Biology · Ch 1 — Sexual Reproduction in Flowering Plants
Apomixis and Polyembryony
Apomixis and Polyembryony
The Big Idea: When Seeds Are Made Without Sex
Most flowering plants reproduce sexually — pollen meets ovule, double fertilisation happens, and a seed forms with a unique genetic mix. But nature has a few shortcuts. Some plants skip the fusion of gametes entirely and still produce viable seeds. Others pack multiple embryos into a single seed. These two phenomena — apomixis and polyembryony — are the final topics of this chapter, and they show that sexual reproduction, for all its elegance, is not the only way to make a seed.
Apomixis: Seed Formation Without Fertilisation
(See the in-text question distinguishing this from apomixis, below.) Apomixis (from Greek apo = away from, mixis = mixing) is a mode of reproduction in which seeds are formed without the fusion of gametes. The embryo develops directly from the maternal tissues of the ovule — usually from the nucellus or the integuments — and not from the zygote. The resulting seed is genetically identical to the mother plant (a clone).
Apomixis is a form of asexual reproduction that mimics sexual reproduction — the plant still makes flowers and seeds, but no fertilisation occurs.
Key features of apomixis:
- No meiosis and no syngamy (fusion of male and female gametes).
- The embryo develops from a diploid cell of the ovule (e.g., nucellus or integument).
- The seed is genetically identical to the parent plant.
- It is common in some grasses, citrus fruits, mango, and many members of the family Asteraceae.
Why does apomixis matter?
- It allows a plant to produce offspring that are exact copies of itself — useful for preserving desirable traits (e.g., seedless fruits, high yield).
- It is a natural way of clonal propagation through seeds.
- It matters economically too: hybrid seed varieties normally have to be purchased fresh every year, because a hybrid's own offspring segregate and lose the parent's superior hybrid traits — and producing hybrid seed commercially is expensive. If apomixis could be introduced into hybrid varieties, farmers could save and resow seed from their own hybrid crop year after year without losing the hybrid vigour, since apomictic seeds are clones of the mother plant. This is why active research is underway to transfer the genes responsible for apomixis into hybrid crop varieties.
Do not confuse apomixis with parthenocarpy. Parthenocarpy is fruit development without fertilisation (e.g., seedless grapes, bananas) — the fruit forms but no seeds develop. Apomixis produces seeds (with embryos), but those embryos are not the result of sexual fusion.
Polyembryony: Many Embryos in One Seed
Polyembryony is the condition in which a single seed contains more than one embryo. This can happen naturally in many plants, especially in citrus (oranges, lemons) and mango.
How does polyembryony arise?
- The zygote (formed by normal fertilisation) divides to produce the main embryo.
- Additional embryos develop from nucellar cells or integument cells — these are apomictic (asexual) in origin.
- So a single seed may contain one sexual embryo and several asexual (nucellar) embryos.
Key points about polyembryony:
- It is often associated with apomixis — the extra embryos arise from the same maternal tissues that give rise to apomictic embryos.
- In citrus, the nucellar embryos are vigorous and often outgrow the zygotic embryo.
- Polyembryony is of practical importance in horticulture: nucellar seedlings are true-to-type (clones of the mother plant) and are used for propagation.
Polyembryony is not the same as multiple seeds in a fruit. It is multiple embryos within a single seed.
(See the orange-seed polyembryony activity, below.)
A Quick Comparison
| Feature | Apomixis | Polyembryony |
|---|---|---|
| What it is | Seed formation without fertilisation | Multiple embryos in one seed |
| Origin of embryo | From maternal tissues (nucellus, integument) | One from zygote + others from maternal tissues |
| Genetic nature | Clone of mother | Mixed (one sexual, rest clones) |
| Example plants | Citrus, mango, grasses | Citrus, mango, onion |