The fruit, apomixis and polyembryony · Biology Class… | StemPy Lab
Course 19.3·Lesson 3 of 3
The fruit, apomixis and polyembryony
After fertilisation the ovary becomes the fruit, and its wall becomes the pericarp. A false fruit includes the thalamus. A parthenocarpic fruit forms without fertilisation. Apomixis is the making of seeds without fertilisation. Polyembryony is the presence of more than one embryo in a seed. Seeds and fruits help the plant to protect, feed and spread its young.
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The ovary becomes the fruit
The changes of the ovule into a seed and of the ovary into a fruit go on at the same time. The ovary wall becomes the wall of the fruit, the pericarp. The pericarp may be fleshy, as in guava, orange and mango. It may be dry, as in groundnut and mustard.
In most plants the other parts of the flower, such as the petals and the sepals, wither and fall off by the time the fruit forms.
A true fruit develops only from the ovary. Mango and orange are true fruits.
A false fruit includes the thalamus too. Apple and strawberry are false fruits.
Fruits without fertilisation
Most fruits are the result of fertilisation, but some are not. A fruit that develops without fertilisation is a parthenocarpic fruit. Banana is an example. Such fruits have no seeds. Parthenocarpy can also be caused by growth hormones, and it is used to make seedless fruits.
Why seeds and fruits matter
Seeds and fruits are a great advantage to flowering plants.
The seed has a food store, so the young plant is fed until it can make its own food.
The hard seed coat protects the young embryo.
Many fruits have ways to spread the seeds, so the plant can reach new places.
A seed is a product of sexual reproduction, so it brings new combinations of genes and variation.
A dry, dormant seed can be stored. People use this for food all year and for sowing the next crop.
Apomixis
Some plants make seeds without fertilisation. This is apomixis. It is a form of asexual reproduction that looks like sexual reproduction, because the plant still makes seeds. It occurs in some species of the family Asteraceae and in some grasses. It happens in two main ways.
Two drawings. In (a), an embryo sac holds a large egg cell and two polar nuclei. An arrow marked no fertilisation leads to a round embryo, and the text says that a diploid egg cell made without reduction division develops without fertilisation. In (b), a large embryo sac sits inside the nucellus. One embryo from the zygote lies at the top of the sac, and three embryos that have grown from cells of the nucellus lie in the sac. The text says that cells of the nucellus divide, bulge into the embryo sac and make extra embryos, which is polyembryony.
A diploid egg cell is made without reduction division, and it grows into an embryo without being fertilised.
Cells of the nucellus around the embryo sac divide, bulge into the embryo sac and grow into embryos. This is common in Citrus and mango.
Because apomixis does not involve meiosis or fertilisation, the offspring are genetically the same as the mother plant.
Polyembryony
Polyembryony is the presence of more than one embryo in a seed. It occurs in Citrus and mango. Squeeze a few orange seeds in water, and look at them under a lens. You will often find several embryos in one seed. In these seeds, one embryo may come from the zygote, and the others from cells of the nucellus.
Why apomixis matters for farmers
Many high-yielding crops are hybrids. A hybrid seed gives very good plants. But if a farmer sows the seeds of these plants, the characters split up and the hybrid is lost. So hybrid seeds must be made again every year, and farmers must buy them.
If a hybrid made apomictic seeds, the offspring would be just like the hybrid. There would be no splitting. A farmer could use the seeds from the crop to grow the next crop. Scientists are trying to put apomixis into hybrid crops.
A flow chart of the life cycle of a flowering plant with the chromosome number marked on each box. The sporophyte plant, 2n, bears a flower with an anther and an ovule. In the anther, a pollen mother cell, 2n, goes through meiosis and mitosis to give a pollen grain, which is the male gametophyte, n, and then two male gametes, n. In the ovule, a megaspore mother cell, 2n, goes through meiosis and three mitoses to give the embryo sac, the female gametophyte, n, with the egg cell and two polar nuclei, n. Fertilisation gives the zygote, 2n, and the primary endosperm nucleus, 3n, which grow into the embryo and the endosperm in the seed. A dashed arrow shows that the seed grows into a new plant.
The chart puts the whole chapter together. The big plant is the diploid sporophyte. The gametophytes, the pollen grain and the embryo sac, are haploid and tiny. Fertilisation brings the diploid number back.
You should find that the heterozygous share falls fast when the seed is made by fertilisation, and it stays at 100 per cent in the apomictic seed.
Try it in Python
The program counts the share of heterozygous plants in each generation, with exact fractions.
Practise with new numbers
Common mistakes
Saying that all fruits come from fertilisation. Parthenocarpic fruits do not.
Calling an apple a true fruit. The thalamus forms most of it, so it is a false fruit.
Saying that apomixis involves meiosis. The embryo comes from a cell that did not go through reduction division.
Saying that polyembryony needs several zygotes. Often the extra embryos come from the nucellus.
Thinking that apomictic seeds vary. They are genetically the same as the mother plant.
Remember these
The ovary becomes the fruit, and the ovary wall becomes the pericarp. True fruit: ovary only. False fruit: with the thalamus (apple, strawberry).
Parthenocarpy: fruit without fertilisation (banana). It gives seedless fruit.
Apomixis: seeds without fertilisation, by a diploid egg or by nucellar cells (Citrus, mango). Offspring are clones of the mother.
Polyembryony: more than one embryo in a seed (Citrus, mango).
Apomictic hybrid seeds could be saved by the farmer, because there is no splitting.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What does the ovary wall become?
What is the difference between a true fruit and a false fruit? Give an example of each.
What is a parthenocarpic fruit?
Give the two ways in which apomixis happens.
Why would apomictic hybrid seed be useful to a farmer?
References
NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.
Urry L. A. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 38.
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