Apomixis and polyembryony

What is apomixis?

In plain words: an apomictic embryo develops without receiving a paternal genome. In the clonal routes described here, the maternal chromosome complement is retained. Beyond the NCERT text: most apomicts still need pollination and fertilisation of the central cell to form endosperm, called pseudogamy; a few form endosperm without it (Mahlandt, Singh and Mercier, Theoretical and Applied Genetics, 2023). So NCERT’s “seeds without fertilisation” (page 22) refers to the embryo (our reading).

Do not mix it up with parthenocarpy, which NCERT’s own question points to (“What is fruit production without fertilisation called?”, page 22): parthenocarpy is a fruit without fertilisation, and the fruit is seedless (page 21); apomixis is a seed without fertilisation.

How apomictic seeds develop

NCERT describes two of the several ways in which apomictic seeds develop (pages 22–23):

1. In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.
2. More often, as in many Citrus and mango varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into embryos. In such species each ovule contains many embryos.

In the normal route, the megaspore mother cell divides by meiosis, so the egg is haploid and the zygote formed at fertilisation is diploid (NCERT pages 10 and 18). For the diploid examples shown, both apomictic routes give an embryo from a diploid maternal cell: an egg that skipped reduction division, or a cell of the nucellus.

Polyembryony

Occurrence of more than one embryo in a seed is referred to as polyembryony.

NCERT Biology (Class 12), Section 1.5, page 23

• Polyembryony follows from the second route: when several nucellar cells grow into embryos, one seed can hold several embryos (pages 22–23). Polyembryony describes the number of embryos, not necessarily their origin: a sexual embryo may coexist with nucellar embryos, so not every embryo in such a seed must be a maternal clone.
• Try it (page 23): take out some seeds of orange and squeeze them. In a polyembryonic variety, some seeds may contain several embryos of different sizes; count those you find rather than assuming every orange seed contains many.

Are apomictic embryos clones? NCERT asks about their genetic nature (page 23). For NCERT’s clonal routes, the embryo-forming lineage avoids meiotic segregation and fertilisation, so apomictic embryos are maternal clones, apart from possible mutations. This statement applies to the nucellar/apomictic embryos; a coexisting zygotic embryo is genetically different.

Apomixis and hybrid seeds

NCERT explains why plant breeders want apomixis (page 23):

• Hybrid varieties of several food and vegetable crops are extensively cultivated, and hybrids have tremendously increased productivity.
• But hybrid seeds have to be produced every year: if seeds collected from hybrids are sown, the progeny segregate and do not keep the hybrid characters.
• Producing hybrid seed is costly, so hybrid seed becomes too expensive for farmers.
• If the hybrids reliably produced clonal apomictic seed, there would be no segregation of characters in the hybrid progeny, and farmers could keep using their own seed year after year.
• Because of this, active research is going on in many laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

Why a hybrid’s own seeds “segregate” is the subject of Mendel’s laws in NCERT Chapter 4. Example 3 below shows the numbers for one and two genes.

Solved examples

Common mistakes to avoid

Quick revision summary

• Apomixis: seeds without fertilisation, a form of asexual reproduction that mimics sexual reproduction; some Asteraceae and grasses.
• Two routes: a diploid egg formed without reduction division grows into the embryo without fertilisation; or nucellar cells round the embryo sac grow into embryos (Citrus, mango).
• Polyembryony: more than one embryo in a seed, as in orange.
• In the diploid clonal routes shown, apomictic embryos retain the maternal genotype; some seeds may also contain a sexual embryo, and some apomicts require fertilisation for endosperm.
• Hybrids made apomictic would not segregate, so farmers could reuse hybrid seed every year.

Questions students ask

What is the difference between apomixis and polyembryony?

Apomixis is the production of seeds without fertilisation, a form of asexual reproduction, while polyembryony is, in NCERT’s words, the “occurrence of more than one embryo in a seed” (NCERT pages 22–23). The two overlap in NCERT’s second apomictic route: when several nucellar cells round the embryo sac grow into embryos, the apomictic seed is also polyembryonic (pages 22–23). A seed whose embryo grew from a diploid egg formed without reduction division can hold a single embryo, so not every apomictic seed is polyembryonic (our reasoning from page 22).

What is the difference between apomixis and parthenogenesis?

Apomixis is seed formation without fertilisation in flowering plants (NCERT page 22), while parthenogenesis is the growth of an unfertilised egg into a new organism, as in the haploid drones of the honey bee (NCERT Chapter 4, page 71; honey bee). The two meet in the apomictic route in which a diploid egg, formed without reduction division, grows into the embryo (page 22): beyond the NCERT text, such an egg “develops parthenogenetically into a maternal embryo” (Springer and Musiał, Plant Reproduction, 2026; CC BY 4.0). The nucellar route involves no egg, so it is apomixis without parthenogenesis.

How can polyembryony be commercially exploited?

Polyembryony can be exploited to raise many uniform plants from seed, because in polyembryonic varieties the extra embryos grow from nucellar cells round the embryo sac rather than from a zygote (NCERT pages 22–23). Beyond the NCERT text, citrus growers use this for rootstocks: a rootstock can affect how the variety grafted onto it develops, so its seedlings must be uniform, and nucellar seedlings have to be told apart from zygotic ones, which differ genetically from the mother (Martínez-Ochoa and others, Scientia Agricola, 2022; CC BY). NCERT’s Summary adds that “Apomicts have several advantages in horticulture and agriculture” (page 24).

What is the difference between apomixis and amphimixis?

Amphimixis, a term beyond the NCERT text, is ordinary sexual reproduction, by meiosis and the fusion of gametes, and plants that reproduce this way are called amphimictic (Springer and Musiał, Plant Reproduction, 2026; CC BY 4.0), whereas apomixis is NCERT’s seed formation without fertilisation (page 22). In an amphimictic flowering plant the embryo grows from a zygote formed by syngamy, so seeds carry new genetic combinations (NCERT pages 18 and 22), while an apomictic embryo comes from a diploid egg formed without reduction division or from a nucellar cell and keeps the mother’s genotype (our reasoning from NCERT pages 22–23).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Name two groups of flowering plants in which NCERT reports apomixis.

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   Some species of Asteraceae and grasses (NCERT page 22).

2. Q2. In which fruits do nucellar cells develop into embryos, giving many embryos in one ovule?

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   In many Citrus and mango varieties (NCERT pages 22–23).

3. Q3. Why do hybrid seeds have to be produced every year?

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   If seeds collected from hybrids are sown, the plants of the progeny segregate and do not maintain the hybrid characters (NCERT page 23).