Twin embryo formation by induced parthenogenesis.

key messageInduction of parthenogenesis (embryo formation from unfertilized egg cells) by embryogenic transcription factors is associated with twin formation at high frequencies, and involves two distinct mechanisms. Synthetic apomixis has been achieved through the induction of parthenogenesis by ectopic expression of the Baby Boom family of transcription factors. An associated phenomenon from this process is the formation of polyembryony including twin progeny at high frequencies, but the underlying mechanisms have not been explored. Here, we provide a brief description of the phenomenon, discuss potential mechanisms for twin formation in flowering plants, propose two possible models for their occurrence, and evaluate the available evidence from both dizygotic and monozygotic twins in relation to these models. The two proposed models are independent, but they can operate in combination. We conclude that both models are required to explain the types of twins and triplets that we and others have observed. These models provide future directions for basic research, as well as suggest possible approaches towards reducing polyembryony when incorporating synthetic apomixis into crop plants such as maize where twinning is not desirable.