Special Modes of Reproduction: Apomixis, Parthenocarpy and Polyembryony

Alongside the standard sexual pathway described so far -- meiosis in both parents, pollination,

double fertilisation, and the post-fertilisation development of a single embryo and endosperm per

seed -- flowering plants also display several special reproductive modes that depart from this

norm in interesting ways. Three of the most important are apomixis, parthenocarpy and polyembryony,

and several are of significant practical and commercial value.

Apomixis is a form of asexual reproduction that nonetheless closely mimics true sexual

reproduction in its outward result: a seed is produced, exactly as in normal sexual reproduction,

but WITHOUT the fusion of male and female gametes ever taking place. This can occur in more than

one way -- for instance, a diploid egg cell may form directly, without undergoing meiosis, and then

develop into an embryo without being fertilised at all, or a diploid cell derived from the

surrounding nucellar tissue (rather than from the embryo sac's own egg cell) may itself begin

dividing and develop directly into an embryo. Because no fertilisation is involved, an apomictically

produced offspring is genetically identical to its single parent -- effectively a clone,

transmitted through seed rather than through a vegetative cutting or graft.

This property makes apomixis extremely valuable in commercial plant breeding, particularly in the

hybrid seed industry. An F1 hybrid variety is normally produced by deliberately crossing two

specific, carefully chosen parent lines, in order to combine desirable traits and to capture hybrid

vigour (heterosis) in the resulting offspring. Ordinarily, such a hybrid cannot be propagated

reliably through its own seed: because normal sexual reproduction involves meiosis and gamete

fusion, growing plants from an F1 hybrid's own seed produces an F2 generation in which the

carefully combined traits segregate and break apart, losing the very combination that made the

hybrid valuable. If, however, a hybrid variety can be engineered or selected to reproduce

apomictically instead, it will produce seed that is genetically identical to itself, generation

after generation -- permanently 'fixing' the hybrid's desirable combination of traits and its

vigour, without the breeder needing to repeat the original, often labour-intensive cross every

single growing season. This is a major, active area of plant breeding research aimed at reducing

the cost of hybrid seed production.

Parthenocarpy is a distinct phenomenon: the development of a fruit from the ovary without prior

fertilisation of its ovules having taken place at all. Because fertilisation never occurs, no zygote

forms, and consequently no seeds develop inside such a fruit -- parthenocarpic fruits are

characteristically seedless. It occurs naturally in a small number of species, such as banana. It

can also be induced artificially: treating the stigma or ovary of an unpollinated flower with

auxins or other plant growth regulators can stimulate the ovary wall to enlarge and mature into a

fruit exactly as though fertilisation had triggered the process, even though it has not.

Commercially, parthenocarpy is deliberately exploited to produce popular seedless fruit varieties,

such as seedless grapes and seedless cucumbers, prized both for convenience of eating and, in many

markets, for higher commercial value; because such varieties bear no viable seed, they must

necessarily be propagated vegetatively rather than by sowing their own fruit's seed.

Polyembryony is the occurrence of more than one embryo within a single seed, rather than the usual

single zygotic embryo. It was first reported by the early microscopist Antonie van Leeuwenhoek in

the genus Citrus, which, together with mango, remains one of the most commonly cited examples.

Polyembryony most commonly arises through nucellar embryony -- a specific form of apomixis in which,

in addition to the normal zygotic embryo formed by syngamy, one or more of the diploid nucellar

cells surrounding the embryo sac independently begin dividing and themselves develop into …