Nucellar embryony is one of the strangest botanical phenomenons that anyone can observe, right in their own home. Essentially it’s a form of seed reproduction where the mother plant creates additional embryos inside the seed, and these extra embryos are clones of the mother plant.
During the germination of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the embryo sac in the ovule can produce additional embryos (polyembryony) which are genetically identical to the parent plant. These nucellar seedlings are essentially clones of the parent [1].
Diagram comparing monoembryonic and polyembryonic seeds, and the resulting number of seedlings. Adapted from [?]
To really spell it out again with an example (because I think if you’re like me, when you first learn about this it seems impossible). If you grew a (for example) Valencia orange seedling from seed, you would 95% of the time, get the exact same Valencia orange variety back out again.
It’s worth noting that nucellar embryony isn’t “on” or “off” – it’s a tendency, and so some plants never do it, some rarely, some half the time, some most of the time, some almost all of the time, etc.
But what does this mean for gardeners?
Breeding new Citrus varieties can be challenging, because many Citrus plants when used as mother plants, will simply appear again as most of the seedlings.
On the flipside, importing new Citrus varieties is easier, because you simply need to grow the seeds and wait until fruiting, and then you’ll have the exact variety again.
Nucellar embryony is a completely separate phenomenon to “breeding true” in for example stonefruit, where there is very minor variation among the seedlings. In that case, the uniformity in the seedlings is related to the genome having high homozygosity, which means that for most genes, both copies of that gene on each chromosome are the same, and so there’s essentially no opportunity to get a different combination. By contrast when the genes are heteozygous (different copies on each chromosome), then there are 3 ways for those 2 genes to combine (aa, aA, AA).
In Citrus
The most well-known examples of nucellar embryony in flowering plants are found in the Citrus genus, where it is found in approximately half of all Citrus varieties. Here’s a short mostly accurate list (will be updated and more references provided later).
Highly Monoembryonic
All pomelos
All finger limes
All citrons*
Nagami kumquat
Makrut lime
NZ Grapefruit (e.g. Golden Special)
Persian lime
Bergamot
Meyer lemon
Most Clementines
Some mandarins (Encore)
Intermediate
Volkamer lemon
Limequat (Eustis)
NZ Lemonade?
Highly Polyembryonic
All sweet oranges
All true grapefruits
Seville / bitter orange
Calamansi / Calamondin
Rangpur
Eureka lemons
Lisbon lemons
Rough lemons (Kaipara lemon)
Yuzu
Trifoliata
Swingle citrumelo
C-35 citrange
Furr C-57 citrandarin
Alemow
Why the asterisk for citrons? Well as I referred to briefly in the Introduction, some plants are highly homozygous (both copies of each gene are the same, in most cases), which means when they self-fertilize themselves the resulting offspring have very little opportunity to receive a new genetic combination. Citrons are both highly homozygous and cleistogamous (self-pollinates itself before the flowers open). So citron seedlings are almost identical to the mother tree, but through the combination of high homozygosity and cleistogamy.
