Polyembryony – Types, Causes, and Importance

Polyembryony refers to the occurrence of multiple embryos developing within a single ovule, seed, or fertilized ovum, and it is observed in both animals and plants. Polyembryony can be influenced by environmental conditions and genetic factors. Environmental factors such as temperature, light, and nutrient availability can cause polyembryony in certain plant species. Polyembryony examples are seen in mango, citrus, and in gymnosperms.

Genetic factors, including specific gene mutations or variations, may also play a role in regulating polyembryony. In this article, we will look into the meaning, causes, types, examples, and significance of polyembryony.

What is Polyembryony in Plants?

Polyembryony is a phenomenon observed in some plants where multiple embryos develop from a single fertilized egg cell or multiple fertilized egg cells within the same seed. This results in the production of multiple offspring (seedlings) from a single seed. In several gymnosperms, polyembryony is so prevalent that it is considered a significant characteristic of this plant group.

In angiosperms (flowering plants), polyembryony is less common but still occurs in some species. For example, in citrus plants like oranges, multiple embryos can develop within a single seed.

Causes of Polyembryony

Polyembryony, the phenomenon where multiple embryos develop from a single fertilized egg or multiple fertilized eggs within the same seed, can be caused by various factors, including genetic, developmental, and environmental influences. Here are some of the major causes of polyembryony:

• Genetic Factors: Genetic mutations or variations in regulatory genes involved in embryo development can lead to the formation of multiple embryos from a single zygote.
• Developmental Abnormalities: Developmental abnormalities may arise due to genetic mutations, environmental stressors, or other factors disrupting normal developmental processes.
• Endosperm Patterning: Abnormalities in endosperm development can influence embryo development and may contribute to the formation of multiple embryos within a seed.
• Environmental Factors: Environmental stressors, such as temperature extremes, nutrient deficiencies, or pathogens, can sometimes trigger polyembryony as a response to stress.
• Pollination and Fertilization Mechanisms: Certain pollination and fertilization mechanisms, particularly in gymnosperms, may contribute to polyembryony. For example, multiple pollen grains or male gametes may fertilize multiple egg cells within the ovule, leading to the formation of multiple embryos.

Types of Polyembryony

Polyembryony can be divided into the following types:

• Induced Polyembryony: This type is induced experimentally, often in laboratory settings. It involves the controlled development of an embryo in a culture medium.
• Spontaneous Polyembryony: This occurs naturally within organisms without external intervention. Herbert John Webber, an American Plant Physiologist, further classified Spontaneous Polyembryony into three subtypes:
  • Cleavage Polyembryony: Cleavage Polyembryony occurs when a young embryo or zygote separates into two or more distinct units. These units then develop into independent embryos. Sometimes, this separation results from the proliferation of the proembryo, which is the series of cells in the ovule of a flowering plant after fertilization but before the formation of the embryo. This type is more common in gymnosperms and relatively rare in angiosperms. An example is Pinus, where the zygote undergoes two divisions to produce four nuclei.
  • Simple Polyembryony: Simple Polyembryony arises from the fertilization of more than one egg or from multiple archegonia, which are haploid structures producing female gametes. Pinus is also an example of Simple Polyembryony.
  • Rosette Polyembryony: Rosette Polyembryony occurs when additional embryos develop from rosette cells in gymnosperms.

Polyembryony in Mango

Polyembryony in mango is a fascinating phenomenon where multiple embryos develop within a single seed. Unlike most plants, where one seed produces one embryo, polyembryony in mangoes results in several genetically identical or fraternal embryos. This occurrence often leads to the production of multiple seedlings from a single seed, enhancing the chances of germination and survival for the mango tree. While the exact mechanisms behind polyembryony in mangoes are not fully understood, it’s believed to be influenced by genetic and environmental factors.

Polyembryony in Gymnosperms

Polyembryony in Gymnosperms includes conifers, cycads, ginkgoes, and genitals, and can manifest in several distinct types. Here are some of the different types of polyembryony observed in gymnosperms:

Polyembryony in Cycadales

• Cycadales, commonly known as cycads, are ancient gymnosperms characterized by their palm-like appearance.
• In Cycadales, polyembryony typically involves the formation of multiple embryos from a single fertilized egg cell.
• The additional embryos may arise through the process of cleavage or the division of the zygote into several cells, each of which can develop into an embryo.
• Polyembryony in Cycadales contributes to the production of multiple seedlings from a single seed, potentially increasing the chances of survival and dispersal for the species.

Polyembryony in Coniferales

• Coniferales, also known as conifers, include familiar trees such as pines, spruces, firs, and cedars.
• In Coniferales, polyembryony can occur through various mechanisms. One common mechanism involves the formation of multiple embryos from several fertilized egg cells within the same seed.
• Polyembryony in conifers can also result from the development of adventitious embryos from tissues other than the fertilized egg, such as the nucellus or integuments.
• Like in Cycadales, polyembryony in Coniferales contributes to the production of multiple seedlings from a single seed, potentially enhancing reproductive success and genetic diversity.

Polyembryony in Taxales

• Taxales, represented by the single extant genus Taxus (yews), are another group of gymnosperms.
• Polyembryony in Taxales is less common compared to some other gymnosperm groups, but it still occurs.
• In Taxales, polyembryony typically involves the formation of multiple embryos from a single fertilized egg cell or multiple fertilized egg cells within the same seed.
• As with other gymnosperms, polyembryony in Taxales can lead to the production of multiple seedlings from a single seed, potentially increasing the chances of successful seedling establishment.

Polyembryony in Gnetales

• Gnetales is a small group of gymnosperms that includes three genera: Gnetum, Ephedra, and Welwitschia.
• Polyembryony in Gnetales is relatively rare compared to other gymnosperm groups.
• In Gnetales, polyembryony can occur through the formation of multiple embryos from a single fertilized egg cell or from multiple fertilized egg cells within the same seed.
• While less common, polyembryony in Gnetales still contributes to the production of multiple seedlings from a single seed, which may enhance reproductive success and dispersal in these plants.

Examples of Polyembryony

Here are some examples of polyembryony in gymnosperms:

• Pinus spp. (Pines): Pines are well-known for exhibiting polyembryony. In some species of Pinus, such as Pinus pinaster (Maritime Pine) and Pinus thunbergii (Japanese Black Pine), cleavage polyembryony is observed.
• Cycas circinalis (Queen Sago): Cycas circinalis, a cycad species, exhibits simple polyembryony. Here, two adjacent archegonia within the same ovule may independently develop into two embryos, occasionally leading to the production of two seedlings.
• Gnetum spp.: Gnetum species, such as Gnetum gnemon, demonstrate high-order polyembryony. Several embryos may arise from each zygote through the branching of primary suspenders and further proliferation of secondary suspenders.
• Podocarpus spp.: Certain species of Podocarpus, a genus within the Podocarpaceae family, exhibit cleavage polyembryony. Each embryonal tetrad functions as an independent embryo in this type of polyembryony.

Difference Between Apomixis and Polyembryony

Apomixis and polyembryony are both phenomena related to plant reproduction, but they involve distinct mechanisms and outcomes:

Feature	Apomixis	Polyembryony
Type of Reproduction	Asexual reproduction	Can occur in sexually reproducing plants
Mechanism	Embryo develops without fertilization; involves parthenogenesis, adventitious embryony, or apogamy	Multiple embryos develop from fertilized egg cells
Genetic Diversity	Offspring are genetically identical to parent plant	Offspring may be genetically identical or diverse
Occurrence	Can occur in various plant taxa	Can occur in a wide range of plant species and may be induced by genetic, developmental, or environmental factors
Seed Development	Seeds develop without pollination or fertilization	Seeds typically develop after fertilization, with multiple embryos
Reproductive Advantages	Ensures genetic uniformity; bypasses the need for pollination and fertilization	May increase genetic diversity; provides multiple offspring from a single seed
Examples	Certain species of grasses, dandelions, and citrus fruits	Cycads, conifers, and various angiosperms

Importance of Polyembryony in Plants

Polyembryony can have ecological and evolutionary implications. It can contribute to increased genetic diversity within populations by producing multiple genetically identical or slightly varied offspring from a single parent. This diversity may enhance the adaptability and resilience of populations to changing environmental conditions and stresses. For example, in parasitic wasps, it allow for the production of numerous offspring from a single egg, increasing reproductive efficiency.

In plants, it can influence genetic diversity within populations and impact traits related to seedling vigor and survival. Overall, polyembryony is a fascinating biological phenomenon that highlights the diverse reproductive strategies evolved by different organisms.

Conclusion – Polyembryony – Types, Causes, and Importance  

Overall, polyembryony is a fascinating biological phenomenon with implications spanning ecology, evolution, agriculture, and conservation. Further research into the mechanisms and ecological significance of polyembryony promises to deepen our understanding of reproductive diversity and strategies in the natural world.

FAQs on Polyembryony – Types, Causes, and Importance  

What is Polyembryony, and how does it Manifest in Gymnosperms?

Polyembryony refers to the phenomenon where multiple embryos develop from a single fertilized egg or within a single seed. In gymnosperms, this can occur through mechanisms such as cleavage polyembryony.

How does Polyembryony Contribute to the Reproductive Strategy of Gymnosperms?

Polyembryony enhances the reproductive efficiency of gymnosperms by increasing the number of potential offspring produced from a single reproductive event.

What are some Examples of Gymnosperms that Exhibit Polyembryony?

Examples include Pinus spp. (pines), which demonstrates both cleavage and simple polyembryony and Gnetum spp., which exhibits high-order polyembryony through suspender branching.

What is False Polyembryony and True Polyembryony?

False polyembryony occurs when two or more nucelli fuses or two or more embryo sacs develop within the same nucellus. In true polyembryony, additional embryos develop in the embryo sac through zygote cleavage or from synergids and antipodal cells.

How does the Study of Polyembryony in Gymnosperms Contribute to Conservation and Forestry Practices?

Understanding polyembryony in gymnosperms is crucial for conservation efforts, as it informs us about strategies for seed collection, propagation, and reintroduction of endangered species.

What is Polyembryony Class 12?

Polyembryony is a botanical phenomenon where multiple embryos develop within a single seed, typically observed in plants like mangoes. This unique trait enhances germination rates and genetic diversity, helping in propagation and cultivation efforts.