | |__________ |

2. Calculation of number of map units from ascospore color gene to centromere in Sordaria:

|% Asci Showing Crossover |( 2 = |Map Distance |

|____________ | |__________ map units |

3. Describe three ways in which mitosis and meiosis differ.

a.

b.

c.

4. Describe the process of crossing over. Be sure to include answers to the following questions: When does it occur? What are the participants? What is the result?

5. What is a map unit?

6. Why are map units a useful genetic tool?

7. Why is Sordaria a good experimental subject for a crossing over study?

8. What is a recombinant organism?

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