What is Apomixis and Polyembryony?

In a normal sexual cycle (amphimixis), the megaspore mother cell (2n) of ovule is converted to haploid gametophytic cells (embryo sac) through meiosis (reductional division). This results in the formation of an egg cell, synergids, antipodal cells and polar nuclei cell. Egg fertilizes with the male gamete to form an embryo. This process occurs in almost all angiospermic plants. However, in some plants, meiotic division and fertilization are eliminated still a functional embryo is formed. The formation of embryo without fertilization (by asexual means) is called Apomixis (Apo = not, mixis = act of mixing) and the seed formed is called Apomictic seed.  In other words, apomixis is a type of asexual reproduction that does not involve meiosis and syngamy (fusion of male and female gametes) for the formation of an  embryo.

Apomixis occurs in about 300 species of angioperms that comprises of at least 35 plant species. It is most common in families such as Asteraceae, Graminaceae, Rosaceae and Rutaceae. Some of the most common examples of apomixis include citrus and mango. 

Types of Apomixis:

1. Gametophytic:

a) Diplospory

b) Apospory

2. Sporophytic 

a) Adventive embryony

DIPLOSPORY:

 Megapore mother cell does not go through meiosis. Besides, egg sac organizes normally but the cells are diploid. It is important to note that the embryo develops without being fertilized. This occues because the egg is already growing. Endosperm development may or may not involve fertilization. 

Diplospory is common in grasses and asteraceae family. 

APOSPORY:

In this case, the MMC (megspore mother cell) undergoes meiosis, but the resulting cells degenerate before forming a functional embryo sac. Additional cells present in the nucellus become cytoplasmic and function as apomictic initials. These cells do not undergo meiosis and form their own embryo sac inside the same ovule. 

Apospory condition is found in families such as Rosaceae, Asteraceae and Grasses.

Advantages of Apomixis:

Seeds and plants produced through apomixis are genetically similar to their parent. This technique is can be used in hybrid seed production. We know that hybrid seeds formed in F1 generation is always superior than its pureline parents. However, the same hybrid seeds cannot be used every year due to segregation of characters. And therefore, farmers need to buy hybrid seeds every year for cultivation. However, forming hybrid seeds is a tough task. Using apomictic technique, hybrid seeds can be easily produced in large amounts.

Seed formation through the apomictic process does not require any pollinating agent. 

ADVENTIVE EMBRYONY (POLYEMBRYONY):

In adventive embryony, the megaspore mother cells (MMC) undergoes normal meiotic division and forms a normal embryo sac. Besides, these cells are fertilized normally with male gametes just like normal embryogenesis.  What happens is that the cells in the nucellus become embryogenic and form multiple embryos in the micropylar region of the ovule. The resulting asexual embryos do not form their own embryos rather they grow inside the sexual embryo. This is common in tropical fruit trees such as citrus and mango.