Parthenocarpy, Polyembryony and Apomixis
Parthenocarpy and seedlessness
In recent years, consumer preference for seedless fruits has been increasing. The seedless nature of some fruits is due to the phenomenon of ‘Parthenocarpy’, which refers to the development of fruits without fertilization or even without the stimulation that comes from pollination. Parthenocarpic fruits are usually seedless but not always.
If a fruit develops without the stimulus of pollination, this phenomenon is called vegetative parthenocarpy. e.g. Banana and Japanese Persimmon.
If a fruit develops only by the stimulus of pollination (but without fertilization), this phenomenon is known as stimulative parthenocarpy. The female flowers of triploid melon require pollen grains from diploid varieties to develop into seedless fruit.
The diploid pollen grain stimulates the guava ovary upon self-pollination, resulting in parthenocarpic fruit development due to stimulation provided by pollen hormones. Seedless varieties of grapes such as Thompson and papaya
Pollination and fertilization take place in the Black Corinth grape variety but the embryo is later aborted, resulting in the fruit becoming seedless. This phenomenon of the development of seedless fruits is called ‘steno-spermocarpy’. Seedless or parthenocarpic fruits are beneficial because consumers have a higher preference for the same type of seedless fruit (such as seedless grapes, guavas, or oranges).
Failure of pollination, male sterility, and self-incompatibility cause no unfruitful problems if a fruit develops parthenocarpically and assures the grower of a good crop (eg banana). One drawback of seedless fruits is that they are usually smaller in size (such as the Black Corinth variety of grape) and irregular in shape (guava).
Induction of seedlessness in fruits
Seedlessness can be induced by the following methods.
1. Use of growth regulators
Seedless fruit development was achieved by applying lanolin paste of 8000 ppm of GA to the cut end of the style of withered guava flowers. Similarly, seedlessness was induced in loquat by spraying GA 100 to 200 ppm on wilted flowers.
2. Changing the ploidy level
Seedlessness was demonstrated for the first time in Japan by crossing tetraploid x diploid varieties to develop a triploid watermelon 2n = 33. Naturally available seedless guava varieties are due to auto polyploidy (triploidy) and not parthenocarpic fruit development.
In some plants, the fruits develop from the parthenocarpically, yet they produce viable seeds. (such as mangosteen and strawberries). This phenomenon is called parthenogenesis. Such fruit plants are genetically identical. In some cases, seeds develop parthenocarpically but seeds are non-viable (such as apples).
Polyembryony means that more than one embryo develops within a single seed. It is also known as adventitious embryony (Nucellar embryony or Nucellar budding).
- Polyembryony means that more than one embryo develops within a single seed. It is also known as adventitious embryoni (nuclear embryony or nuclear budding).
- Polyembryony can develop for many different reasons. Specialized cells in the nucellus or sometimes in the integument contain the embryo. Genetically, these embryos have the same genotype as the parent plant and are apomictic.
- The adventitious embryo occurs in many plant species but is most common in citrus and mango. Both zygotic and apomictic embryos are produced in these species. In other species (such as Opuntia), no pollination or fertilization is required.
- Polyembryony is common in mangoes and citrus. In Trifoliate Orange, several plants emerge from a single seed.
- Typically of these plants, one plant may be the weaker sexual, and others arise apomorphically from cells in the nucellus, which are diploid copies of the parent plant.
Horticultural significance of polyembryony
The Nucellar shoots in citrus are completely free of viruses, as the embryo sac and adjoining tissue are impregnated at the time of flowering produce some unknown substance that kills all viruses. For the urgent need of planting material, growth from nucellar lines is the fastest and easiest way. The major potential horticultural applications of polyembryony are:
- Nucellar seedlings are of true type.
- Such plants are genetically identical and can be used as virus-free rootstocks
- More vigorous seedlings – continued vegetative propagation leads to a decline in citrus vigor
- Development of virus-free shoots and bud wood.
- Importance in a breeding program.
- Winkler (1908) defined apomixis as “the replacement of sexual reproduction or any other asexual reproduction process that does not involve nuclear or cellular fusion (i.e. fertilization)”. It was first reported in citrus seeds by Leuwenhoek in early as 1719.
- In some species of plants, an embryo develops from the diploid cells of the seed, and not as a result of fertilization between the ovule and the pollen. This type of reproduction is known as apomixis and the plants produced in this way are known as apomicts.
- Asexual plants are similar to the mother plant and similar to those grown by other vegetative methods, as such plants have the same genetic makeup as the parent plant.
- Apomixis is widely distributed in higher plants. More than 300 species belonging to 35 genera are apomixis. It is most common in Gramineae, Compositae, Rosaceae and Rutaceae. Among the major cereals maize, wheat, and pearl millet have apomictic relatives.
- Such plants are completely free from viruses. Plants that produce only apomictic embryos are known as obligate apomicts and those that produce both apomictic and sexual plants are called facultative apomicts.
Types of Apomixis:
- Recurrent Apomixis: In this type of apomixis, the embryo develops from diploid egg cells or without fertilization from diploid cells of the embryo sac. As a result, the egg has the same normal diploid number of chromosomes as the mother plant. Recurrent apomixis commonly occurs in species such as Parthenium, Rubus, Malus, Allium, Rudbeckia, Poa, Taraxacum, etc.
- Non-Recurrent Apomixis: In this case, the embryo develops either from a haploid egg cell or from some other haploid cells of the embryo sac. From this, haploid plants are produced, which have only one set of chromosomes from the mother plant. Therefore, haploid plants are sterile in nature and normally cannot be produced in the next generation. Non-recurrent apomixis occurs only in some species such as Solanum nigrum, Lilium etc.
- Nucellar Embryony or Adventitious Embryony: In this type of apomixis, embryos arise from diploid sporophytic cells i.e. cells of the nucellus, integuments, etc. outside the embryo sac. This type of apomixis is quite common in some varieties of citrus and mango, where fertilization occurs normally and multiple apomixis (nuclear) embryos develop simultaneously with the sexual.
- Vegetative apomixis or bulbils: In some species of plants, such as allium, agave, poa, etc., the flowers in the inflorescence are replaced by bulbils or vegetative buds, which also germinate when they are still on the mother plant and grow into new daughter plants.
- Ensures reproduction in extreme environments such as the absence of pollinators,
- Maternal energy is not wasted in unsuitable offspring (endurance of meiosis)
- Some apomictic plants (but not all) avoid the loss of male energy in pollen production.
- Cannot control the accumulation of harmful genetic mutations
- Usually confined to narrow ecological regions, lacking the ability to adapt to a changing environment
The term clone can be defined as a group of genetically identical individuals, the individual produced which is derived from one original form either sexually or by mutation and is maintained exclusively by asexual methods from one ancestor.
Bud- sports or bud mutations
When mutations occur and appear suddenly as an occurrence in a plant branch, it is called bud sport or bud mutation, as they appear to have arisen within the same bud.
When a mutation occurs within a single cell of a clone, it initially generates an ‘island’ of mutant cells within the growing point of a stem. The plant becomes a mixture of two different genotypes. This structural arrangement is known as a chimera.