Polyembryony

Polyembryony is the occurrence of more than one embryo developing within a single seed, a phenomenon first recorded by the early microscopist Antonie van Leeuwenhoek in 1719, in seeds of the citrus genus. Because each of these multiple embryos can independently give rise to a seedling upon germination, a single polyembryonic seed is capable of producing several separate plants at once.

The additional embryos found in a polyembryonic seed arise through the differentiation and development of various maternal or zygotic tissues associated with the ovule, and the condition is described as either 'true' or 'false' polyembryony depending on whether the multiple embryos arise within the same embryo sac or in physically different embryo sacs of the same ovule.

Two principal mechanisms generate polyembryony:

• Adventive polyembryony: Additional embryos develop directly from diploid somatic cells of the nucellus or the integuments, entirely outside the normal embryo sac lineage, growing alongside the normal zygotic embryo, as seen in Citrus. This nucellar, adventive form of polyembryony is of particular horticultural significance, since the extra embryos are genetically identical to the maternal parent and so give rise to true-to-type seedlings.
• Cleavage polyembryony: The zygote-derived proembryo itself divides (cleaves) into several separate parts or units at an early stage, and each such unit subsequently develops independently into its own complete embryo. …