Apomixis and Polyembryony in Plants Explained Simply

Apomixis is a type of asexual reproduction that occurs through seeds without fertilisation. The embryo in the seed develops from a diploid cell of the ovule, so the seedling is genetically identical to the mother plant. Polyembryony is the occurrence of more than one embryo in a single seed.

These phenomena matter in agriculture because apomixis could fix hybrid vigour permanently. If a high yielding hybrid could produce seeds by apomixis, farmers would not need to buy fresh hybrid seed every year. Polyembryony is common in citrus and mango.

Key Concepts

Apomixis: Seed formation without meiosis and fertilisation, producing clonal seeds.

Hybrid vigour: Superior performance of a hybrid compared to its parents, normally lost in the next seed generation.

Polyembryony: Presence of two or more embryos in one seed.

Nucellar embryos: Extra embryos formed from nucellus tissue around the embryo sac in citrus.

Diploid: Having two sets of chromosomes, represented as 2n. Apomictic embryos remain diploid like the mother.

Adventive embryony: Embryo arising directly from nucellar or integument cells.

How It Works

1. In normal sexual reproduction, meiosis makes haploid egg and fertilisation restores diploid embryo.
2. In apomixis, meiosis is bypassed or an unreduced egg develops without fertilisation.
3. Alternatively, somatic cells of the nucellus start dividing and form embryos.
4. These embryos use the seed coat and sometimes endosperm to develop as normal seeds.
5. In polyembryony, one embryo may be sexual and others are nucellar clones.
6. On germination, multiple seedlings can emerge from a single seed.

Worked Example

A citrus seed is sown and produces 4 seedlings. One seedling is weak and different, likely the sexual embryo, while 3 are vigorous and identical to the mother. Percentage of clonal seedlings is 3 divided by 4 multiplied by 100 equals 75 percent. If a farmer sows 200 such seeds and each gives an average of 3 clonal seedlings, total clonal plants expected are 200 multiplied by 3 equals 600 plants. These can be used as uniform rootstocks.

Common Misconceptions

• Apomixis is the same as vegetative propagation. Apomixis uses seeds as the unit of propagation, though genetics is clonal.
• Polyembryony means all embryos are identical. One may be sexual and variable, while nucellar ones are clones.
• Apomictic seeds are sterile. They germinate normally and produce fertile plants identical to the mother.

Real-World Applications

Citrus nurseries use nucellar seedlings to get uniform, virus free rootstocks. Some grasses reproduce by apomixis and maintain forage quality. Plant breeders are trying to introduce apomixis into rice and maize to preserve hybrid vigour. Mango varieties like Olour show polyembryony and give multiple seedlings per stone.

Apomixis turns seeds into clones, and polyembryony packs several embryos into one seed. Both challenge the idea that seeds always mean genetic variation, and both offer tools for uniform crop production.