S

Figure 5.7 Spontaneously shot ascospore octads from a Neurospora crassa cross insertional translocation X normal. (a) All normal (black). (b) Four normal : four defective (white). (c) Six normal : two defective. From Perkins (1974). With permission of Genetics Society of America.

Standard sequence

Chromosome 1

Chromosome 2

Deficiency (Chr. 1)

Insertional translocation

Reciprocal translocation a b c g -□-

Figure 5.8 Common types of chromosomal rearrangements found in N. crassa. From Davis (2000). With permission from the Oxford University Press.

L refer to the right and left arms of the linkage groups. The compound chromosomes have visible genetic markers near the junction point: albino-1 (al, white color), colonial temperature sensitive-1 (cot, button-shaped colony at 34°C) and yellow (yellow color). Segregation of these markers among progeny helps in the detection of the linkage of an unknown gene to any of these markers. The use of this linkage tester facilitates the rapid mapping of new mutations on a particular chromosome, thereby greatly reducing the time required to find the linkage of an unknown gene than if it is to be found by crossing the unknown to each of the seven linkage group tester strains. Consider the example of a mapping mutant senescent strain that ceases to grow in four to five subcultures. A cross of a wild strain with conidia from a senescent strain produced half wild type (+) and half senescent progeny, showing the segregation of a single gene pair according to Mendel's principle and identifying the "death" phenotype to be due to a single gene, called senescent (sen). A single cross of the mutant senescent to alcoy gave an essentially random recombination of sen with al-1 (41%), eliminating linkage groups (LG) I and II from consideration. On the other hand, sen showed 22% recombination with cot-1, indicating that sen is either on LG IV or V. A follow-up cross of sen to a strain having cot-1 marker on the normal chromosome produced parental and recombinant classes in nearly equal frequency (48 and 52%), showing that sen is not on LG IV. From a cross of sen to a strain containing two mutant markers, cycloheximide-2 resistant and albino-3 in LGV, the frequency of recombination between cyh-2 and sen (37%) and between sen and al-3 (11%), sen was positioned between cyh-2 and al-3 (Figure 5.10).

Nearly a thousand genes have been mapped in N. crassa (Perkins et al., 2000). It has the most saturated genetic map of all fungi. The gene maps show that functionally related

Prophase I orientation and crossing over

Ascus Viable: deficient constitution ascospores

Alternate centromeres to same pole

Adjacent centromeres to same pole

Alternate centromeres to same pole

Adjacent centromeres to same pole

Prophase I orientation and crossing over

Alternate centromeres to same pole

Adjacent centromeres to same pole

Alternate centromeres to same pole

Adjacent centromeres to same pole

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