Lecture 16: Fungal systems for studying recombination

(version 22 September 1999)

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since 20 July 1999

Fungi have special structures for examining recombination

Looking at the products for a single meiosis

The final products from a single meiosis together in an ascus, which is either a tetrad (4 spores) or an octad (8 spores).

linear and unordered

Centromere mapping

Because of the linear segregation in linear tetrad/octads we can use single markers to map distance from centromere

Key is first- versus second -division segregation

In 1st division segregation, the two different alleles segregate at the 1st meiotic division

In 2nd division segreation, the two different alleles segregate at the 2nd meiotic division

Centromere mapping

Key is that half of all gametes from 2nd div seg are recombinant

Recombination fraction between marker and centromere

= (1/2)*% (2nd division segregation)

Upper limit on centromere mapping

The precent 2nd division segregation reaches a limiting value of 2/3. Why?

Mapping with unordered tetrads

Cross AB/ab

three types of spores (no order)

Parental ditypes (PD) AB, AB, ab, ab

Nonparental ditypes (NPD) Ab, Ab, aB, aB

Tetratypes (T) AB, ab, Ab, aB

PD, NPD, T

parental ditypes -- all four gametes are parental

nonparental ditypes -- all four gametes are recombinant

Tetratypes -- two parental, two recombinant

What is recombination frequency?

Example

Putting the pieces together: mapping two loci and the centromere

Note that the parental genome is t + / +m, as this is the most common tetrad.

1. Consider the recombination frequency between the t and m loci.

   Tetrad	t + t + + m + m	t + t m + + + m	t + + m t + + m	t + + + t m + m	t m t m + + + +	t m + + t + + m
   Number	260	76	4	54	1	5
   Type	PD	T	PD	T	NPD	T

   Hence, the t-m recombination frequency is (1/2) Freq(T) + Freq(NPD) = (1/2)(76+54+5)/400 + 1/400 = 0.17

2. Now consider the t-centromere distance

   Tetrad	t + t + + m + m	t + t m + + + m	t + + m t + + m	t + + + t m + m	t m t m + + + +	t m + + t + + m
   Number	260	76	4	54	1	5
   Division type for t locus	1st	1st	2nd	2nd	1st	2nd

   Hence, frequency 2nd divsion segregation = (4+54+5)/400 = 0.1575, giving the t-centromere distance as (1/2)*0.1575 = 0.079

3. Now consider the m-centromere distance

   Tetrad	t + t + + m + m	t + t m + + + m	t + + m t + + m	t + + + t m + m	t m t m + + + +	t m + + t + + m
   Number	260	76	4	54	1	5
   Division type for m locus	1st	2nd	2nd	1st	1st	2nd

   Hence, frequency 2nd division segregation = (76+4+5)/400 = 0.213, giving the t-centromere distance as (1/2)*0.23 = 0.106

   

   What type of tetrad expected from a double-crossover?

   Crossovers can involve 2, 3 or all 4 strands. To illustrate the idea, here are particular 2- and 3-strand double crossovers and the resulting tetrads.

   

   One four-strand double