Introduction
For a long period of time filamentous fungi like Sordaria have been employed in the food and pharmaceutical sectors of industry as major producers of recombinant proteins. This use has arisen from many of these filamentous fungi, like Sordaria being given the classification of generally recognized as safe (GRAS), making them suitable candidates for the production of recombinant proteins. (1).
Biology of Sordaria
Sordaria is a member of the group of fungi known as Ascomycetes. A feature of Sodaria is the presence of branched filaments that are haploid. Sexual reproduction in Sodaria occurs between the adjacent hyphae. During this sexual reproduction the nuclei in one hypha migrate into the adjacent hypha and fuse in pairs. Every diploid nucleus that is the consequence of this fusion generates a perithecium that is made up of a group of spore-producing infrastructure that are termed as asci. (2).
To start with every ascus contains a single diploid nucleus. This single diploid nucleus goes through three repeated divisions, in which the initial two divisions are meiotic and may be termed as meiotic I and meiotic II, while the third division is mitotic. The consequence of these three repeated divisions is that the ascus now has eight haploid spores, which are known as ascospores. Inside the ascus the eight acospores are so arranged to make up a linear sequence that reflects the order in which they were formed. This means that the spores containing two nuclei as a result of any particular nuclear division can be seen adjacent to one another.
The color of the acosphores is not the same and is varied, where some of the acosphores are seen as black, while the others appear as white. This color variance is because of the difference in genetic constitution. The white spores have an allele which can be termed as W, which is different from the allele that is possessed by the black spores and termed as B. (2).
Recombination Frequency
Linked genes are known to demonstrate some frequency of recombination that ranges from almost nil to a maximum of fifty percent and is dependent on the particular genes involved. This recombination that is seen in linked genes can be explained as the result of the crossing over between chromatids that occurs during the diplotene stage of meiosis. This makes it possible that the difference between the alleles segregating at the time of meiosis becomes the marker for the gene and also the chromosome locus where it is found. (3).
In the Sordaria species it is seen that the tetrads demonstrating recombination between two linked markers are normally tetratypes, which means that they are made up of the two parental and the two reciprocally constituted recombinant types. Tetrads that have all the four spores as recombinant are seen at a low frequency and this can be explained due to the double crossing over of all the four chromatids. Better understanding of the nature of crossover is obtained by analysing the tetrads that segregate at three linked loci, which make up the two chromosome intervals. When there is a crossover at each interval, the participation of a chromatid in one crossover does not hinder the chances of involvement in a second crossover.
The usual proportion in which doubles wherein the second crossover occurs as a result of the same two chromatids as the first crossover; the two chromatids that did not participate in the previous crossover; and one same and one different occur is 1:1:2. (3).
This random participation of chromatids in successive crossovers makes it possible to explain the maximum fifty percent recombination occurring between distant genes. When one chiasma is found to be formed always in one specific interval the consequence will be fifty percent recombination between the ends. When a second chiasma is formed, the chances that a chromatid that had crossed over at first remains the same for it not to cross back again, which means that any crossover occurring in excess of one will normally on an average cancel out as many recombinants as the crossover makes.
Chiasmata and the crossovers that are linked to these chiasmata are not the same and vary in position in meiotic cells. At times two linked markers may be recombined and at other times they may not be. The chances of recombination occurring is dependent on the distance between them. This makes it possible for the recombination frequency to be used as the basis for mapping. The usual standard employed as a mapping unit is one percent recombination, which is also known as a centimorgan.
Recombination frequency can be considered as an additive measure of map distance in the limited frame of it being representative of the total crossover frequency for the interval being evaluated. Thus this remains true only in the case of the short intervals, where more than one crossover is virtually nonexistent. In the case of longer intervals, where the chances of double or multiple crossover are high recombination frequency tends to underestimate crossover frequency, as two or more crossovers on an average produce the same proportion of recombinant meiotic products. (3).
Works Cited
- Kuck, Ulridge & Poggler, Stephanie. “Sordaria macrospora”. Production of Recombinant Proteins: Novel Microbial and Eukaryotic Expression Systems. Ed. Gerd Gelissen. Weinheim, Germany: Wiley-VCH, 2005. 215-230.
- Roberts, M. B. V. & King, T. J. Biology a Functional Approach: Students’ Manual. Second Edition. Thomas Nelson and Sons Ltd., Cheltenham, U.K, 1987.
- Fincham, J. R. S. Genetic Analysis: principle scope and objective. Blackwell Science Limited, Oxford, UK, 1994.