Sexual reproduction provides a mechanism to produce genetic variation, as the genes of two different individuals are arranged in various ways. This requires a reduction in the chromosome number of the parent cell, normally diploid, to half that, or haploid, in somatic cells. The type of cell division resulting in half the chromosome number of the parent cell is called meiosis.
In meiosis, a germ cell divides into four haploid gametes. When two gametes, typically an egg and sperm for most animals, combine during fertilization to form a zygote, the diploid chromosome number is restored. Meiosis consists of one DNA replication and two nuclear divisions, meiosis I and II. This results in the formation of four daughter cells, each with only half the number of chromosomes of the
parent.
Genetic variability is further increased by a process called crossing over. In the early stages of meiosis, the homologous pairs of chromosomes move close together in such a way that all four chromatids are entwined, forming a tetrad. This process, known as synapsis, allows for the exchange of chromosome sections between the homologous pairs.
The example that will be used in the investigation is Sordaria fimicola. S. fimicola is an ascomycete fungus that is haploid for the bulk of its life cycle; the haploids comprise the individual fungal filaments, called hyphae, which normally exist in a mass called a mycelium representing the “body” of the fungus, and the ascospores, from which mycelia develop. The only diploid portion of the life cycle of S. fimicola occurs when the nuclei of specialized hyphae come together.
These hyphae, which belong to different strains of the species, fuse to form a zygote. This zygote then undergoes meiosis to produce the haploid ascospores, yielding four haploid nuclei contained in a sac called an ascus. After meiosis, the four nuclei undergo mitosis, resulting in an ascus containing eight haploid ascospores. Many asci form inside a fruiting body called a perithecium (Figure 1).
One type of genetic variability in S. fimicola is the color of the ascospores. Most strains have dark brown, or wild-type, ascospores although there are variants. Certain strains have tan or gray ascospores. A tan ascospore strain mated with the wild-type variety produces a series of perithecia containing asci with four tan and four wild-type ascospores each.
How these ascospores are arranged within the ascus is a direct representation of whether or not crossing over has occurred between the centromere and the site for the gene for ascospore color. If no crossing over has occurred, the ascospores will be arranged in a 4:4 manner. If crossing over has occurred, they will occur in a 2:4:2 or 2:2:2:2 manner.
In the example displaying no crossing over (Figure 2), homologous chromosomes line up in the first stages of meiosis. Two chromatids of one chromosome carry the wild-type gene for spore color, and two chromatids carry the mutant gene for spore color. In meiosis I, two cells, each containing only one type of gene for color, are produced. After segregation of these two cells from meiosis I, meiosis II occurs and produces four cells, each containing the haploid number of chromosomes. The third and final step is a mitotic division that duplicates the four resultant cells from meiosis II. This leaves eight spores, in a 4:4 pattern.
The second example is of meiosis with crossing over (Figure 2). Crossing over has occurred between the gene for spore color and the centromere, interrupting the linear relationship between the two homologous chromosomes. When meiosis I occurs and the homologous chromosomes are separated, each chromosome will contain both genes for spore color. Since each chromosome contains both genes, segregation has not occurred. Meiosis II occurs and the genes for spore color are segregated. The resulting mitotic division creates eight spores arranged in either a 2:2:2:2 or 2:4:2 pattern. Any of these patterns indicates that crossing over has occurred.
By observing the ascospore arrangement, the percentage of asci exhibiting crossover can be determined. This frequency appears to be significantly affected by the distance from the gene to the centromere. From the crossover frequency, the distance in map units from the gene for ascospore color and the chromosome centromere can be calculated.
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Sordaria Genetics Lab Activity - Students will be able to identify genetic differences between diploid and haploid organisms, explain how genetic variability arises from sexual reproduction, and study the roles of mitosis and meiosis. Students will grow wild type and mutant Sordaria strains on mating agar and examine the ascospores under a microscope (available separately). They will then calculate the distance in map units between the gene for ascospore color and the chromosome centromere.- Sordaria Cross Demonstration Plate Living Specimen - For your convenience we have performed a cross between wild type and tan mutant strain.
Sordaria Cultures - Demonstrates crossing over during cell division. Ascospores, the microscopic spores within the fungi’s ascus, exhibit characteristic color patterns based on mutant type and are readily observable under a microscope. The arrangement of ascospores is used to estimate percentage of crossover between the centromere and the gene that controls spore color.
