Apomixis and polyembryony

Apomixis means forming seeds without fertilisation; it occurs in some species of Asteraceae and grasses. It is asexual reproduction that imitates the sexual route.

In some species the egg cell forms without reduction division, so it is diploid, and it develops into an embryo without fertilisation.

More often, as in many Citrus and mango varieties, some nucellar cells around the embryo sac divide, push into the embryo sac and develop into embryos, so one ovule holds many embryos.

Polyembryony is the occurrence of more than one embryo in a seed; squeezing an orange seed shows several embryos of different sizes. Nucellar embryos come from the parent's diploid tissue, so they are genetically identical to it: clones.

Hybrid seed has to be bought every year, because seed saved from a hybrid gives progeny that segregate and lose the hybrid characters, and producing hybrid seed is costly.

If hybrids were made apomictic, the hybrid progeny would not segregate and farmers could reuse their own seed year after year. This is why laboratories are studying the genetics of apomixis and trying to transfer apomictic genes into hybrid varieties.