8 min readConcept sheet1-mark MCQ and assertion-reason, and a 2 or 3-mark question on why apomixis matters to the hybrid seed industry; also a NEET favourite

Special modes: apomixis, parthenocarpy and polyembryony

Apomixis forms seeds without fertilisation; parthenocarpy forms seedless fruits without fertilisation. Polyembryony means more than one embryo in a seed, as in citrus and mango.

ApomixisFormation of seeds without fertilisation; an asexual process that mimics sexual reproduction (some Asteraceae and grasses)

ParthenocarpyFormation of fruit without fertilisation; the fruit is seedless, e.g. banana; can be induced by growth hormones

PolyembryonyOccurrence of more than one embryo in a seed, e.g. many varieties of Citrus and mango

Nucellar embryoAn embryo formed from a diploid nucellar cell that grows into the embryo sac; it is a copy of the mother plant

Two ways apomictic seeds form

Diploid egg without fertilisationThe egg cell forms without reduction division, so it is diploid. It develops into an embryo without fertilisation.

Nucellar embryosMore often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac start dividing. They push into the embryo sac and grow into embryos. One ovule then holds many embryos.

No male gamete is involved, so these embryos are genetically identical to the mother plant.

Apomixis, parthenocarpy and polyembryony compared

Feature	Apomixis	Parthenocarpy	Polyembryony
What it is	Seed formed without fertilisation	Fruit formed without fertilisation	More than one embryo in one seed
Are there seeds?	Yes	No, the fruit is seedless	Yes
Examples	Some species of Asteraceae and grasses; Citrus, mango	Banana; can be induced with growth hormones	Citrus (for example orange), mango

Why apomixis matters to the hybrid seed industry

1
Hybrid varieties of many food and vegetable crops give high yields and are widely grown.
2
But if seeds collected from a hybrid crop are sown, the plants segregate: the hybrid characters are lost in the next generation.
3
So farmers must buy new hybrid seed every year. Hybrid seed is costly to produce, so it is expensive for farmers.
4
If a hybrid is made apomictic, its seeds form without fertilisation, so there is no segregation of characters in the progeny.
5★
Farmers could then save seed from their own hybrid crop and sow it year after year. Scientists are studying the genetics of apomixis to transfer apomictic genes into hybrid varieties.

★ key step

Activity: seeing polyembryony in an orange seed

1
Take a few seeds of an orange and soak them in water for a few hours.
2
Peel off the seed coat and gently squeeze each seed between your fingers.
3★
Observe: several embryos of different sizes and shapes come out of a single seed. Count and record them.
4
Conclude: one seed has more than one embryo, so the orange shows polyembryony. Most of the extra embryos are nucellar embryos.

★ key step

In real life

The banana in your lunch box has no seeds because its fruit develops without fertilisation. Seedlings grown from the nucellar embryos of citrus are true copies of the mother plant, which helps nurseries keep a variety unchanged.

Exam angle

3 marks: “What is apomixis? Explain its importance in the hybrid seed industry.” Seed formation without fertilisation, e.g. some Asteraceae and grasses. Hybrid seed must be bought every year because hybrid characters segregate in the next generation. Apomictic hybrids would show no segregation, so farmers could reuse their own seed.

2 marks: “Differentiate between parthenocarpy and apomixis.” Parthenocarpy: fruit forms without fertilisation and is seedless (banana). Apomixis: seed with an embryo forms without fertilisation (some grasses, Citrus).

1 marks: “Name one fruit that develops without fertilisation.” Banana.

Remember it

Apomixis: seed apart from sex. Parthenocarpy: 'virgin fruit', no seeds. Polyembryony: many embryos.