I didn’t see a thread specifically on the topic of seeds that are “true to type” via nucellar embryony. I am mostly familiar with the phenomenon in connection with mangos and citrus, but I assume there are other plants that sometimes produce such seeds?
My understanding is that when a citrus or mango seed is polyembryonic, then at most one of the embryos is zygotic (i.e., the product of pollination and not genetically identical to mother plant). Is this correct, or can there be multiple zygotic embryos in a polyembryonic seed? Do polyembryonic seeds ever lack a zygotic embryo entirely?
Also, I can’t seem to find much information about whether monoembryonic seeds are ever nucellar in cultivars with both mono- and polyembryonic seeds. Does nucellar embryony always produce seeds with multiple embryos, or can it sometimes result in a seed with only one (nucellar) embryo?
That says “a zygotic embryo can divide to produce multiple embryos” but the source supporting that doesn’t really discuss it outside a very specific type of citrus, and it’s unclear if that is commonplace or rare.
Look up the seed structure in beets. A single “seed” contains about 10 embryos, all to my knowledge zygotic. The point to be made is that some species undergo modified embryogenesis resulting in a cluster of embryos. Here are the things that I have read:
Multiple clonal embryos develop in the seed of some species, citrus being a prime example.
Most of the time, clonal embryos are more vigorous than zygotic embryos. I’d love to see proof of this!
A typical citrus seed contains about 8 embryos of which 3 or 4 will typically develop if conditions are favorable.
I can extrapolate a few things from the information available. The process for developing clonal embryos and zygotic embryos must be very similar with critical differences vs other species at the cellular super-cluster stage of development. In most plants, a single cell divides via mitosis into 2 cells, then 4, then 8, then 16. At the 16 cell stage, it is referred to as a super-cluster. Within the super-cluster, and this varies by species, either all 16 cells undergo meiosis forming 32 haploid reproductive cells, or a small group either 4 or 8 cells undergoes meiosis forming haploid reproductive cells. In a normal embryo, 8 of the haploid cells form the proto-embryo with 1 haploid cell prepared to receive pollen, 2 haploid cells joined to become the proto-endosperm, and 5 more cells that perform specific functions in the process of pollination and embryo development. When a pollen grain is deposited on the pistil, it is composed of 2 haploid cells which split with 1 merging with the endosperm and the other fertilizing the zygotic female cell to become the embryo. The 5 “helper” cells variously disappear after fertilization, the endosperm proceeds to accumulate starch and nutrients to feed the embryo when it germinates, and the embryo goes into a resting stage awaiting favorable growing conditions. Here is the speculation. I suspect nucellar embryos occur at the super-cluster stage of seed development. If so, some number of the super-cluster cells must abort the process of producing haploid reproductive cells and instead must form a nucellar embryo. Dig deep enough in the literature and a description similar to this likely will be found.
Having collected beet seeds I think the idea that each seed contains multiple embryos is kind of a layman way of looking at it. In reality what we think of as the beet “seed” is the whole seed pod. Because it’s not practical to break it apart to separate the actual seeds inside it’s easier to just call the whole pod a seed.
Thank you for the reply. The “how it works” stuff is very fascinating, but I’m afraid tends to go slightly over my head. I guess my question was a little more along the lines of this:
If I grow multiple seedlings from a single polyembryonic seed, can I be reasonably certain that one or more of those seedlings are from nucellar embryos?
In particular, I have three seedlings of a “TDE” mandarin that all came from the same seed, and two seedlings of an Ataulfo mango, both from the same seed. So I’m expecting that at most, one of each of those could be zygotic, but one or more are nucellar. I’m not sure there’s any practical way to know, though.
Yes, there is a way to know, but it is not what you think. Separate the plantlets and watch them as they grow. If one is zygotic and the others are clonal, there will come a point where one of them grows different than the others. As I stated above, it is very common for zygotic embryos to be less vigorous than clonal embryos. It can also happen that the zygotic embryo is more vigorous. With citrus, you can also tell to some extent by juvenile traits in zygotic plants. They often have different expression of chlorophyll, anthocyanin, or physical traits such as thorns. Nucellar embryos often retain the adult characteristics where thorns are minimal and anthocyanin is not expressed.