During ascus formation in Neurospora, any ascospore with a chromosomal deletion

Question

During ascus formation in Neurospora, any ascospore with a chromosomal deletion dies and appears white in color. How many of the eight ascospores in the ascus would be white if the octad came from a cross of a wild-type strain with a strain of the opposite mating type carrying each of the following. Briefly explain your answer.

a. a paracentric inversion, and no crossovers occurred between normal and inverted chromosomes?






b. a pericentric inversion, and a single crossover occurred in the inversion loop?






c. a paracentric inversion, and a single crossover occurred outside the inversion loop?






d. a reciprocal translocation, and an adjacent-1 segregation occurred with no crossovers between translocated chromosomes?






e. A reciprocal translocation, and alternate segregation occurred with no crossovers between translocated chromosomes?

Explanation / Answer

A) A paracentric inversion is when the inversion takes place in chromosomal arms and doesn't involve centromere. In the given case a WT is crossed to a paracentric inversion, there will be eight ascospores formed. All the eight will be viable as no-crossing over has occurred, so no deletion. Four will be WT and other four ascospores will be having a paracentric inversion.

B) A pericentric inversion involves centromere in the inverted region. Given here is a pericentric inversion crossed with a WT. During meiosis, one cross-over have occurred in the inverted region. Due to single cross-over, the arms will be exchanged. Spores formed will be

Normal WT- chromatid where crossing over has not occurred = 2 in number = viable

Paracentric mutant, from chromatid that did not participate in crossing over = 2 in number = viable

Duplicated and deleted, from two chromatids where crossing over took place and arms are exchanged = 4 in number = inviable

C) A crossing over outside the inversion will yield- 4 WT ascospores and 4 paracentric inversion ascospore= all viable.

D) A reciprocal translocation involving one chromatid from each chromosome will produce 8 ascospores. Each of these chromatids with reciprocal translocation will have duplication and deletion of some gene. In this case, since adjacent chromatids are involved in translocation, central four ascospores will die while other the two on either end will survive. The four ascospores surviving will have normal WT chromosomal gene order with no reciprocal translocation. Only ascospores with reciprocal translocation will have deletions and duplications, so will be inviable.

E) Reciprocal translocation with one sister chromatid involved in exchange is alternate chromatids on two chromosomes. So, four ascospores will die but these will be alternating with two viable WT ascospores.

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