Apomixis

What is Apomixis?

Apomixis can be defined as a means of asexual reproduction where fertilisation. The term was first coined by botanist Hans Winkler. The seed of a plant is developed without fertilisation. 


The Features of Apomixis are: 

  • It is an asexual means of plant reproduction and is found in many species especially in fruit crops.

  • The process does not involve the union of male and female gametes and therefore there is no genetic recombination. Apomixis does not lead to the generation of variability in a population.

  • Apomixis does not permit gene flow which means it does not permit the combination of genes from different sources.

  • It helps in the rapid development of pure lines and can be achieved by haploid parthenogenesis.

  • Apomixis can be genetically controlled and is useful in conserving superior genotypes and hybrid vigor.

  • It has been reported in more than 300 plant species of 35 families.


Apomixis can be classified based on

  1. Cell involved

  2. Occurrence

  3. Frequency

Apomixis types:

Classification

Types of apomixis

Description

Cell involved apomixis

Parthenogenesis

The embryo develops from an egg cell


Apogamy apomixis

  • The embryo originates from synergids or antipodal cells.

  • It develops from the normal embryo sac cells. 

  • It may be haploid or diploid.


Diplospory 

The unreduced embryo sac is derived from a megaspore mother cell by aborted meiotic division or a direct mitotic division. 


The major types of diplospory apomixis are Taraxacum, Ixeris, and Antennaria.


In Taraxacum, the meiotic prophase starts but then the phase is aborted which results in two unreduced dyads one of which gives rise to the embryo sac by mitosis


In Ixeris, two further mitotic division of the nuclei give rise to an octa-nucleate embryo sac which follows an equational division following meiotic prophase.


In Antennaria, a total of three mitotic divisions form the megagametophyte.


Apospory

The nuclear cells that give rise to apomictic embryo sacs which are known as aposporos initials are distinct from the ameiotic megasporocyte.


The aposporos initial cells may differentiate close to the ameiotic megasporocyte and transform into an apomictic embryo sac.


After they differentiate then enter mitotic cell division to produce an embryo sac. Some ovules can contain several embryo sacs and, depending on the plant species, the form of the embryo sac may be - different from that seen in the sexual reproduction process.


The initiation apospory embryo sac can occur alongside a sexual one or it can inhibit sexual embryo sac formation.


Adventive embryonic 

It is also known as sporophytic apomixis. In this type of apomixis, there may be a megagametophyte in the ovule. However, the embryos do not rise from the cells of the gametophyte. They rise from the cells of nucellus.

It can be observed in species of Garcinia, Mangifera indica, etc

Occurrence

Recurrent apomixis

It is also called gametophytic apomixis The megagametophyte has the same number of chromosomes as the mother plant. This is due to incomplete meiosis. It generally arises from an archesporial cell or some other part of the nucellus.


Non-recurrent apomixis

The embryo sac consists of haploid cells and the embryo develops from a haploid cell.

Frequency

Obligate apomixis

The reproduction takes place  by apomictic means


Facultative apomixis

The process of reproduction is carried out by both apomictic and sexual means.

Apomixis Examples:

Apomixis can be observed in hawthorns, shadbush, Sorbus, brambles, and blackberries, meadow grasses, mat grass, hawkweeds, etc.

Applications of Apomixis

Apomixis has many applications in the produce sector. Some of them are explained below:

  • Development of Pure Lines: Apomixis is a means of rapid production of pure lines in plants. Haploid apogamy and parthenogenesis give rise to haploid plants which after being treated by colchicine treatment can produce diploid pure lines. These pure lines can be used in the production of high yielding cultivars and hybrids.

  • Maintenance of Purity: Obligate apomixis breeds retain the characteristics of the mother plant which are useful in maintaining genetic purity from generation to generation. It can maintain a genotype for several generations. 

  • Conservation of Heterosis: Obligate recurrent apomixis is useful in conserving hybrid vigor for numerous generations. As apomixis does not permit segregation, heterosis can be easily conserved.

  • Easy Hybrid Seed Production: Apomixis offers a simple way of hybrid seed production. These seeds are automatically produced by apomictic means and there is no need for crossing. Once a hybrid is developed using an apomictic line as one of the parents, the hybrid seed production will occur automatically. It is also more affordable than conventional methods of hybrid seed production.

Advantages of Apomixis:

Apomictic plants conserve the genetic structure of their carriers which enables them to maintain heterozygote advantages for many generations. It offers a great advantage in plant breeding where genetic uniformity is maintained for both homo and heterozygosity.  The advantages of apomixis are:

  • Rapid multiplication of genetically uniform progenies without the risk of segregation.

  • Hybrid vigor or heterosis can be fixed permanently in crop plants

  • If maternal characteristics are present in resultant progenies then it can be exploited from generation to generation.

The significance of Apomixis

The significance or the importance of apomixis in the plant breeding industry is massive. It is a method that develops seeds without fertilization. It can be referred to as a means of asexual reproduction which mimics sexual reproduction. It helps in the production of hybrid seeds and is cost-efficient when it comes to large-scale production.  There is an increase in yield as well. Also, since there is no cross-fertilization, apomixis helps in preserving the good characteristics of a crop plant.

FAQ (Frequently Asked Questions)

1. What are the salient features of recurrent and non-recurrent apomixis?

Recurrent apomixis

  • The embryo-sac is developed from diploid cells without a reduction in chromosome number.

  • The plants produced by recurrent apomixis have diploid (2n) number of chromosomes.

  • All recurrent apomixis produced plants are fertile

Non-recurrent apomixis

  • The embryo-sac has haploid cells (n).

  • Plants produced by non-recurrent apomixis are haploid and sterile.

  • The method includes haploid apogamy and haploid parthenogenesis 

  • This method is used in the development of inbred lines.

2. What are the limitations of apomixis?

The limitation of apomixis are:

  • Genetic diversity: Other than obligate apomictic lines that show low genetic diversity, apomictic lines are genetically highly uniform. The diversity shows through mutation.

  • Lack of gene flow: Re-combinations do not occur in such apomictic populations, therefore there is a lack of gene flow.

  • Selection is ineffective: Selection is ineffective in obligate apomictic lines due to lack of heritable variation. All the plants of an apomictic line have the same genotype. It can either be homozygous or heterozygous.

  • Low frequency: Apomictic lines have very low frequency and are available in limited crop plants. However, these lines are more frequent in fruit crops.