Crossing Over in Sordaria Online 1

Crossing Over in Sordaria

Background

Sordaria fimicola is an ascomycete fungus that can be used to demonstrate the results of crossing over during meiosis. Sordaria is a haploid organism for most of its life cycle. It becomes diploid only when the fusion of the mycelia of two different strains results in the fusion of the two different types of haploid nuclei to form a diploid nucleus. The diploid nucleus must then undergo meiosis to resume its haploid state.

Meiosis, followed by mitosis, in Sordaria results in the formation of eight haploid ascospores contained within a sac called an ascus (plural, asci). Many asci are contained within a fruiting body. When ascospores are mature the ascus ruptures, releasing the ascospores. Each ascospore can develop into a new haploid fungus. The life cycle of Sordaria fimicola is shown in Figure 1.

To observe crossing over in Sordaria, one must make hybrids between wild‐type and mutant strains of Sordaria. Wild‐type (+) Sordaria have black ascospores. One mutant strain has tan spores (tn). When mycelia of these two different strains come together and undergo meiosis, the asci that develop will contain four black ascospores and four tan ascospores. The arrangement of the spores directly reflects whether or not crossing over has occurred.

Below, no crossing over has occurred.

Formation of non‐crossover asci

If crossing over occurs, it will occur in the region between the gene for spore color and the centromere. The homologous chromosomes then separate during meiosis I. Meiosis I (MI) will result in two cells, each containing both genes (1 tan, 1 wild‐ type); therefore, the genes for spore color have not yet segregated. Meiosis II (MII) results in segregation of the two types of genes for spore color. A mitotic division will result in the formation of 8 spores with an arrangement different than seen above.
Meiosis with crossing over

The frequency of crossing over appears to be governed largely by the distance between genes, or in this case, between the gene for spore coat color and the centromere. The probability of a crossover occurring between two particular genes on the same chromosome (linked genes) increases as the distance between those genes becomes larger. The frequency of crossover, therefore, appears to be directly proportional to the distance between genes.

A map unit is an arbitrary unit of measure used to describe relative distances between linked genes. The number of map units between two genes or between a gene and the centromere is equal to the percentage of recombinants. Customary units cannot be used because we cannot directly visualize genes. However, due to the relationship between distance and crossover frequency, we may use the map unit.

Objectives:

To determine the map unit distance of a gene to centromere based on the crossover frequency.

Procedure

1. Using the website provided, determine if crossing over has occurred in at least 50 hybrid asci. Record the data.

2. Based on your counts, determine the percentage of asci showing crossover.

(Percent crossover asci = bttb + tbbt + tbtb + btbt /Total interstrain asci X 100)

4. Divide the percent showing crossover by 2. This is your gene to centromere distance. (The percentage of crossover asci is divided by 2 because only half of the spores in each ascus are the result of a crossover event.)

(Distance between centromere and locus of tan gene = % Crossover /2)

Data

Number of 4:4 asci
Number of Crossover
Total Asci
% Showing Crossover
Gene to Centromere

Asci

Distance 19
35
54
64.81%
32.405

Analysis

1. Published results indicate that the map distance from the centromere of the gene for spore color in S. fimicola is 26 map units (corresponds to 52% crossover frequency). How close are your results to the published results?

My results were 12.81 away from the results that the published results.
2. Describe what could account for the differences in the published versus your results.
My data was off from the published edition because I did not count all of the data provided for me and I skipped over the asci that I could not see.