Outbreeding Devices

Repeated self-pollination across many generations tends to reduce a population's genetic diversity

and, with it, its overall vigour and fertility -- a phenomenon sometimes called inbreeding

depression. To counter this, flowering plants have evolved a range of outbreeding devices: floral

mechanisms whose collective effect is to discourage or prevent self-pollination (autogamy) and to

favour, instead, cross-pollination (xenogamy) with genetically different individuals.

One important class of outbreeding device is dichogamy, which operates through time rather than

space: the anthers and the stigma of a single flower mature at different times, so that pollen

release from that flower's own anthers and stigma receptivity in that same flower never coincide.

If the anthers mature and shed pollen before the stigma of the same flower becomes receptive, the

condition is called protandry; if the stigma matures and becomes receptive before the anthers

mature, the condition is called protogyny. Either pattern is common in many members of the family

Asteraceae, among others, and effectively prevents a flower from fertilising itself even though it

is structurally bisexual.

A second class of device is herkogamy, which operates through space rather than time: the anthers

and stigma of a bisexual flower may both be fully mature and functional at the same time, but they

are structurally arranged, or physically separated, in a way that prevents self-pollen from being

deposited on the stigma of that same flower. This kind of physical separation is well developed in

many orchids, where highly specialised floral architecture keeps the pollen-bearing structures and

the receptive stigmatic surface mechanically isolated from one another, so that transfer between

them essentially requires the intervention of a specific pollinator.

A third, biochemical rather than structural or temporal mechanism is self-incompatibility, a

genetic system -- well documented in families such as Brassicaceae and Solanaceae -- by which the

pistil is able to recognise pollen from the same plant (or from a genetically similar individual)

as 'self' and to actively prevent it from achieving fertilisation, even after it has landed on an

otherwise receptive stigma. This rejection typically occurs at one of two points: either the

self-pollen fails to germinate on the stigma at all, or, if it does germinate, its pollen tube's

growth is arrested somewhere within the tissue of the style before it can travel far enough to

reach an ovule. Because the male gametes carried by such pollen never reach the embryo sac, neither

syngamy nor triple fusion can occur, and self-pollination in a self-incompatible plant therefore

never results in seed formation -- even though the very same stigma would readily accept, and be

fertilised by, pollen arriving from a genetically different individual of the same species.

A further, more drastic outbreeding device found in some species is dioecy -- the condition in

which male flowers (bearing only a functional androecium) and female flowers (bearing only a …