Abstract Sexual reproduction in organisms is a cause for genetic variation. This can be seen through the process of meiosis in Sordaria fimicola because of the effects of crossing over and independent assortment that occur in meiosis I. Before performing this experiment we hypothesized that we would be able to see that crossing over did indeed occur in the S. fimicola. We tested our hypothesis by growing our own culture and observing it under a microscope. By counting and recording the types of asci we were able to find that our hypothesis had been correct.
This has now led us to know that sexual reproduction causes an increase in genetic variation in organisms such as, Sordaria fimicola. Introduction Organisms reproduce in two ways: asexually and sexually. Sexual reproduction can be defined as the reproduction involving the union of gametes or when genetic material from two parents combine to form offspring (Cyr). Offspring produced from sexual reproduction have a unique genetic make-up, which can either be beneficial or destructive to them (Saleem). Close to all known organisms use this kind of reproduction during some moment in their lifetime.
If this is true, however, why is not every species seemingly genetically identical? To answer this, one must observe the chromosome behavior during the sexual reproduction life cycle also known as meiosis. Meiosis is the process of cell division in which gametes are produced. It results in four haploid (IN) cells from two diploid (2N) cells (Cyr). In meiosis I, crossing over and independent assortment of the chromosomes occur. These two processes increase the genetic variation within the cell either benefiting or hurting the cell’s ability to adapt (natural selection).
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After these two events have occurred, the chromosomes will then proceed through all four stages and produce two daughter cells (Cyr). In meiosis II, the two daughter cells each again proceed through all four stages and produce a final four distinctly different daughter cells (Cyr). Sordaria fimicola allows us to see observe the process of meiosis. This is because its life cycle is rapid and allows scientists to observe many generations in a short amount of time (Meiosis). Also, the size of S. fimicola makes it easily viewable under a microscope.
Experiments with “Evolution Canyon” have shown exactly how S. fimicola is a representative organism for crossing over. Evolution Canyon represents the whole idea of different locations in Israel containing two mountain slopes exposed to vastly different climatic conditions that converge with a valley between them (Meiosis). In each of these cases, one slope has been exposed to harsh conditions while the other has been exposed to temperate conditions (Saleem). Because each slope undergoes different conditions, we can observe how genetic variation is affects the S. imicola that live on each slope. Scientists gathered samples of the organism living on both slopes and analyzed the differences in crossing over and the differences in crossing over frequencies. The purpose of our lab is to observe the different cross over frequencies. We want to compare and contrast the crossover frequencies in different color strains of S. fimicola. We hypothesize that in this experiment we will observe crossing over occur in the Sordaria fimicola after two weeks of growth in the agar plates.
To test our hypothesis, we will grow our own asci spores in an agar plate and observe the organism under the microscope while recording the different ascus types which can either be type A (4:4), type B (2:4:2), or type c (2:2:2:2). Materials and Methods From the ‘Meiosis and Genetic Diversity in Sordaria’ handout, we found the materials and procedure for this experiment. Equipment that we needed to perform the first part of this experiment were two agar plates, a marking pen, a scalpel, and two different color strands of S. imicola. The first step in this lab was to mark two separate agar plates with the marking pen in such a way as to create four separate quadrants on each agar plate. Second, each plate needed to be labeled, identifying it as either the plate with tan fungi or gray fungi. Next, using the scalpel, a small piece of the samples of S. fimicola had to be sliced and placed in the corresponding spots in the agar plates. The plates then were taped and allowed two weeks to grow. After two weeks, the samples had flourished.
We first scooped up small portions of each of our samples and put them on a slide with a drop of water, creating squashes, to be viewed under the microscope. When viewing under the microscope, each of the four group members counted and recorded their own twenty asci. While recording, each member categorized whether the asci they found had represented type A, type B, or type C recombination type. Last, when each member had attained their individual data, we combined our data to create a total data for our entire group. Then we proceeded to combine our data to find a section and a course total.
Results Four separate sectors analyzed the data of this experiment, the individual, the group, the class, and the section. The results were as follows: Cross Over Frequency Percent of Cross Over = (Number of Recombinant Asci / Total Number of Asci ) X 100% Individual Cross Over Frequency = 12/20 X 100% = 60% (gray) Combined Group Cross Over Frequency = 24/40 X 100% = 60% (gray) Combined Group Cross Over Frequency = 28/40 X 100% = 70% (tan) Combined Section Cross Over Frequency = 128/220 X 100% = 58% (gray) Combined Section Cross Over Frequency = 163/260 X 100% = 62. % (tan) Combined Course Cross Over Frequency = 4054/7066 X 100% = 57% (gray) Combined Course Cross Over Frequency = 8277/13946 X 100% = 59% (tan) After examining the cross over frequencies of the four sectors of data, we can see that for both the gray and tan spores an average of 60% were recombinant. This means that on average, 60% of the time S. fimicola will cross over resulting in spores of type B (2:4:2) or type C (2:2:2:2) Map Distances Map Distance from Cross Over = Percent Cross Over / 2 Individual Map Distance = 60%/2 = 30 mu (gray) Combined Group Map Distance = 60%/2 = 30 mu (gray) Combined Group Map Distance = 70%/2 = 35 mu (tan)
Combined Section Map Distance = 58%/2 = 29 mu (gray) Combined Section Map Distance = 62. 6%/2 = 31. 3 mu (tan) Combined Course Map Distance = 57%/2 = 28. 5 mu (gray) Combined Course Map Distance = 59%/2 = 29. 5 mu (tan) Again, the map distances for both the gray and tan spores averaged about 30 mu. This means that on average there were 30 units between the cross over and the centromere of the chromosome. The fact that all of the map distances are around the same number also helps to represent the accuracy in our results. Discussion After observation of the Sordaria fimicola, we were able to depict the different types of crossing over.
This supports our hypothesis– crossing over did occur in the S. fimicola. Because we were able to see that crossing over did occur and examine the three different cross over types, we can now say that sexual reproduction attributes to increases in genetic variation. We can see this by the 60% cross over frequency of the recombinant (type B and type C) spores in both the gray and tan strands. This example of the process of meiosis shows us that independent assortment and crossing over attributes to the variety of offspring that are produced in Sordaria fimicola.
This large variety leads us to know natural selection plays a large role in the life cycle of fungus and especially S. fimicola. Our experiment showed baseline data under the same conditions as ‘Evolution Canyon. ’ While ‘Evolution Canyon’ showed the effects of cross over frequency as the two spore strands adapted to their different environmental conditions, our experiment showed the cross over frequency of S. fimicola under normal conditions. This baseline allowed for the comparison of the strand types that had become adapted to their different environmental conditions on ‘Evolution Canyon. ’
Errors that could have occurred during this experiment were the recounting of asci. There may have been replication of certain asci strands. The experiment, however, is still reliable due to the large number of spores counted. This large number of 7066 gray spores and 13946 tan spores allowed for a reliable average to still be found. Future experiments may find this information useful because it gives insight into the how often crossing over occurs and therefore at what rate genetic variation is happening. By knowing more about genetic variation rate, we can learn more about evolution and how that effects natural selection.
Experimenters could use this information to compare the rate of genetic variation to the effects it has on natural selection.
- Cyr, R. 2002. Heredity and the Life Cycle. In, Biology 110: Basic concepts and biodiverity course website. Department of Biology, The Pennsylvania State University. http://www. bio. psu. edu/ Meiosis and Genetic Diversity in the Model Organism, Sordaria. Written by Hass, C. and Ward, A. 2010.
- Department of Biology, The Pennsylvania State University, University Park, PA. Saleem, Muhammad. 2001.
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