Q.15 Self incompatibility prevents self fertilization and this mechanism involves:
- S₁ – pollen grain from an S₁ S₂ parental sporophyte cannot fertilize eggs of an S₁ S₂ flower
- S₁ – pollen grain from an S₁ S₂ parental sporophytes can fertilize eggs of an S₁ S₂ flower
- S₂ pollen grain can fertilize eggs of S₁ S₂ and S₂ S₃ flower
- S₁ – pollen grain from an S₁ S₂ parental sporophytes can fertilize eggs of an S₁ S₃ flower
Self-incompatibility (SI) in plants prevents self-fertilization through genetic recognition at the S-locus, where matching S-alleles between pollen and pistil block fertilization. The first statement correctly describes this mechanism.
Introduction
Self incompatibility prevents self fertilization by recognizing matching S-alleles in pollen and pistil, promoting genetic diversity in flowering plants. This gametophytic or sporophytic mechanism is key for exams like GATE Life Sciences. This analysis evaluates each option precisely.
Statement Analysis
Each option describes gametophytic self-incompatibility (GSI), where pollen phenotype matches its own haploid genotype (common in Solanaceae, Rosaceae).
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S₁ – pollen grain from an S₁ S₂ parental sporophyte cannot fertilize eggs of an S₁ S₂ flower: Correct. In GSI, S1 pollen (from S1S2 parent) matches S1 in pistil, triggering rejection (pollen tube inhibition via S-RNase). No fertilization occurs.
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S₁ – pollen grain from an S₁ S₂ parental sporophytes can fertilize eggs of an S₁ S₂ flower: Incorrect. This would allow self-fertilization, contradicting SI; matching alleles block it.
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S₂ pollen grain can fertilize eggs of S₁ S₂ and S₂ S₃ flower: Incorrect. S2 pollen rejects S1S2 pistil (S2 match) but fertilizes S2S3 only if no full match—wait, partially tricky, but S2S3 has S2, so S2 pollen rejected by S2S3 too. Cannot fertilize both.
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S₁ – pollen grain from an S₁ S₂ parental sporophytes can fertilize eggs of an S₁ S₃ flower: Correct for compatible (S1 pollen rejects only if pistil has S1; S1S3 lacks matching full block for S1 alone? No—in GSI, S1 pollen fertilizes S1S3 (no S1 in pollen matching pistil’s other? Standard: pollen rejected only if its allele matches pistil’s allele. S1 pollen on S1S3 pistil matches S1, rejected. Actually incorrect. Wait—clarify: pollen rejected if its S matches ANY S in pistil.
Standard GSI rule: Pollen with allele matching either pistil allele is rejected. So S1 pollen from S1S2 on S1S3 pistil matches S1, rejected. On S2S3, no match, accepted. Thus this is incorrect; first is precise.
Mechanism Details
SI operates via S-locus with multiple alleles (S1, S2…).
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Gametophytic SI: Pollen haploid genotype determines fate. S1 pollen rejected by S1_ pistil.
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Sporophytic SI: Diploid parent genotype controls (e.g., Brassica); dominance possible.
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Molecular: Pistil S-RNase degrades pollen RNA; non-self pollen detoxifies.
Rule: Fertilization blocked if pollen shares allele with pistil.
Applications
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Promotes outcrossing, hybrid vigor.
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Breeding: Overcome via mentor pollen, heat, CO2.
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~50% angiosperms; absent in apomicts.
Exam Tip
GATE focus: GSI—pollen rejected on matching S-parent. Visualize Punnett: S1S2 × S1S2 yields 50% incompatible pollen.
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