Q.44 Determine the correctness or otherwise of the following Assertion (a) and the Reason (r)
Assertion (a):
Cytoplasmic male sterility (CMS) is invariably due to defect(s) in mitochondrial function.
Reason (r):
CMS can be overcome by pollinating a fertility restoring (Rf) plant with pollen from a non-CMS plant.
Options:
(A) Both (a) and (r) are true and (r) is the correct reason for (a)
(B) Both (a) and (r) are true and (r) is not the correct reason for (a)
(C) (a) is false but (r) is true
(D) (a) is true but (r) is false
Correct Answer: (C) (a) is false but (r) is true.
CMS is maternally inherited via mitochondrial DNA mutations creating chimeric ORFs that disrupt pollen development. However, not all CMS cases stem from mitochondrial defects—some involve nuclear-mitochondrial interactions or other cytoplasmic factors, making (a) false. Reason (r) is true: Rf genes (nuclear) restore fertility in CMS lines via pollen from restorer plants, but this doesn’t cause CMS or link directly to (a).
Core Concepts: CMS and Fertility Restoration
CMS prevents self-pollination in crops, enabling hybrid vigor (heterosis). It arises from mitochondrial genome rearrangements producing novel proteins (e.g., ORFH79 in rice) that impair tapetum function in anthers.
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Standard restoration: Nuclear Rf genes (e.g., Rf1, Rf2 in maize) suppress CMS effects by altering mitochondrial transcripts or proteins—no pollination directionality required.
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Why (r) holds: Crossing CMS female (sterile) with Rf-male (restorer) pollen yields fertile hybrids; reverse cross fails.
This system powers 90%+ of commercial maize hybrids.
Detailed Option Breakdowns
Option (A): Both true, (r) correct reason for (a)
Incorrect. (a) overstates “invariably”—CMS is typically mitochondrial but exceptions exist (e.g., nuclear modifiers in some wheat lines). (r) doesn’t explain CMS causation; Rf restores despite CMS, not because of it.
Option (B): Both true, (r) not correct reason
Incorrect. Fails because (a) is false. While Rf mechanisms are diverse (e.g., RNA cleavage by Rf proteins), they don’t universally tie to mitochondrial “defects” alone.
Option (C): (a) false, (r) true
Correct. CMS is cytoplasmic (mitochondrial DNA), but “invariably mitochondrial function defects” ignores cases like chloroplast-influenced sterility or epigenetic factors. (r) accurately describes standard breeding: Rf pollen on CMS plants restores fertility via gametophytic/sporophytic mechanisms.
Option (D): (a) true, (r) false
Incorrect. (r) is true—Rf lines pollinate CMS to produce F1 restorers. Misreading (r) as “Rf plant as female” trips students; it’s pollen donor.
| Option | Assertion (a) | Reason (r) | Causal Link? | Why Wrong/Correct |
|---|---|---|---|---|
| (A) | True | True | Yes | (a) false |
| (B) | True | True | No | (a) false |
| (C) | False | True | N/A | Matches facts |
| (D) | True | False | N/A | (r) true |
Exam Tips for Plant Genetics
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Mnemonics: CMS = “C”ytoplasmic (mtDNA), Rf = nuclear “Restorers.”
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Breeding Application: CMS × Rf → Fertile F1; ideal for bioinformatics sims (e.g., sequence ORFs via BLAST).
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Common Trap: “Invariably” flags absolutes—question them.
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Advanced: Rf acts via PPR proteins processing mt-transcripts.
Master this for molecular breeding interviews or CSIR-NET. References draw from plant genetics reviews like those in PMC and Nature.
