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Sordaria Fimicola’s Ascuses
Námsbraut: Raunvísindadeild; Líffræðiskor
Námskeið: 09.51.35 Erfðafræði
Nafn kennara: Ólafur S. Andrésson, Zophonías O. Jónsson, Sigríður H. Þorbjarnardóttir og
Bryndís K. Gísladóttir.
Vikudagur, hópur: Fimmtudagur, síðari kennslustund, Hópur 2 og 4.
Tilraun framkvæmd: 30. október og 06. nóvember, 2003
Skýrsluskil til kennara: 13. nóvember, 2003
Skýrsluskil til stúdenta:
Einkunn:
Nöfn stúdenta: Bjarki Steinn Traustason, Egill Guðmundsson, J. Gabriel-Rios Kristjánsson,
Marcella Manerba og Nicoletta Palmegiani.
________________________________
Bjarki Steinn Traustason
________________________________
J. Gabriel-Rios Kristjánsson
________________________________
Egill Guðmundsson
________________________________
Marcella Manerba
________________________________
Nicoletta Palmegiani
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7
Erfðafræði
Sordaria Fimicola’s Ascuses
Introduction:
Sordaria fimicola is an ascomycetes related to Neurospora. Sordaria fimicola’s asuses,
where meiosis progresses in, are examined, but they are many together in co called
peritheca. When meiosis has concluded, the total of 4 spores have developed, but each
nucleus, subsequently, undergoes mitosis, meaning, there is the total of 8 spores in
each ascus, where two and two spores are identical if no mutation or gene conversion
has occured.
Two Sordaria-strains, one with dark spores (d+), and the other with light
spores (d−), are cultured together on the same petri plate with mais agar. Specimens,
perithecas, where obtained form where the tvo cultures meet, peripheral, on the petri
plate, wherein one can assume that some of spores have intermixed. When separation
in former cell division has occured, 4 dark spores are in the one half of the ascus, and
4 light spores are in the other (FIGURE 01 ■). But when separation in latter division
takes place, because of interconversion between the d-gene and the centromere,
identical spores pair, leading to one dark pair and one light pair located alternately in
the ascus (FIGURE 01 ■).
There are exception to these form of ascuses. One example is when ascuses
contain equal number of dark spores end light spores, but the order of the spores
indicates that separation has occured after meiosis. Also, ascuses can form, where the
number of dark and light spores is unequal. The most prevalent ratio is then, 5d+:3d−,
3d+:5d−, 6d+:2d−, 2d+:6d−.
One can explane these irregluar ascuses with the formation of heteroduplexes
and during recombination, and with restoration because of heteroduplexes.
FIGURE 01, DIFFERENT ORDER OF DARK (d+) AND LIGHT (d–) SPORES:





4d :4d

4d :4d


4d :4d 4d :4d
AA







4d :4d

4d :4d

4d :4d

4d :4d


4d :4d 4d :4d 4d :4d 4d :4d
BB
CC
A. Separation
after division I., B. Schematic drawing from a microscope, C. Separa-tion after division II.
□
Aims/Hypothesis:
The Aim of this experiment, is to determine the distance between the d-gene and the
centromere in the chromosome. Here, interpreting the distance in units of recombination, RU.
Design and Methods:
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Reference to work sheets in manual booklet, for present exercise (p.29-30).
Results:
FIGURE 02, DIFFERENT TYPES OF ASCUSES:
Type I

4d :4d
Type II


4d :4d
Type III



4d :4d
Type IV

4d :4d

Type V

4d :4d
Type VI


4d :4d

Information for this figure is interpreted in TABLE 01.
□
TABLE 01, NUMBER FOR EACH TYPE OF ASCUS, GIVEN RESULTS:
Number
Type I
Type II
Number
52
60
25
18
20
25
Type III
Type IV
Type V
Type VI
TABLE 02, THE NUMBER OF ASCUSES
Separation I.
Separation II.
Number §
Σ
52 + 60
18 + 25 + 20 +25
112
88
dcentromere to d-gene = 22 RU *
These numbers are given data, because the disired separation didn’t occur.
*Formula used for the calculationsn of d, is described in Formula 01.
§
The distance between the centromere and the d-gene is 22 RU.
FORMULA 01; DISTANCE BETWEEN THE CENTROMERE AND THE d-GENE:
General Formula:
d = ((ntotal,II. + (ntotal,I. + ntotal,II.))/2) ∙ 100
d : distance between the centromere and the d-gene [RU]
[RU] : units of recombination
ntotal,I. : the total number of ascuses after separation I.
ntotal,II. : the total number of ascuses after separation II.
Eg,:
dcentromere tod- gene = ((88 + (112 + 88))/2) ∙ 100 = 22 RU
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Conclusion/Discussion:
FIGURE 03, EXAMPLES OF IRREGULAR ASCUSES:
4d:4d 4d:4d
5d:3d 3d:5d 6d:2d 2d:6d
A
B
C
A. Segregation has occured after meiosis (PMS)., B. Result from gene conversion., C. Schematic drawing from a
microscope.
□
Looking at FIGURE 03 ■, one can see some examples of mutated ascuses, called
irregular ascuses. Occasionally heteroduplexes are formed according to Hollidays
model of recombination. They are DNA-helices hybrids that contain, in this instance,
the d+- and d−- gene.
When irregular ascuses have 4 dark spores and 4 light spores, 4d+:4d−, then no
additional chanegs have occured in the heteroduplexes formed. Therewithal,
segregation has occured after meiosis (PMS, post meiotic segregation), as
interconverion between the d-genes has taken place. When the ratio of d+ to d− is
unequal, ie, 5d+:3d−, 3d+:5d−, 6d+:2d−, or, 2d+:6d−, it is accounted for as gene
conversion, taking place after the heteroduplexes has been repaired. Gene conversion
is a process which is an exception from the Mendel’s Law.
■
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