Pollination from an Evolutionary Point of View

Gymnosperm pollination is invariably anemophilous (primary); only recently evolved genera as Ephedra and Welwitschia are pollinated by insects. There is general agreement that early angiosperms were pollinated by Coleoptera and Diptera. Woody and herbaceous secondary anemophilous angiosperms may descend from zoophilous species. Hydrophily is probably derived from

Table 2 Features of major pollen vectors. Honeybees are the best pollinators because they visit flowers of a given species for as long as they are available, and store pollen for their progeny, unlike nonsocial insects

Pollen vectors

Specificity (with respect to a flower attractant) Efficiency3

Distancesa (minimum and maximum)

Animals

Air currents Water

Insects

Honeybees Solitary bees and wasps Flies

Butterflies and moths Coleopterab

Birds

Flying animals and small marsupials Breezes Strong winds

Saltc Fresh

Very high High

Low Lowd

Moderate Low

Low® Very low

Very low Very low

Very high Several cm ! few hundred m

High Several cm ! few hundred m

Moderate Several cm ! few hundred m

Lowd Several cm ! few hundred m

Low Several cm ! few hundred m

Moderate- Few m ! few hundred m high

Low Several cm ! few hundred m

Low Few cm ! few hundred m

Very low Few hundred m ! several km

Very low Some m ! several hundred m

Very low Few dm ! several hundred m aBiotic and abiotic environmental parameters may radically affect efficiency and distance traveled. bColeoptera are not good pollinators because they have few hairs and their chewing mouthparts damage flowers. cUnderwater pollination occurs in all seaweeds.

^Efficiency is low because these animals feed only on nectar and must visit different species in order to have a balanced diet. eSpecificity is low but species with this pollination syndrome often grow close to each other, as in the case of grasses and tress.

Figure 1 Scheme of hermaphrodite and unisexual flowers and possible crosses in angiosperms according to plant sexual expression H = plant with hermaphrodite flowers; M = monoecious plant with flowers of both sexes; D = dioecious plant with male and female flowers on different plants. Two plants of the same and one of another species are shown for each type of sexual expression. a = autogamy, b = geitonogamy, and c = heterogamy are legitimate pollination styles though they imply different genetic reassortment; d = xenogamy is illegitimate pollination and gives rise to hybrids in the absence of barriers. Plants with hermaphrodite flowers have all possible crosses; these reduce progressively in monoecious and dioecious plants. Pollen vectors determine pollen cross types. Air and water currents are agents of all types of crosses. Social bees are commonly responsible for a, b, and c, rarely d. Butterflies and moths are most often agents of d, because they only feed on nectar and must visit different flowers to have a balanced diet.

anemophily. Hydrophilous pollen of seagrasses characterized by submarine pollination is 2-3 mm long and a few dozen microns wide. The genus Callitriche has terrestrial, amphibious, and submerged freshwater species; their pollen is spherical and that of submerged species is devoid of exine, as in all species with submarine pollination.

Competition to attract pollinators is high when many entomophilous species bloom at the same time and pollinators are few. This is avoided by different blooming periods and anemophily. Examples of entomophilous families with anemophilous members are: Ranunculaceae, Thalictrum; Euphorbiaceae, Mercurialis and castor bean (Ricinus); Asteraceae, ragweed (Ambrosia) and Artemisia. Few entomophilous species bloom at the same time in January and February in Northern Hemisphere Mediterranean environments, when few insects are active. Helleborus bocconei and H. foetidus grow in similar environments and share the same pollinator, but pollen attaches to different parts of the pollinator body, so that useless pollination is avoided. Anemophilous species of Juniperus growing in the same environments disperse pollen in different periods.

Preparing for Dispersal

A fluid fills the anther cavity in which pollen develops. It disappears by evaporation and/or resorption when pollen is ripe, prior to pollen dispersal. Pollen sacs, anthers, and pollen also lose some water. Pollen could be damaged if it were released with a high water content and high metabolic activity. In order to avoid this possibility, pollen is dispersed in a quiescent state, in which cell division and metabolism are arrested. Two classes of pollen, with different levels of developmental arrest, are determined on the basis of their water content at dispersal: partially dehydrated pollen (PDP) has a water content of less than 30% and partially hydrated pollen (PHP) has a water content of more than 30%. The former has metabolic devices to keep its low water content constant; the latter does not have devices and loses water quickly, especially in dry environments. PDP and PHP may be transported by animals or air currents. Both have advantages and disadvantages as well as devices to ensure successful pollination. PDP survives longer at low relative humidity, whereas PHP dries out readily and dies. However, the latter germinates quickly, taking only a few minutes after landing on a stigma. PDP is more common in dry temperate environments and is dispersed during the dry hours of the day. PHP is more common in the Tropics and wet environments; in temperate regions, it is presented for dispersal when RH is high, such as at night or in winter and autumn. Species belonging to these two groups have physiological and ecological strategies to ensure safe pollination journeys, which are quick and short in the case of PHP.

Pollen Presentation and Dispersal Mechanisms

When the anther opens, pollen is presented for dispersal and may (1) be launched from the anther by ballistic movements of the anther or stamen and dispersed by air currents, as in Parietaria and castor bean (Ricinus communis); (2) be scattered from the anther by flower movements caused by an insect in search of nectar, and loaded on insect hairs as in Spartium junceum and other Phaseolaceae; (3) be dropped by the anther for lack of any forces to keep it attached, as in grasses and many herbaceous and woody anemophilous species; (4) remain stuck to the anther by pollenkitt or other sticky fluids or threads, pending removal by animals or air currents; (5) be kept in anthers having only small apertures, being released in small doses when the flower is

shaken by animals or breezes, as in tomato (Solanum lycoper-sicum) and Ericaceae; and (6) be dislodged from the anther and presented in another part of the flower (secondary presentation), when flowers are small and disposed in inflorescences where there is no space to expose pollen in the anther, as in daisies such as Bellis.

Pollen may be presented for different lengths of time: for zero time, when it is launched, leaving the anther when it opens, as in many anemophilous species having PHP; for a few hours to a month, as in many entomophi-lous species having PDP; for longer, in the case of orchids.

Pollen presentation ceases when the flower closes, as in some species with PHP, probably to avoid dispersal of unviable pollen, or when the anthers are discarded.

Animals are attracted to flowers by the prospect of rewards or shelter, or by misleading messages (deception). Common rewards include pollen and nectar, both rich in nutrients: the former contains more proteins whereas the latter contains more carbohydrates. Pollen is collected actively by animals that feed on it and/or passively by those collecting nectar. Each visit to a flower is associated with pollen uptake and discharge.

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