Polyembryony is the phenomenon of two or more embryos 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 and typical sexual reproduction. 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 vertebrates, invertebrates, and plants.
Armadillos are the most well studied vertebrate that undergoes polyembryony, with six species of armadillo in the genus Dasypus 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.
A more striking example of the use of polyembryony as a competitive reproductive tool is found in the parasitoid Hymenoptera, 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. 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 (Macrocentrus), Platygasteridae (Platygaster), Encyrtidae (Copidosoma), and Dryinidae.
The term is also used in botany to describe the phenomenon of seedlings emerging from one embryo. Around 20 genera of gymnospores undergo polyembryony, termed "cleavage polyembryony," where the original zygote 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 to be programmed cell death (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, and therefore the now dormant seed, while the other embryos are destroyed through PCD.
The genus Citrus has a number of species that undergo polyembryony, where multiple nucellar-cell-derived embryos exist alongside sexually-derived embryos. Antonie van Leeuwenhoek 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 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.
- Filonva, L. H.; von Arnold, S.; Daniel, G.; Bozhkov, P. V. (2002). "Programmed cell death eliminates all but one embryo in a polyembryonic plant seed". Nature. 9 (10): 1057–1062. doi:10.1038/sj.cdd.4401068. PMID 12232793.
- Craig, Sean F.; Slobodkin, Lawrence B.; Wray, Gregory A.; Biermann, Christiane H. (1997-03-01). "The 'paradox' of polyembryony: A review of the cases and a hypothesis for its evolution". Evolutionary Ecology. 11 (2): 127–143. doi:10.1023/A:1018443714917. ISSN 0269-7653.
- Batygina, T. B.; Vinogradova, G. Iu (2007-05-01). "[Phenomenon of polyembryony. Genetic heterogeneity of seeds]". Ontogenez. 38 (3): 166–191. ISSN 0475-1450. PMID 17621974.
- Loughry, W. J.; Prodöhl, Paulo A.; McDonough, Colleen M.; Avise, John C. (1 January 1998). "Polyembryony in Armadillos: An unusual feature of the female nine-banded armadillo's reproductive tract may explain why her litters consist of four genetically identical offspring". American Scientist. 86 (3): 274–279. doi:10.1511/1998.25.824. JSTOR 27857027.
- E., Beckage, Nancy (1997-01-01). Parasites and pathogens : effects on host hormones and behavior. Chapman & Hall. ISBN 978-0412074011. OCLC 875319486.
- Strand, Michael (2009). Encyclopedia of Insects (2nd edition). Oxford, UK: Elsevier.
- Jenkins, Helen L.; Waeschenbach, Andrea; Okamura, Beth; Hughes, Roger N.; Bishop, John D. D. (2017-01-17). "Phylogenetically Widespread Polyembryony in Cyclostome Bryozoans and the Protracted Asynchronous Release of Clonal Brood-Mates". PLoS ONE. 12 (1): e0170010. doi:10.1371/journal.pone.0170010. ISSN 1932-6203. PMC 5240946. PMID 28095467.
- Nakano, Michiharu; Shimada, Takehiko; Endo, Tomoko; Fujii, Hiroshi; Nesumi, Hirohisa; Kita, Masayuki; Ebina, Masumi; Shimizu, Tokurou; Omura, Mitsuo (2012-02-01). "Characterization of genomic sequence showing strong association with polyembryony among diverse Citrus species and cultivars, and its synteny with Vitis and Populus". Plant Science. 183: 131–142. doi:10.1016/j.plantsci.2011.08.002. ISSN 1873-2259. PMID 22195586.
- Kishore, Kundan; N., Monika; D., Rinchen; Lepcha, Boniface; Pandey, Brijesh (2012-05-01). "Polyembryony and seedling emergence traits in apomictic citrus". Scientia Horticulturae. 138: 101–107. doi:10.1016/j.scienta.2012.01.035.
- 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 in Trichogramma 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. doi:10.1023/A:1018443714917.