Polyembryony refers to the biological phenomenon where two or more embryos develop from a single fertilized egg. This process occurs in both the plant and animal kingdoms, leading to the formation of genetically identical offspring. While sometimes observed in humans, it is a more common reproductive strategy found across various other species.
Natural Occurrences
Polyembryony is observed widely in nature. A well-known animal example is the nine-banded armadillo, which consistently produces litters of four genetically identical young from a single fertilized egg. This specific reproductive pattern makes them a unique subject for studying developmental biology. In the insect world, parasitic insects like certain parasitoid wasps demonstrate an even more extensive form of this process, where a single egg can give rise to thousands of individual offspring.
In the plant kingdom, polyembryony also presents in diverse forms. Some gymnosperms exhibit “cleavage polyembryony,” where a single embryo divides into multiple distinct embryos. The genus Citrus provides another example, often developing multiple embryos within a single seed. These can include embryos derived from nucellar cells, existing alongside embryos that result from sexual reproduction.
How Polyembryony Develops
Polyembryony can arise through different mechanisms, broadly categorized as spontaneous or induced. Spontaneous polyembryony occurs naturally without external intervention, as seen in the armadillo. Induced polyembryony, conversely, is triggered by specific environmental factors or experimental manipulations.
Webber’s classification system further details these developmental pathways. Cleavage polyembryony involves the splitting of a single proembryo or young embryo into several independent embryos, each capable of developing into a complete organism. Simple polyembryony describes the formation of multiple embryos within the same ovule, but from different origins, such as several egg cells or synergid cells. Rosette polyembryony is characterized by the development of accessory embryos from suspensor cells.
The underlying causes of polyembryony are explained by various theories. The necrohormone theory suggests that the breakdown products of degenerating cells can stimulate the formation of additional embryos. The hybridization theory proposes that the interbreeding of different species can disrupt normal developmental processes, leading to polyembryony. These theories explain how multiple embryos can arise.
Why Polyembryony Matters
Polyembryony offers advantages, particularly in an evolutionary context for certain species. In parasitoid wasps, for instance, it allows for an increase in offspring numbers from a single egg, which is especially beneficial for smaller females who might have limited egg-laying capacity. This reproductive strategy maximizes reproductive output and aids species propagation.
Beyond evolutionary benefits, polyembryony holds practical significance in agriculture and plant breeding. The genetic uniformity of polyembryonic offspring is valuable, as it maintains desirable traits across generations. This characteristic is useful in propagating fruit trees like citrus, where growers can ensure that new trees retain the genetic makeup of a parent plant. This supports uniform crop production and cultivar preservation.
Polyembryony Compared to Apomixis
While both polyembryony and apomixis involve forms of asexual reproduction in plants, they represent distinct biological processes. Polyembryony is the development of two or more embryos from a single fertilized egg or from associated tissues, such as nucellar cells. This results in multiple plantlets emerging from one seed.
Apomixis, by contrast, is the formation of seeds without fertilization. In apomixis, the embryo develops directly from an unfertilized egg cell or other maternal cells within the ovule, bypassing the need for pollen. The difference lies in the initiation: polyembryony starts with one or more embryos from a fertilized ovule, while apomixis produces embryos without sexual fusion.