Apomixis and Polyembryony
Apomixis and polyembryony are special reproductive mechanisms in plants that deviate from the normal sexual reproduction process but help in maintaining and propagating desirable traits.
1. Apomixis
Apomixis is a type of asexual reproduction in plants where seeds are formed without fertilization. The embryo develops without the fusion of gametes, leading to offspring that are genetically identical to the parent.
Types of Apomixis:
Agnospermy – Embryo forms from diploid sporophytic cells of the ovule instead of the zygote (e.g., Citrus, Mango).
Parthenogenesis – Embryo forms directly from an unfertilized egg cell (e.g., Apple, Pear).
Apogamy – Embryo develops from synergid or antipodal cells instead of the egg (e.g., Alchemilla).
Adventive Embryony – Embryo arises from nucellar or integumentary tissues (e.g., Citrus, Mango).
Significance of Apomixis:
Produces genetically identical (clonal) offspring.
Helps in maintaining hybrid vigor (heterosis) in crops.
Saves time and energy as fertilization is bypassed.
Diagram: Types of Apomixis in Plants
(Illustration showing parthenogenesis, apogamy, and adventive embryony.)
2. Polyembryony
Polyembryony is the phenomenon where more than one embryo develops from a single seed. This can occur due to the development of multiple embryos within an ovule.
Types of Polyembryony:
True Polyembryony – Multiple embryos arise from fertilized egg cells (e.g., Orchids).
Simple Polyembryony – Extra embryos develop from other cells of the embryo sac (e.g., Citrus).
Adventive Polyembryony – Embryos develop from nucellar or integumentary tissues (e.g., Mango).
Cleavage Polyembryony – A single fertilized egg splits into multiple embryos (e.g., Gymnosperms like Pinus).
Significance of Polyembryony:
Ensures higher survival rates for plants.
Used in commercial plant propagation.
Helps in maintaining genetic uniformity.
Diagram: Types of Polyembryony in Plants
(Illustration showing true, simple, adventive, and cleavage polyembryony.)
Conclusion
Both apomixis and polyembryony play crucial roles in plant reproduction and agriculture. Apomixis ensures clonal propagation, while polyembryony enhances seed viability, making them valuable in crop improvement and hybrid seed production.