Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Polyembryony occurs regularly in many plants and animals, and humans as well. The nine banded armadillo, for instance, usually gives birth to four identical young. Polyembryony is best known among parasitoid insects of the order Hymenoptera, families Encyrtidae, Dryinidae, Platygastridae and Braconidae. The term is also used in botany to describe the phenomenon of two seedlings emerging from one seed.
In plants, polyembryony often gives rise to the enigma of a single offspring. The mechanism underlying this phenonomenon is programmed cell death (PCD) which removes all but one embryo.
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.
Polyembryony also occurs in Bryozoa.