Special Modes-Apomixis

Introduction

Flowering plants follow three modes of reproduction: 

• cross-pollination
• self-pollination
• special modes – apomixis (asexual reproduction)

The phenomenon of apomixis occurs in over 35 different plant families, including cereals such as maize and wheat. The word ‘apomixis’ has Greek origin – ‘APO’ means “away from” and ‘MIXIS’ means “the act of mixing or mingling”. Antonie van Leeuwenhoek discovered the phenomenon of apomixis in citrus seeds in 1719. 

Special Modes-Apomixis 

Apomixis in plants refers to a mode of asexual reproduction that occurs via seeds, in which embryos develop without fertilization. That means that the reproductive organs of related structures take part, but no zygote is formed through gametic fusion. Apomixis is, therefore, a special kind of asexual reproduction that replaces normal sexual reproduction. The resulting embryo, including the baby plant, is genetically identical to its mother. Furthermore, these seedlings are completely free from harmful viruses.

Certain plant species may produce an embryo without the fertilisation of ovule and pollen. This phenomenon is called apomixis in plants, and the seedlings produced by this process are known as ‘apomicts’. The word, ‘apomixis’ was coined by botanist Hans Winkler in 1908, as a special mode of asexual reproduction ‘without fertilization’. It should be noted that the normal asexual reproduction in plants, such as propagation from leaves, cannot be considered apomixis. However, replacing seeds with plantlets or replacing the flower with a bulbil is classified as apomixis. 

Features of apomixis 

• Apomixis is an asexual means of reproduction in plants. This phenomenon occurs in over 300 plant species of 35 families
• The process does not involve the fusion of male and female gametes; thus, there is no genetic recombination

• It helps develop pure lineage, which can be achieved by haploid parthenogenesis. Haploid parthenogenesis is a rare form of parthenogenesis that occurs in a few species of bees, nematodes, and plants – offsprings develop from haploid eggs to produce haploid adults
• Apomixis can be manipulated genetically, which makes it useful for preserving superior genes

Special Modes-Apomixis Classification

Depending on the cell involved, occurrence, and frequency ,Apomixis is categorised into four groups  .

• Recurrent apomixis or Gametophytic apomixis

In this apomixis ,the embryo sac develops without meiosis from diploid cells. .A diploid cell has paired chromosomes. Both egg and embryo are diploid. Hence, this new egg has the normal number of chromosomes equivalent to the mother plant. The embryo subsequently develops from the egg nucleus without fertilisation. Examples of plant species are ‘Crepis’ and ‘Allium’.

• Non-recurrent apomixis

In non-recurrent apomixis, the embryo develops directly from the sac’s haploid egg cell or other haploid cells without fertilisation. A haploid cell has a single set of unpaired chromosomes in biological terms. Hence, the embryo developed is also haploid and sterile. This type of apomixis is rare in plant species and depends heavily on genetic interest. Examples of plant species are ‘Solanum nigrum’ and ‘Lilium’.

• Adventitious embryony

It is also known as sporophytic budding or nucellar embryony. It is a type of sporophytic apomixis. The embryo develops directly from the nucellus or a group of cells of the nucellus or integuments, resulting in the formation of a bud-like structure. The cells of the resultant embryo are diploid since the nucellus cells are diploid. Nucellar embryony commonly occurs in tropical and sub-tropical tree species, including citrus and mango.

• Vegetative apomixis or bulbils

In this type of apomixis, vegetative buds are called bulbils that grow in place of flowers. These bulbils are easily reproducible and may develop into new plants while attached to the mother plants. This type of apomixis is quite common in ‘Allum’, ‘Poa’, ‘Bulbosa,’ and some grasses.

Common Examples of Apomixis

Apomixis occurs commonly in sweet oranges, grapefruits, lemons, dandelions, hawthorns, blackberries, meadow grasses, and hawkweeds.

Advantages of Apomixis

Apomictic plants inherit the genetic structure of their mother plants, which in turn enables them to maintain genetic uniformity for many generations. The main advantages of apomixis are as follows:

• Assured reproduction in extreme environments as external pollinators do not contribute to reproduction
• It helps in hybrid seed production for apomictic plants
• Cost-effective in seed production
• Apomixis prevents the loss of specific hybrid characteristics
• Apomixis can be genetically engineered to preserve superior genotypes and hybrid characteristics

Disadvantages of Apomixis

The major disadvantages of special modes-apomixis are as follows:

• It cannot control the growth of harmful genetic mutations
• The phenomenon typically occurs in small ecological niches. The process cannot adapt to rapid changes in environments

Significance of Apomixis in Plant breeding

Special modes-apomixis has proven to be beneficial for the plant-breeding industry. This method helps develop seeds without involving fertilisation. Since there is no meiosis during seed formation, the superior traits of the seeds remain intact. These good traits are passed onto the next generation, thus retaining the purity of the hybrid seeds.

Hybrid seeds are expensive to grow. However, special modes-apomixis helps farmers grow crops throughout the year without purchasing hybrid seeds. Therefore, the process of apomixis proves to be cost-efficient for farmers.

Apomixis is also a time-saving process. Hybrid seeds allow farmers to grow crops year after year because they provide a high yield in a short period of time.

Conclusion

Special mode- apomixis involves the formation of a seed without fertilisation. Genetic recombination is not found in apomixis since there is no mixing between the male and the female gametes. This process proves to be effective for retaining the hybrid character of the seeds as there is no meiosis involved. Apomixis is a cost-effective and time-efficient process of production of seeds that helps farmers cultivate hybrid seeds. In addition, it can prove beneficial on the commercial front as superior traits can be passed down to newer generations.