Neurospora Tetrad Analysis Confirms that Recombination occurs at the Four Strand Stage

1. Neurospora Tetrad Analysis
Confirms that Recombination
occurs at the Four Strand Stage
by
Dr.K.Raveendra Retnam
Assistant Professor of Botany
Sethupathy Government Arts College
Ramanathapuram - 623 502

2. Introduction
Neurospora is a genus of Ascomycetes fungi
In Ascomycetes each meiotic tetrad is formed with in a sac like structure
called Ascus. Each spore is called ascospore.
Neurospora is useful for genetic analysis Because
 It has a haploid life cycle. Therefore Genotype is directly expressed in
Phenotype that makes genetic analysis simple
 They are produced in large numbers and therefore it is easy to detect
variations.
 Its life cycle is very short
 Zygote is the only diploid stage and undergoes meiosis soon after it is
formed resulting into haploid ascospores.

3. • Neurospora an ideal organism for studying mutation,
chromosomal rearrangements, and recombination .
• Neurospora cannot self fertilize
• Asexual spores can develop again into multicellular
hyphae, completing the cycle.
• Asexual spores can also function as male gametes in
the sexual reproductive cycle.
• Zygote is the only diploid cell in its life cycle. After
meiosis, each haploid cell undergoes another mitosis,
producing eight haploid ascospores in the ascus (an
octad)
• The ordered arrangement of spores in asci reflects the
geometry of the meiotic and mitotic spindles.
• In the absence of Crossing over, ascospores are
arranged linearly in the form of 4 : 4 Pattern.

4. Life Cycle of Saccharomyces cerevisia
The products of meiosis i.e. four
haploid ascospores are arranged
randomly in unordered manner
Yeast cells can grow vegetatively
either as haploids or diploids.

5. Zygote is the only diploid cell in its life cycle. After meiosis, each haploid cell undergoes
another mitosis, producing eight haploid ascospores in the ascus (an octad) The ordered
arrangement of spores in asci reflects the geometry of the meiotic and mitotic spindles.
Life Cycle of Neurospora crassa

6. In yeast, each tetrad is unordered; that is, the four meiotic products, known as spores, are
arranged at random within the ascus.
But In Neurospora crassa, Ascus is very narrow and linear. Therefore the cells cannot slide
around each other and each tetrad is ordered, with the four pairs, or eight haploid spores.

7. When crossover does not takes place
Two Spore patterns are Possible
First Pattern is

8. When crossover does not
takes place the Second
Pattern is

9. Two Spore patterns are
50% 50%

10. When crossover takes place Four Patterns of Spores are possible
They are
25% 25% 25% 25%

15. Conclusion
Tetrad Analysis Confirms that Recombination
occurs at the Four Strand Stage
Thank you