Polyembryony is the phenomenon of two or moreembryos developing from a single fertilized egg.^[1][2][3] Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents.^[1][2] The genetic difference between the offspring and the parents, but the similarity among siblings, are significant distinctions between polyembryony and the process ofbudding and typicalsexual reproduction.^[2] Polyembryony can occur in humans, resulting inidentical twins, though the process is random and at a low frequency.^[1] Polyembryony occurs regularly in many species ofvertebrates,invertebrates, andplants.

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.^[4] 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.^[5][4]

The potential advantages of polyembryony in competing invasive plant species has been studied as well.^[6]

Armadillos are the most well studied vertebrate that undergoes polyembryony, with six species of armadillo in the genusDasypus that are always polyembryonic.^[7] Thenine 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.^[2] It is thought that nine banded armadillos evolved to be polyembryonic because of the latter.^[7]

Chondrichthyans, namely the skatesBeringraja binoculata andBeringraja pulchra, in addition tospotted catsharks, have exhibited multiple embryos in a singleegg case.^[8]

A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoidHymenoptera, familyEncyrtidae.^[9] 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.^[9] These latter larvae patrol the host and kill any other parasitoids they find with the exception of their siblings, usually sisters.^[9]

Obligately polyembryonic insects fall in two classes: Hymenoptera (certain wasps), andStrepsiptera.^[10] From one egg, these insects can produce over thousands of offspring.^[10] Polyembryonic wasps from the Hymenoptera group can be further subdivided into four families includingBraconidae (Macrocentrus),Platygastridae (Platygaster),Encyrtidae (Copidosoma), andDryinidae.^[10]

Polyembryony also occurs inBryozoa.^[2][11] Through genotype analysis and molecular data, it has been suggested that polyembryony happens in the entire bryozoan orderCyclostomatida.^[11]

The term is also used inbotany to describe the phenomenon of multipleseedlings emerging from one embryo. Around 20 genera ofgymnosperms undergo polyembryony, termed "cleavage polyembryony," where the originalzygote splits into many identical embryos.^[1][3] In some plant taxa, the many embryos of polyembryony eventually gives rise to only a single offspring.^[1] The mechanism underlying the phenomenon of a resulting single (or in some cases a few) offspring is described inPinus sylvestris to beprogrammed cell death (PCD), which removes all but one embryo.^[1] Originally, all embryos have equal opportunity to develop into full seeds, but during the early stages of development, one embryo becomes dominant throughcompetition, and therefore the nowdormant seed, while the other embryos are destroyed through PCD.^[1]

The genusCitrus has a number of species that undergo polyembryony, where multiple nucellar-cell-derived embryos exist alongside sexually-derived embryos.^[12][13]Antonie van Leeuwenhoek first described polyembryony in 1719 when the seed inCitrus was observed to have two germinating embryos.^[3] InCitrus, polyembryony is genetically controlled by a shared polyembryony locus among the species, determined bysingle-nucleotide polymorphism in the genotypes sequenced.^[12] The variation within the species of citrus is based on the amount of embryos that develop, the impact of the environment, and gene expression.^[13] As with other species, due to the many embryos developing in close proximity, competition occurs, which can cause variation in seed success or vigor.^[13]

• Monoembryony

• Juan Manuel Alvarez A. (15 April 1997)."Chapter 26 — Largest Parasitoid Brood".Book of Insect Records.University of Florida.
• J. Bronté Gatenby (1918)."Memoirs: Polyembryony in Parasitic Hymenoptera: A Review".Quarterly Journal of Microscopical Science. s2-63:175–196.
• J. Bronté Gatenby (1918)."The Segregation of the Germ-cells inTrichogramma evanescens".Quarterly Journal of Microscopical Science. s2-63:161–174.
• Craig, S.F.; Slobodkin, L.B.; Wray, G. (1997). "The 'Paradox' of Polyembryony: A review of the cases and a hypothesis for its evolution".Evolutionary Ecology.11 (2):127–143.Bibcode:1997EvEco..11..127C.doi:10.1023/A:1018443714917.S2CID 5556785.