20. Following are certain statements regarding apomixis in plants:
A. Apomixis cannot be used to maintain hybrid vigor over many generations in plants.
B. In sporophytic apomixis maternal genotype is maintained.
C. There is an event of meiosis during gametophytic apomixis and is also referred as apomeiosis.
D. In diplospory, meiosis of the megaspore mother cell is aborted, resulting in two unreduced spores, out of which one forms the female gametophyte.
Which one of the following combinations is correct?
(1) A and B                                                    (2) A and C
(3) B and C                                                    (4) B and D

 

The correct combination of statements regarding apomixis in plants is:

(4) B and D

Explanation:

• (B) In sporophytic apomixis, the maternal genotype is maintained — This is correct. Sporophytic apomixis (adventive embryony) produces embryos directly from somatic cells of the sporophyte, bypassing genetic recombination and maintaining the maternal genotype.

• (D) In diplospory, meiosis of the megaspore mother cell is aborted, resulting in two unreduced spores, out of which one forms the female gametophyte — This is correct. Diplospory involves bypassing normal meiosis (apomeiosis), leading to unreduced spores that give rise to a diploid embryo sac.

• (A) Apomixis cannot be used to maintain hybrid vigor — Incorrect. Apomixis, especially synthetic apomixis, has potential to fix and maintain hybrid vigor over multiple generations by clonal seed formation.

• (C) There is an event of meiosis during gametophytic apomixis and is also referred to as apomeiosis — Incorrect. Apomeiosis refers to the bypass or alteration of meiosis, not the occurrence of meiosis.

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Introduction

Apomixis is an asexual mode of seed formation that allows certain plants to bypass sexual reproduction mechanisms like meiosis and fertilization. This process is categorized into types such as sporophytic and gametophytic apomixis, with diplospory and apospory as subtypes of gametophytic apomixis. Understanding the specifics of these processes is crucial for their potential exploitation in agriculture, particularly for fixing hybrid vigor.

Sporophytic Apomixis (Adventive Embryony)

• In sporophytic apomixis, embryos develop directly from somatic cells of the maternal sporophyte such as nucellus or integuments without fertilization.

• This leads to clonal propagation, maintaining the genetic identity of the maternal plant across generations.

• This type of apomixis is observed in several plant families and is of significant interest for crop improvement.

Diplospory and Apomeiosis in Gametophytic Apomixis

• Diplospory involves development of the embryo sac from an unreduced megaspore mother cell due to aborted or modified meiosis known as apomeiosis.

• This results in the formation of unreduced spores that preserve the maternal genotype.

• It allows the generation of embryos genetically identical to the mother plant without sexual recombination.

Misconceptions about Hybrid Vigor and Meiosis

• Contrary to statement (A), apomixis provides a strategy to maintain hybrid vigor or heterosis in crops by producing genetically identical seeds from superior hybrids.

• Statement (C) incorrectly suggests meiosis occurs during gametophytic apomixis, but apomeiosis is the suppression or alteration of meiosis resulting in unreduced spores.

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Summary Table: Apomixis Statements Validity

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Conclusion

The correct valid statements about apomixis in plants are (B) sporophytic apomixis maintains maternal genotype and (D) diplospory involves abortion of meiosis resulting in unreduced spores.