Polyembryony

Polyembryony is defined as the occurrence of more than one embryo within a single seed, and its first documented case - in certain oranges - was reported by Anton van Leeuwenhoek as far back as 1719. Based on the origin of the extra embryo(s), polyembryony is divided into four categories: cleavage polyembryony, in which a single zygotic embryo itself splits during early development to give rise to more than one embryo (as in Orchids); formation of an embryo from a cell of the embryo sac other than the egg, for instance from a synergid (Aristolochia), an antipodal cell (Ulmus, illustrated in Figure 1.24, where a normal zygotic embryo develops alongside a second embryo arising from an antipodal cell) or even from endosperm tissue (Balanophora); development of more than one embryo sac within a single ovule, whether derived from the same megaspore mother cell or from two or more separate megaspore mother cells (as in Casuarina); and activation of sporophytic cells of the ovule itself, specifically the nucellus or integuments, to form additional embryos (Citrus, Syzygium) - this last category overlaps with the adventive embryony described under apomixis. Polyembryony has two recognised practical applications: seedlings raised from the nucellar-derived …