Apomixis and Polyembryony

Seeds Without Fertilisation — Is That Even Possible?

Throughout this chapter, we have seen that seeds are the products of fertilisation. A male gamete fuses with the egg, a zygote forms, and that zygote grows into an embryo packaged inside a seed. But what if a plant could skip all of that and still produce a perfectly functional seed? Surprisingly, some flowering plants do exactly this.

A small number of species, including some members of the Asteraceae (the sunflower/daisy family) and certain grasses (family Poaceae), have evolved a special trick. They produce seeds without any fertilisation taking place. This phenomenon is called apomixis (literally “away from mixing,” referring to the absence of genetic mixing).

At first glance, apomixis looks like sexual reproduction because the outcome is a seed. But look closer, and you realise that no fusion of gametes has occurred. There is no mixing of genetic material from two parents. So, apomixis is really a form of asexual reproduction that mimics sexual reproduction in its output.

A related but different concept is worth noting here: the production of fruit without fertilisation is called parthenocarpy (think seedless bananas). Apomixis, on the other hand, is about producing seeds without fertilisation. The two should not be confused.

How Do Apomictic Seeds Form?

There is more than one route by which a plant can pull off this feat. Two key pathways stand out:

Pathway 1 — A Diploid Egg That Skips Meiosis

In some species, the egg cell itself forms differently from the normal process. Normally, the megaspore mother cell undergoes meiosis (reduction division), producing haploid cells that eventually give rise to a haploid egg. In this apomictic pathway, meiosis simply does not happen. The egg cell is produced without reduction division, so it retains the full diploid ($2n$) chromosome set of the parent. This diploid egg then develops directly into an embryo, with no need for a sperm to contribute the missing half of the genome. The missing half was never missing in the first place.

Pathway 2 — Nucellar Embryony

This is the more commonly observed pathway, particularly in many varieties of Citrus (oranges, lemons, grapefruits) and Mango. Here, the embryo sac may develop normally and even undergo regular fertilisation. But alongside the normal embryo, something extra happens. Some of the nucellar cells (the cells of the nucellus, the tissue that surrounds the embryo sac inside the ovule) start dividing on their own. These dividing nucellar cells push into the embryo sac and develop into additional embryos.

The result? A single ovule ends up carrying more than one embryo. You might find a normal fertilised embryo alongside several nucellar embryos, all packed into the same seed.

Polyembryony — Many Embryos in One Seed

When a seed contains more than one embryo, the condition is called polyembryony (from “poly” meaning many, and “embryo”). This is a direct consequence of nucellar embryony in many cases. The extra embryos come from the nucellar cells that invaded the embryo sac, so one seed can house several embryos of different sizes and shapes.

You can observe this yourself with a simple experiment. Take a few orange seeds and squeeze them open. Inside, you will find not one but several small embryos of varying sizes. Count them and compare across seeds. The number can vary quite a bit.

What Is the Genetic Nature of These Embryos?

This is an important question. Since apomictic embryos form without meiosis and without fertilisation, there is no recombination of genes and no contribution from a second parent. Every apomictic embryo is genetically identical to the mother plant. In other words, they are clones. This genetic uniformity is the key reason why apomixis has attracted so much attention from agricultural scientists.

Why Apomixis Matters for Agriculture — The Hybrid Seed Problem

To understand why apomixis excites plant breeders, you first need to understand a major headache in modern farming: the hybrid seed problem.

Hybrid varieties of many food and vegetable crops are grown extensively because they show hybrid vigour. They grow faster, yield more, and often resist diseases better than their parent lines. This has led to a tremendous increase in agricultural productivity worldwide.

But hybrids come with a catch. When a farmer collects seeds from a hybrid plant and sows them the next season, the offspring do not maintain the same hybrid characteristics. Why? Because during sexual reproduction, the genes from the two original parent lines segregate (separate and recombine according to Mendelian principles). The result is a mixed population of plants, some with desirable traits and many without. The uniformity and vigour of the original hybrid is lost.

This means fresh hybrid seeds must be produced every year by repeating the original controlled cross between the two parent lines. This process is labour-intensive and expensive, making hybrid seeds costly for farmers.

The Apomixis Solution

Now imagine if these high-performing hybrid plants could reproduce through apomixis instead of normal sexual reproduction. Since apomixis involves no meiosis and no fertilisation, there would be no segregation of genes in the progeny. Every seed produced by the hybrid would be a genetic copy of the parent hybrid, carrying all its desirable traits intact.

Farmers could simply save seeds from their harvest and replant them year after year, getting the same excellent hybrid performance every time. There would be no need to buy expensive fresh hybrid seeds each season. The cost saving for farmers, especially smallholder farmers in developing countries, would be enormous.

Current Research

Because of this potential, active research is underway in laboratories around the world. Scientists are working to understand the genetic basis of apomixis, to identify the genes that control it, and to find ways to transfer apomictic genes into hybrid crop varieties. If successful, this would revolutionise agriculture by making hybrid vigour permanently available through saved seeds.