Polyembryony

What is Polyembryony?

An exciting phenomenon in plants occurs in which a single seed or ovule gives rise to several embryos. This process has great implications for reproduction in plants, agriculture, and plant development studies. Polyembryony helps to understand the plasticity and adaptability of a species of plant.

Polyembryony is the development of two or more embryos in a single seed or ovule. The process can be induced artificially in vitro culture, but it also occurs naturally in some plant species. More than one embryo develops from a single seed or ovule, forming genetically different individuals.

Characteristics of Polyembryony

Polyembryony involves the development of multiple embryos within a single seed. These embryos may arise from the same zygote or different maternal tissues like the nucellus. Depending on their origin, the polyembryony would lead to genetic variation among the resultant embryos.

Asexual Reproduction: In a way, polyembryony can be seen as falling under the category of asexual reproduction because this process makes it possible for an organism to produce offspring that are genetically identical without using gametes.

Importance in Agriculture: There are many uses of polyembryony in agriculture, especially in clonal planting material production and cultivar development.

Types of Polyembryony

It is mainly classified into zygotic, nucellar, and adventitious polyembryony. Polyembryony can result through different mechanisms, leading to the formation of multiple embryos within a single seed or ovule:

Zygotic Polyembryony

In zygotic polyembryony, the division of the zygote leads to the formation of two or more embryos that result in genetically identical individuals. This kind of polyembryony is frequent in citrus fruits and in some other plant species.

Nucellar Polyembryony

Nucellar polyembryony consists of the development of embryos from the nucellar tissue surrounding the embryo sac. These embryos are genetically identical to the maternal plant since they originate from somatic cells. Nucellar polyembryony is found in numerous plants of citrus fruits, mango, and a few other tropical and subtropical plants.

Adventitious Polyembryony

Adventitious embryony occurs when additional embryos develop from the remaining parts of the ovule, such as the integuments or the suspensor. These embryos may or may not be genetically identical to the primary embryo or each other. Examples of plants with this type of polyembryony are some orchids and ferns.

Importance of Polyembryony

One of the most important advantages of this phenomenon is that polyembryony enables the generation of offspring with similar genetic material. This can, therefore, be utilised in crops for the propagation of identical genomes carrying desirable features.

Genetic Diversity: Still, on occasions, polyembryony may lead to variations between the formed embryos, hence offering the advantages of adaptation to plant species.

Embryological Studies: Polyembryony offers scope for the studies of plant embryology and the mechanisms of development of embryos.

Crop Improvement: The knowledge of polyembryony can be utilised to create new crop forms with such deliberate characteristics as high yield or disease resistance.

Problems and Limitations of Polyembryony

Polyembryony may restrict selective breeding, complicate hybrid seed production, and reduce genetic diversity due to uniformity in offspring. The various limitations of polyembryony is discussed below-

Unequal Development: In some cases, the several embryos included in a seed or ovule may not be equally developed, and consequently may be of different sizes and very unequal in vigour.

Low Seed Viability: This polyembryony may sometimes depress the overall viability of seeds because the seed's resources are divided among the many embryos.

Difficulty of Identification: Genetically different embryos are hard to distinguish from clonal ones, especially in the early stages of development.

MCQs on Polyembryony

Q1. In many citrus fruits, poly-embryonic seed formation takes place where one or two nucellar cells start growing as embryos along with the zygotic embryo. As a result, seed germinates to give rise to as many as two or three plantlets. If one wants to grow the plant with the same characteristics as the fruit from which the seed is obtained, which of the plantlets can be used?

Option 1: The one from the zygotic embryo is the best choice as it has hybrid vigour.

Option 2: The nucellus embryo plantlet is a clone of the mother plant.

Option 3: As there will be competition for survival among the three plantlets, the most healthy and largest in size is the best choice.

Option 4: Any of the three as all will have the same phenotypes.

Correct answer: 2) The nucellus embryo plantlet is a clone of the mother plant.

Explanation:

In the case of poly-embryonic seed formation in citrus fruits, where multiple embryos develop within a single seed, the nucellus embryo plantlet can be used to grow a plant with the same characteristics as the mother plant.

The nucellus embryo is derived from the maternal tissue, specifically the nucellus, rather than the fertilization process. Therefore, it is genetically identical or very similar to the mother plant, making it a clone. By using the nucellus embryo plantlet, you can ensure that the resulting plant will have the same characteristics as the fruit from which the seed was obtained.

In contrast, the zygotic embryo is the result of fertilization and contains genetic material from both the male and female parents. It may not have the same characteristics as the mother plant, as genetic variation can occur due to genetic recombination during fertilization.

Therefore, if you want to grow a plant with the same characteristics as the fruit from which the seed is obtained in the case of poly-embryonic citrus fruits, it is preferable to use the nucellus embryo plantlet.

Hence, the correct answer is option 2. The nucellus embryo plantlet is a clone of the mother plant.

Q2. Nucellar polyembryony is reported in species of

Option 1: Citrus

Option 2: Gossypium

Option 3: Triticum

Option 4: Brassica

Correct answer: 1) Citrus

Explanation:

Polyembryony refers to the presence of more than one embryo within a single seed. In plants like Citrus and mango, certain nucellar cells surrounding the embryo sac undergo division and develop into additional embryos, alongside the zygotic embryo. This phenomenon, known as nucellar polyembryony, results in ovules containing multiple embryos, a characteristic feature of these species. Polyembryony has practical significance in horticulture as it often produces genetically identical seedlings from nucellar embryos.

Hence, the correct answer is option 1) Citrus

Q3. Which of the following is true about polyembryony?

Option 1: It is a type of asexual reproduction in plants.

Option 2: It results in the formation of genetically identical offspring.

Option 3: It involves the development of seeds without fertilisation.

Option 4: A single fertilised egg gives rise to multiple embryos in polyembryony.

Correct answer: 4) A single fertilised egg gives rise to multiple embryos in polyembryony.

Explanation:

Polyembryony is a phenomenon where a single fertilised egg gives rise to multiple embryos in polyembryony. It is a type of reproductive strategy found in some plants, where the fertilised egg divides multiple times to form multiple embryos, resulting in the development of multiple seedlings from a single seed.

Hence, the correct option is 4) A single fertilised egg gives rise to multiple embryos in polyembryony.

Recommended video on Polyembryony