Polyembryony is the phenomenon of two or more

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

s developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic difference between the offspring and the parents, but the similarity among siblings, are significant distinctions between polyembryony and the process of

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

and typical

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

. Polyembryony can occur in humans, resulting in identical twins, though the process is random and at a low frequency. Polyembryony occurs regularly in many species of

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, and

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

Evolution of polyembryony

The evolution of polyembryony and the potential evolutionary advantages that may entail have been studied. In parasitoid wasps, there are several hypotheses surrounding the evolutionary advantages of polyembryony, one of them being that it allows female wasps that are small in size to increase the number of potential offspring in comparison to wasps that are mono embryonic. There are limitations to monoembryony, but with this method of development, multiple embryos can be derived from each of the individual eggs that are laid. The potential advantages of polyembryony in competing invasive plant species has been studied as well.

Vertebrates

Armadillo Armadillos () are New World placental mammals in the order (biology), order Cingulata. They form part of the superorder Xenarthra, along with the anteaters and sloths. 21 extant species of armadillo have been described, some of which are dis ...

s are the most well studied vertebrate that undergoes polyembryony, with six species of armadillo in the genus ''

Dasypus ''Dasypus'' is the only extant genus in the family Dasypodidae. Its species are known as long-nosed or naked-tailed armadillos. They are found in South America, South, Central America, Central, and North America, as well as on the Caribbean islan ...

'' that are always polyembryonic. The nine banded armadillo, for instance, always gives birth to four identical young. There are two conditions that are expected to promote the evolution of polyembryony: the mother does not know the environmental conditions of her offspring as in the case of parasitoids, or a constraint on reproduction. It is thought that nine banded armadillos evolved to be polyembryonic because of the latter. Chondrichthyans, namely the skates ''Beringraja binoculata'' and ''Beringraja pulchra'', in addition to spotted catsharks, have exhibited multiple embryos in a single egg case.

Invertebrates

A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid

Hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

, family Encyrtidae. The progeny of the splitting embryo develop into at least two forms, those that will develop into adults and those that become a type of soldier, called precocious larvae. These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters. Obligately polyembryonic insects fall in two classes: Hymenoptera (certain wasps), and

Strepsiptera The Strepsiptera () are an order of insects with eleven extant families that include about 600 described species. They are endoparasites of other insects, such as bees, wasps, leafhoppers, Zygentoma, silverfish, and cockroaches. Females of most s ...

. From one egg, these insects can produce over thousands of offspring. Polyembryonic wasps from the Hymenoptera group can be further subdivided into four families including

Braconidae The Braconidae are a family of parasitoid wasps. After the closely related Ichneumonidae, braconids make up the second-largest family in the order Hymenoptera, with about 17,000 recognized species and many thousands more undescribed. One analysis ...

('' Macrocentrus''),

Platygastridae The hymenopteran family Platygastridae (sometimes incorrectly spelled Platygasteridae) is a moderate-sized group (about 2000 described species) of exclusively parasitoid wasps, mostly very small (1–2 mm), black, and shining, with genicul ...

('' Platygaster''), Encyrtidae ('' Copidosoma''), and Dryinidae. Polyembryony also occurs in

Bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

. Through genotype analysis and molecular data, it has been suggested that polyembryony happens in the entire bryozoan order

Cyclostomatida Cyclostomatida, or cyclostomata (also known as cyclostomes), are an ancient order of stenolaemate bryozoans which first appeared in the Lower Ordovician. It consists of 7+ suborders, 59+ families, 373+ genera, and 666+ species. The cyclostome ...

Plants

The term is also used in

botany Botany, also called plant science, is the branch of natural science and biology studying plants, especially Plant anatomy, their anatomy, Plant taxonomy, taxonomy, and Plant ecology, ecology. A botanist or plant scientist is a scientist who s ...

to describe the phenomenon of multiple

seedling A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embry ...

s emerging from one embryo. Around 20 genera of

gymnosperms The gymnosperms ( ; ) are a group of woody, perennial Seed plant, seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include Pinophyta, conifers, cycads, Ginkgo, and gnetoph ...

undergo polyembryony, termed "

cleavage Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

polyembryony," where the original

zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...

splits into many identical embryos. In some plant taxa, the many embryos of polyembryony eventually gives rise to only a single offspring. The mechanism underlying the phenomenon of a resulting single (or in some cases a few) offspring is described in ''

Pinus sylvestris ''Pinus sylvestris'', the Scots pine (UK), Scotch pine (US), Baltic pine, or European red pine is a species of tree in the pine family Pinaceae that is native plant, native to Eurasia. It can readily be identified by its combination of fairly s ...

'' to be

programmed cell death Programmed cell death (PCD) sometimes referred to as cell, or cellular suicide is the death of a cell (biology), cell as a result of events inside of a cell, such as apoptosis or autophagy. PCD is carried out in a biological process, which usual ...

(PCD), which removes all but one embryo. Originally, all embryos have equal opportunity to develop into full seeds, but during the early stages of development, one embryo becomes dominant through

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

, and therefore the now dormant seed, while the other embryos are destroyed through PCD. The genus ''

Citrus ''Citrus'' is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

'' has a number of species that undergo polyembryony, where multiple nucellar-cell-derived embryos exist alongside sexually-derived embryos.

Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch art, science and technology. A largely self-taught man in science, he is commonly known as " ...

first described polyembryony in 1719 when the seed in ''Citrus'' was observed to have two germinating embryos. In ''Citrus'', polyembryony is genetically controlled by a shared polyembryony locus among the species, determined by

single-nucleotide polymorphism In genetics and bioinformatics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a ...

in the genotypes sequenced. The variation within the species of citrus is based on the amount of embryos that develop, the impact of the environment, and gene expression. As with other species, due to the many embryos developing in close proximity, competition occurs, which can cause variation in seed success or vigor. Syzygium Polyembryony seeds

References

External links