Types of Apomixis

There are several types of apomixis in plants. The basic classification of apomixis is gametophytic and sporophytic apomixis based on the origin of the embryo. The gametophytic apomixis is caused by the direct development of an embryo from the egg cell of an unreduced embryo sac. Based on the precursor cell used to form the embryo sac, it can divided into diplospory and apospory.

Sporophytic apomixis is formed by the direct development of an embryo from the mitotic division of the somatic cell of the ovule. Sporophytic apomixis is a typical phenomenon in citrus plants in which sexual reproduction coexists with adventitious embryony.

Gametophytic Apomixis

• Diplospory: It is a type of gametophytic apomixis where the unreduced embryo sacs are formed from the megaspore mother cell in which the meiosis is suspended. From the above diagram (fig 2), we can see that an unreduced embryo sac develops from a megaspore mother cell. The unreduced embryo sac contains of diploid egg which develops into an embryo. It can be observed in Tripsacum, Erigeron, and Taraxacum officinale species.
• Apospory: It is a type of gametophytic apomixis where an unreduced embryo sac originates from somatic cells instead of germ line cells of the ovule. From the above diagram, we can see the development of unreduced embryo sacs from nuclear cells known as aposporos initial cells. It is seen in Panicum, Poa, Pennisetum, and Hieracium spp.

Sporophytic Apomixis

Adventitious Embryony: It is also known as nucellar embryony. It is a type of sporophytic apomixis where the embryo is formed from a cell or a group of cells either in the nucellus or in the integuments (somatic cells) of the ovule. It takes place outside the embryo sac. It can also cause a condition called polyembryony, where multiple embryos are formed in the same ovary. This condition is due to the simultaneous development of an embryo from sexual reproduction within the same ovary. It is commonly seen in citrus fruits.

In most apomictic plants, both sexual and asexual reproductive processes take place simultaneously in the same ovule; this is known as facultative apomixis. In others, gametophytic apomixis replaces amphimixis and this is considered obligate apomixis. In both diplosporous and aposporous ovules, the endosperm can grow spontaneously without fertilization or by pseudo-fertilization, supplying nutrition for the embryo.

Vegetative Apomixis

This type of apomixis is rare in nature. So it is not frequent in fruit crops. But in certain situations, such as Poa bulbosa and various Agave, and grass species, vegetative buds or bulbils are formed instead of flowers in the inflorescence. The bulblet reproduction can also be considered as vegetative apomixis.

Based on the egg cell ploidy, apomixis can also be classified into recurrent and non-recurrent apomixis:

• Recurrent Apomixis: The embryo develops directly from a diploid megaspore mother cell or egg cell without fertilisation. Here both the egg and embryo are diploid(2n). In general, somatic apospory (apospory- development of a gametophyte from a sporophyte without meiosis seen in certain bryophytes), diploid parthenogenesis, and diploid apogamy (apogamy- development of a sporophyte from a gametophyte without fertilization seen in some pteridophytes) are considered recurrent apomixis. It is observed in Taraxacum, Poa, and Allium spp.
• Non-recurrent Apomixis: The embryo develops from a haploid egg cell without being fertilized. This sort of apomixis is very rare. The egg cell and embryo are haploid(n) in nature. This category includes generative apospory, haploid parthenogenesis, and haploid apogamy.
• Parthenogenesis: This is a type of apomixis that can occur in plants, animals or microbes. It is the development of the unfertilised egg into an embryo. Parthenogenesis in honey bees is a common example.

Apomixis is the formation of an embryo without the fusion (mixing) of gametes. Apomixis example is Taraxacum officinale. The term was first coined by Albert Winkler in 1908 when he noticed a single female Alchornea ilicifolia plant producing seeds even when there were no male equivalents in the United Kingdom. In this article, we will learn about the apomixis definition, types, characteristics, advantages, and disadvantages of apomixis in plants in detail.