Q.59 Cybrids may arise through the
(A) fusion of a normal protoplast with another nucleated protoplast of different origin
(B) fusion between a normal protoplast and a protoplast containing viable nucleus
(C) elimination of one of the nuclei from heterokaryon formed from two protoplasts of
different origin
(D) fusion of a normal cell with another nucleated cell
Cybrids arise through the elimination of one nucleus from a heterokaryon formed by fusing protoplasts of different origins. This process is key in plant biotechnology for creating cytoplasmic hybrids. The correct answer is option (C).
Option Analysis
Cybrids, or cytoplasmic hybrids, form when protoplasts (wall-less cells) fuse, creating a heterokaryon with two nuclei and mixed cytoplasm, followed by loss of one nucleus. This retains the nucleus from one parent and cytoplasm (including organelles like mitochondria and chloroplasts) from both.
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(A) Fusion of a normal protoplast with another nucleated protoplast of different origin: This describes initial protoplast fusion leading to a heterokaryon, but not cybrid formation, as both nuclei remain initially, potentially forming a somatic hybrid instead.
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(B) Fusion between a normal protoplast and a protoplast containing viable nucleus: Both protoplasts have viable nuclei, so this again forms a heterokaryon without specifying nucleus elimination, which is required for cybrids.
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(C) Elimination of one of the nuclei from heterokaryon formed from two protoplasts of different origin: Correct. After fusion of two nucleated protoplasts from different species, one nucleus is selectively eliminated (e.g., via irradiation or sorting-out), yielding a cybrid with one nucleus and mixed cytoplasm.
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(D) Fusion of a normal cell with another nucleated cell: Incorrect, as intact cells with walls do not fuse easily; cybridization requires protoplasts.
Cybrids in protoplast fusion represent a breakthrough in plant biotechnology, enabling the creation of cytoplasmic hybrids without full nuclear mixing. Cybrids arise through the elimination of one of the nuclei from heterokaryon formed from two protoplasts of different origin, making this essential knowledge for CSIR NET Life Sciences aspirants tackling questions like Q.59.
Cybrid Formation Process
Protoplasts from two species are isolated, fused (using PEG or electrofusion) to form a heterokaryon—a binucleate cell with mixed cytoplasm. One nucleus is then eliminated via methods like X-ray irradiation of the donor or natural sorting-out, resulting in a cybrid: nucleus from one parent, cytoplasm from both.
Key steps include:
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Protoplast isolation using enzymes.
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Fusion induction.
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Selection and regeneration into plants.
Applications in Biotechnology
Cybrids transfer cytoplasmic male sterility (CMS) for hybrid seed production, disease resistance, or herbicide tolerance across species barriers. Examples include Solanum cybrids for potato improvement.
CSIR NET Relevance
This MCQ tests somatic hybridization concepts from Unit 5 (Cell Communication and Signaling) or Unit 9 (Biotechnology). Focus on distinguishing cybrids from somatic hybrids.
