Q.How polyembryony can be commercially exploited?

Citrus, orange and mango seeds often contain more than one embryo — polyembryony.

The occurrence of more than one embryo in a single seed is called polyembryony. In many varieties of citrus fruits (including orange, which is itself a citrus) and in mango, extra embryos develop from the nucellus or integuments (adventive/apomictic embryos) in addition to the zygotic embryo. So all the listed seeds can show polyembryony.

Apomixis matches with (e) Grass family, as this asexual seed-formation mechanism is common among grasses.

Apomixis is a form of asexual reproduction that mimics sexual reproduction — seeds are produced without the fusion of gametes (without fertilisation), often because the diploid egg cell develops into an embryo directly, or a nucellar cell forms an embryo. This trait is exploited commercially for producing hybrid seeds that don't segregate in subsequent generations, and it is naturally very common among members of the grass (Poaceae) family, as well as in some Citrus and mango varieties.

Polyembryony means more than one embryo forms in a single seed; option (iv) reverses the relationship (an ovule contains an embryo, not the other way round), so it is the incorrect statement.

Polyembryony is the occurrence of more than one embryo in a seed. It arises in genuine, documented ways:

• More than one egg cell forming in the embryo sac (e.g. when extra archesporial cells develop), with more than one egg getting fertilised — statement (i) is a real cause.

• Embryos developing from different cells/parts of the ovule other than the zygote (e.g. synergids also getting fertilised, or developing parthenogenetically) — statement (ii) is a real cause.

• Nucellar embryony — embryos developing directly from the diploid nucellus tissue surrounding the embryo sac, without fertilisation, seen commonly in Citrus and Mango — statement (iii) is a real, well-known cause.

• Statement (iv), "presence of more than one ovule in the embryo," is biologically backwards — an ovule is the structure that, after fertilisation, develops INTO a seed containing the embryo; an embryo cannot contain an ovule. This statement does not describe any real mechanism of polyembryony at all.

The occurrence of more than one embryo in a seed is called polyembryony.

The presence of more than one embryo in a single seed is called polyembryony.

Polyembryony occurs in some plants (e.g. Citrus, mango) when multiple embryos develop within a single ovule/seed, often due to the fertilized egg dividing to give more than one embryo, or nucellar cells surrounding the embryo sac developing into additional embryos (adventive/nucellar embryony), giving rise to genetically identical (clonal) seedlings alongside the sexual one.

Polyembryony is the presence of more than one embryo in a seed, seen commonly in citrus and mango.

Normally a seed has a single embryo formed from the fertilised egg (zygote). In polyembryony, additional embryos form either by fertilisation of more than one egg cell/synergid in the same embryo sac, or by development of embryos directly from nucellus or integument cells without fertilisation (nucellar/adventive embryony, a form of apomixis). It is commercially useful in citrus and mango because nucellar embryos are genetically identical to the mother plant, giving disease-free, true-to-type seedlings.

Polyembryony — more than one embryo developing in a single seed — is classically seen in Citrus.

In most flowering plants a seed contains a single embryo, but in some species more than one embryo may develop within a seed, a condition called polyembryony. It arises in two main ways: (i) fertilisation of more than one egg/synergid cell within the same embryo sac, or (ii) more commonly, the formation of additional 'adventive' embryos from the diploid cells of the nucellus surrounding the embryo sac — a form of apomixis that does not require fertilisation. Citrus (and certain Mango varieties) are the standard textbook examples of such nucellar polyembryony; a seed from these plants can germinate to give rise to more than one seedling, one of which is the true sexual (zygotic) embryo and the rest are clonal nucellar embryos. Tomato, Potato and Banana are not characterised by this phenomenon.

More than one embryo developing within a single seed/ovule.

When a single seed develops multiple embryos instead of one, the phenomenon is called polyembryony.

Normally a seed contains a single zygotic embryo. In polyembryony, more than one embryo develops within a single seed — this can occur, for example, when nucellar cells surrounding the embryo sac start dividing and develop into extra embryos (as commonly seen in citrus fruits), in addition to the zygotic embryo.

Polyembryony is a phenomenon in which a single seed contains more than one embryo, often arising when cells other than the fertilized egg (e.g., nucellar cells) also develop into embryos.

Definition and mechanism

Normally, a single fertilized egg (zygote) develops into one embryo per seed. Polyembryony, however, is the occurrence of more than one embryo within the same seed. This can happen through different mechanisms:

• Occasionally, in addition to the egg cell, one or more synergids or antipodal cells also get fertilized/develop into embryos.
• More commonly (as in Citrus and mango), diploid cells of the nucellus surrounding the embryo sac start dividing and give rise to additional embryos without undergoing fertilization — this is called nucellar polyembryony, and it is a form of apomixis (asexual seed formation).

Polyembryony is commercially exploited in horticulture, since nucellar embryos are genetically identical (clonal) to the mother plant, allowing propagation of hybrid vigour without variation from sexual recombination.

Apomixis produces a seed without any fertilisation event, while polyembryony refers to a single seed containing multiple embryos, whether or not fertilisation occurred.

Apomixis is a special asexual mode of seed formation in which an embryo develops without the actual fusion of male and female gametes (no fertilisation occurs), so the resulting seedling is a genetic clone of the mother plant — often via a diploid cell of the nucellus developing directly into an embryo. Polyembryony, in contrast, refers to the occurrence of more than one embryo within a single seed; this can arise either as a form of apomixis (e.g., additional embryos developing from nucellar cells alongside the zygotic embryo, as commonly seen in citrus fruits) or, less commonly, from other developmental irregularities, and does not by definition require the total absence of fertilisation (the seed may still contain one fertilisation-derived embryo alongside additional apomictic ones).

Apospory, apogamy and adventive embryony are all forms of agamospermy; geitonogamy is instead a mode of self-pollination and does not belong here.

Agamospermy is a form of asexual reproduction in flowering plants in which seeds are produced without the fusion of gametes (without meiosis and fertilization), so the resulting embryo is genetically identical to the parent plant. It occurs mainly by:

• Apospory: a diploid somatic (sporophytic) cell of the ovule directly develops into an embryo-sac-like structure, bypassing meiosis.
• Apogamy: the egg cell or another cell of the embryo sac develops directly into an embryo without fertilization.
• Adventive embryony: an embryo develops directly from a diploid cell of the nucellus or integument, outside the embryo sac (e.g., in Citrus, Mango).

Geitonogamy, by contrast, is a mode of self-pollination in which pollen from one flower is transferred to the stigma of another flower on the same plant. It is genetically a form of self-pollination (though pollinating agents are involved), not a mode of asexual seed formation, and is therefore not a type of agamospermy.