Apomixis and Polyembryony

Seeds Without Fertilisation: The Shortcut Called Apomixis

Throughout this lesson, we have followed the step-by-step journey of sexual reproduction in flowering plants: pollination, pollen tube growth, double fertilisation, and finally, seed formation. But what if a plant could skip the fertilisation step entirely and still end up with a viable seed? That is exactly what a small number of flowering plants have figured out.

This alternative route is called apomixis (a mechanism of seed production in which the embryo forms without fertilisation, bypassing the fusion of male and female gametes). Some species of the Asteraceae family (the daisy and sunflower family) and certain grasses are well-known examples of plants that use this strategy.

Despite producing seeds, apomixis is classified as a form of asexual reproduction. The reason is straightforward: seeds normally result from the fusion of two gametes (one male, one female), which shuffles genetic material from both parents. In apomixis, that fusion never happens. The embryo develops on its own, carrying only the mother’s genetic information. In this sense, apomixis mimics sexual reproduction on the outside (a seed is produced) while being asexual on the inside (no genetic mixing takes place).

A useful distinction to keep clear: producing a fruit without fertilisation is called parthenocarpy (think of the seedless banana from the previous topic). Producing a seed without fertilisation is apomixis. The two are different processes with different outcomes.

Two Routes to an Apomictic Seed

There is more than one way a plant can pull off this trick. The two main pathways are:

• Diploid egg cell pathway — In some species, the megaspore mother cell does not undergo meiosis (the reduction division that normally halves the chromosome count). Instead, it develops directly into an embryo sac containing a diploid egg cell (one with the full set of chromosomes). Because this egg already has the complete chromosome complement, it can grow into an embryo without needing a sperm cell to contribute the missing half. No fertilisation is required.

• Nucellar embryony — This is the more commonly observed route, seen in many varieties of Citrus (oranges, lemons) and Mango. Here, cells from the nucellus (the nutritive tissue that surrounds the embryo sac inside the ovule) begin to divide on their own. These dividing nucellar cells push into the embryo sac and develop into embryos independently. Since nucellar cells are ordinary somatic (body) cells of the parent plant, every embryo they produce is a genetic copy of the mother.

Polyembryony: Many Embryos, One Seed

When nucellar embryony occurs, something remarkable happens inside the ovule. The normal sexually produced embryo (if fertilisation also took place) sits alongside one or more additional embryos that grew from nucellar cells. The result is a single seed that contains multiple embryos.

This phenomenon is called polyembryony (the occurrence of more than one embryo within a single seed). If you were to take an orange seed and gently squeeze it open, you would find several embryos of different sizes and shapes packed together. Some of these are nucellar in origin (clones of the parent tree), while one may be the product of normal fertilisation.

Clones by Nature

Since apomictic embryos develop without any contribution from a second parent, every one of them carries the exact same genetic makeup as the mother plant. They are, in every meaningful sense, clones (genetically identical copies). Whether the embryo arose from a diploid egg cell that skipped meiosis or from a nucellar cell that started dividing independently, the outcome is the same: the offspring is a carbon copy of the parent.

This is fundamentally different from seeds produced through normal sexual reproduction, where each embryo carries a unique mix of genes from two parents.

Why Apomixis Matters for Agriculture: The Hybrid Seed Problem

To appreciate why scientists around the world are deeply interested in apomixis, you need to understand a practical problem that farmers face every growing season.

Hybrid crop varieties have transformed modern agriculture. When breeders cross two carefully selected parent lines, the first-generation offspring (the hybrid) often shows dramatic improvements in yield, disease resistance, or quality. This advantage, known as hybrid vigour, has boosted productivity for many of our food and vegetable crops.

Here is the catch. If a farmer collects seeds from these hybrid plants and sows them the following year, the next generation does not stay uniform. The offspring segregate: the favourable combination of traits that made the hybrid special breaks apart as the genes from the two parent lines sort independently. The progeny will be a mixed bag, some good, some poor, none reliably matching the original hybrid.

This means farmers cannot simply save hybrid seeds from one harvest and replant them. They must buy freshly produced hybrid seeds every single season. Producing those seeds is expensive because it requires controlled crosses between the two parent lines each time, using techniques like emasculation and bagging. That cost is passed directly to the farmer.

Now imagine if these high-performing hybrids could be made apomictic. Every seed the hybrid plant produced would be a genetic clone of itself. There would be no segregation, no loss of hybrid characters, and no need for fresh crosses. A farmer could harvest seeds from the current crop and use them for the next season, year after year, without any decline in quality.

This is precisely the goal driving active research in laboratories worldwide. Scientists are working to identify and understand the genes responsible for apomixis so that they can be transferred into commercially important hybrid varieties. If this becomes possible, it would revolutionise the hybrid seed industry, making high-yielding seeds accessible and affordable for farmers everywhere.