Introduction to Sordaria Lab Report

1 January 2017

The lab experiment examined meiosis and genetic diversity through the model organism, Sordaria fimicola. Meiosis is part of the sexual life cycle and occurs in all sexually reproducing organisms. It is a method of cell division that produces gametes. Meiosis has two parts: meiosis I and meiosis II. Both parts undergo the same four stages of prophase, metaphase, anaphase, and telaphase. The process begins with one single parent diploid cell (that contain homologous chromosomes) that divides into four daughter haploid cells which each contain half the number of chromosomes that the original parent cell contained.

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Both independent assortment and crossing over occur in meiosis I. Crossing over rearranges the DNA sequences that are then inherited and passed down to future offspring. This rearrangement, or recombination results in genetic variation within a species. The mechanisms controlling these crossover events are undefined. Recent existing evidence argues that harsh environmental pressures may lead to heritable changes in mechanisms controlling recombination.

Much of this evidence has come from studies done at the Evolution Canyons in Israel. There are four “Evolution Canyons”, each of which consists of two mountain slopes with varying climates. Evolution Canyon I, located in Lower Nahal Oren, Mount Carmel, contains a south facing slope (SFS) which experiences harsh climatic conditions, such as high temperatures and drought. The opposing north facing slope (NFS) is characterized by mild climatic conditions, including cooler temperatures and higher humidity.

Evolution Canyon serves as a model for microevolution and can be used to study how mutation and recombination contribute to adaptation and genetic diversity. The study published in the Genetics Journal and referred to in the biology laboratory manual studied whether there is a natural genetic variation for recombination frequencies and whether it correlates to environmental conditions and adaptation. The research was conducted on Evolution Canyon I and focused on the fungi organism, Sordaria fimicola. Several asexual filaments of S. imicola were collected on different levels of each slope.

The specimens were grown in the laboratory, where wild type spores from these crosses were then self-crossed to create a second generation of wild type spores. First, the filaments were grown on cornmenal agar with sodium acetate at 18 degrees and then crosses were completed on minimal medium at 17. 5 degrees. Spontaneous spore color mutations appeared as non-black spores in the wild type strains.

Also, the researchers plated the parithecia (the fruiting body of the S. imicola that contains ascospores) on growth medium containing acriflavin (a fungicide) and spontaneous acriflavin-resistant mutants formed. The strains were used to study varying mutation frequencies of wild type strains from the two different slopes on Evolution Canyon. Also, these strains were used to study the variations in crossover and gene conversion frequencies on the two opposing slopes. Their results revealed that wild type strains from the SFS experienced higher mutation rates than those strains obtained from the NFS.

Although much less apparent, there were also slight differences in crossover frequencies within slopes. The results provide evidence that mechanisms controlling mutation and recombination may adapt heritable changes in response to the harsh climatic demands, particularly with the SFS. Therefore, increased genetic diversity within a species may depend on the organisms’ environmental conditions. By understanding the factors controlling recombination, more can be known about genetic variation within a species.

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