The essentials
Some plants produce seeds or fruits without the usual sequence of fertilisation. Apomixis is the formation of seeds without fertilisation; it is a form of asexual reproduction that mimics sexual reproduction. It is seen in some species of Asteraceae and grasses. In some apomictic plants, a diploid egg cell forms without reduction division and develops into an embryo without fertilisation. In others, such as many Citrus and mango varieties, nucellar cells around the embryo sac divide and push into the embryo sac to form extra embryos.
Polyembryony is the occurrence of more than one embryo in a seed. In Citrus, the extra embryos from nucellar cells are genetically identical to the parent. Parthenocarpy is the development of fruit without fertilisation; such fruits are seedless. Banana is a natural example, and it can be induced by applying growth hormones.
Key points
- Apomixis keeps hybrid characters fixed, so farmers could reuse hybrid seed year after year instead of buying new seed.
- Hybrid seed of normal crops segregates in the next generation and loses hybrid vigour.
- Nucellar embryos are clones of the mother plant.
- Parthenocarpic fruits: no seeds; apomictic fruits: seeds present.
Exam tips
Distinguish carefully: apomixis gives seeds without fertilisation, parthenocarpy gives fruits without fertilisation. Questions often ask why apomixis interests the hybrid seed industry.
Common mistakes
- Polyembryony is not the same as having many seeds in a fruit; it means several embryos in one seed.