Dispersal of seeds and pollen

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13.4.1 Seed dispersal mutualisms

Very many plant species use animals to disperse their seeds and pollen. About 10% of all flowering plants possess seeds or fruits that bear hooks, barbs or glues that become attached to the hairs, bristles or feathers of any animal that comes into contact with them. They are frequently an irritation to the animal, which often cleans itself and removes them if it can, but usually after carrying them some distance. In these cases the benefit is to the plant (which has invested resources in attachment mechanisms) and there is no reward to the animal.

Quite different are the true mutualisms between higher plants and the fruits birds and other animals that feed on the fleshy fruits and disperse the seeds. Of course, for the relationship to be mutualistic it is essential that the animal digests only the fleshy fruit and not the seeds, which must remain viable when regurgitated or defecated. Thick, strong defenses that protect plant embryos are usually part of the price paid by the plant for dispersal by fruit-eaters. The plant kingdom has exploited a splendid array of morphological variations in the evolution of fleshy fruits (Figure 13.7).

Mutualisms involving animals that eat fleshy fruits and disperse seeds are seldom very specific to the species of animal concerned. Partly, this is because these mutualisms usually involve long-lived birds or mammals, and even in the tropics there are few plant species that fruit throughout the year and form a reliable food supply for any one specialist. But also, as will be apparent when pollination mutualisms are considered next, a more exclusive mutu-alistic link would require the plant's reward to be protected and denied to other animal species: this is much easier for nectar than for fruit. In any case, specialization by the animal is important in pollination, because interspecies transfers of pollen are disadvantageous, whereas with fruit and seed it is necessary only that they are dispersed away from the parent plant.

13.4.2 Pollination mutualisms

Most animal-pollinated flowers offer nectar, pollen or both as a reward to their visitors. Floral nectar seems to have no value to the plant other than as an attractant to animals and it has a cost to the plant, because the nectar carbohydrates might have been used in growth or some other activity.

Presumably, the evolution of specialized flowers and the involvement of animal pollinators have been favored because an animal may be able to recognize and discriminate between different flowers and so move pollen between different flowers of the same species but not to flowers of other species. Passive transfer of pollen, for example by wind or water, does not discriminate in this way and is therefore much more wasteful. Indeed, where the vectors and flowers are highly specialized, as is the case in many orchids, virtually no pollen is wasted even on the flowers of other species.

There are, though, costs that arise from adopting animals as mutualists in flower pollination. For example, animals carrying pollen may be responsible for the transmission of sexual diseases as well (Shykoff & Bucheli, 1995). The fungal pathogen Microbotryum violaceum, for example, is transmitted by pollinating visitors to the

Superior ovary

Sepal

Superior ovary

Sepal

Types Dispersal Seeds And Fruits

Achene (dry ovary with 1 seed inside)

Mulberry

(Moraceae)

Cherry

(Rosaceae)

Figure 13.7 A variety of fleshy fruits involved in seed dispersal mutualisms illustrating morphological specializations that have been involved in the evolution of attractive fleshy structures.

Achene (dry ovary with 1 seed inside)

Mulberry

(Moraceae)

Cherry

(Rosaceae)

Figure 13.7 A variety of fleshy fruits involved in seed dispersal mutualisms illustrating morphological specializations that have been involved in the evolution of attractive fleshy structures.

Mutualisms

Figure 13.8 Pollinators: (a) honeybee (Apis mellifera) on raspberry flowers, and (b) Cape sugarbird (Promerops cafer) feeding on Protea eximia. (Courtesy of Heather Angel.)

Raspbery Seed Dispersal

Figure 13.8 Pollinators: (a) honeybee (Apis mellifera) on raspberry flowers, and (b) Cape sugarbird (Promerops cafer) feeding on Protea eximia. (Courtesy of Heather Angel.)

flowers of white campion (Silene alba) and in infected plants the anthers are filled with fungal spores.

Many different kinds of animals have entered into pollination liaisons with flowering plants, including hummingbirds, bats and even small rodents and marsupials (Figure 13.8). However, the pollinators par excellence are, without doubt, the insects. Pollen is a nutritionally rich food resource, and in the simplest insect-pollinated flowers, pollen is offered in abundance and freely exposed to all and sundry. The plants rely for pollination on the insects being less than wholly efficient in their pollen consumption, carrying their spilt food with them from plant to plant. In more complex flowers, nectar (a solution of sugars) is produced as an additional or alternative reward. In the simplest of these, the nectaries are unprotected, but with increasing specialization the nectaries are enclosed in structures that restrict access to the nectar to just a few species of visitor. This range can be seen within the family Ranunculaceae. In the simple flower of Ranunculus ficaria the nectaries are exposed to all visitors, but in the more specialized flower of R. bulbosus there is a flap over the nectary, and in Aquilegia the nectaries have developed into long tubes and only visitors with long probosces (tongues) can reach the nectar. In the related Aconitum the whole flower is structured so that the nectaries are accessible only to insects of the right shape and size that are forced to brush against the anthers and pick up pollen. Unprotected nectaries have the advantage of a ready supply of pollinators, but because these pollinators are unspecialized they transfer much of the pollen to the flowers of other species (though in practice, many general-ists are actually 'sequential specialists', foraging preferentially on one plant species for hours or days). Protected nectaries have the advantage of efficient transfer of pollen by specialists to other flowers of the same species, but are reliant on there being sufficient numbers of these specialists.

Charles Darwin (1859) recognized that a long nectary, as in Aquilegia, forced a pollinating insect into close contact with the pollen at the nectary's mouth. Natural selection may then favor even longer nectaries, and as an evolutionary reaction, the tongues of the pollinator would be selected for increasing length - a reciprocal and escalating process of specialization. Nilsson (1988) deliberately shortened the nectary tubes of the long-tubed orchid Platanthera and showed that the flowers then produced many fewer seeds - presumably because the pollinator was not forced into a position that maximized the efficiency of pollination.

Flowering is a seasonal event in most plants, and this places strict seasonality limits on the degree to which a pollinator can become an obligate specialist. A pollinator can only become completely dependent on specific flowers as a source of food if its life cycle matches the flowering season of the plant. This is feasible for many short-lived insects like butterflies and moths, but longer lived pollinators such as bats and rodents, or bees with their long-lived colonies, are more likely to be generalists, turning from one relatively unspecialized flower to another through the seasons or to quite different foods when nectar is unavailable.

insect pollinators: from generalists to ultraspecialists

Species Dispersal
Figure 13.9 Fig wasps on a developing fig. Reproduced by permission of Gregory Dimijian/Science Photo Library.

13.4.3 Brood site pollination: figs and yuccas

Not every insect-pollinated plant provides its pollinator with only a take-away meal. In a number of cases, the plants also provide a home and sufficient food for the development of the insect larvae (Proctor et al., 1996). The best studied of these are the complex, largely species-specific interactions between figs (Ficus) and fig wasps (Figure 13.9) (Wiebes, 1979; Bronstein, 1988). Figs bear many tiny flowers on a swollen receptacle with a narrow opening to the outside; the receptacle then becomes the fleshy fruit. The best-known species is the edible fig, Ficus carica. Some cultivated forms are entirely female and require no pollination for fruit to develop, but in wild F. carica three types of receptacle are produced at different times of the year. (Other species are less complicated, but the life cycle is similar.) In winter, the flowers are mostly neuter (sterile female) with a few male flowers near the opening. Tiny females of the wasp Blastophaga psenes invade the receptacle, lay eggs in the neuter flowers and then die. Each wasp larva then completes its development in the ovary of one flower, but the males hatch first and chew open the seeds occupied by the females and then mate with them. In early summer the females emerge, receiving pollen at the entrance from the male flowers, which have only just opened.

The fertilized females carry the pollen to a second type of receptacle, containing neuter and female flowers, where they lay their eggs. Neuter flowers, which cannot set seed, have a short style: the wasps can reach to lay their eggs in the ovaries where they develop. Female flowers, though, have long styles so the wasps cannot reach the ovaries and their eggs fail to develop, but in laying these eggs they fertilize the flowers, which set seed. Hence, these receptacles generate a combination of viable seeds (that benefit the fig) and adult fig wasps (that obviously benefit the wasps, but also benefit the figs since they are the figs' pollinators).

Following another round of wasp development, fertilized females emerge in the fall, and a variety of other animals eat the fruit and disperse the seeds. The fall-emerging wasps lay their eggs in a third kind of receptacle containing only neuter flowers, from which wasps emerge in winter to start the cycle again.

This, then, apart from being a fascinating piece of natural history, is a good example of a mutualism in which the interests of the two participants none the less appear not to coincide. Specifically, the optimal proportion of flowers that develop into fig seeds and fig wasps is different for the two parties, and we might reasonably expect to see a negative correlation between the two: seeds produced at the expense of wasps, and vice versa (Herre & West, 1997). In fact, detecting this negative correlation, and hence establishing the conflict of interest, has proved elusive for reasons that frequently apply in studies of evolutionary ecology. The two variables tend, rather, to be positively correlated, since both tend to increase with two 'confounding' variables: the overall size of fruit and the overall proportion of flowers in a fruit that are visited by wasps. Herre and West (1997), however, in analyzing data from nine species of New World figs, were able to over-come this in a way that is generally applicable in such situations. They controlled statistically for variation in the confounding variables (asking, in effect, what the relationship between seed and wasp numbers would be in a fruit of constant size in which a constant proportion of flowers was visited) and then were able to uncover a negative correlation. The fig and fig wasp mutualists do appear to be involved in an on-going evolutionary battle.

A similar, and similarly much studied, set of mutualisms occurs between the 35-50 species of Yucca plant that live in North and Central America and the 17 species of yucca moth, 13 of which are newly described since 1999 (Pellmyr & Leebens-Mack, 2000). A female moth uses specialized 'tentacles' to collect together pollen from several anthers in one flower, which she then takes to the flower of another inflorescence (promoting outbreeding) where she both lays eggs in the ovaries and carefully deposits the pollen, again using her tentacles. The development of the moth larvae requires successful pollination, since unpollinated flowers quickly die, but the larvae also consume seeds in their immediate vicinity, though many other seeds develop successfully. On completing their development, the larvae drop to the soil to pupate, emerging one or more years later during the yucca's flowering season. The reproductive success of an individual adult female moth is not, therefore, linked to that of an individual yucca plant in the same way as are those of female fig wasps and figs.

A detailed review of both seed dispersal and pollination mutualisms is given by Thompson (1995), who provides a thorough account of the processes that may lead to the evolution of such mutualisms.

figs and fig wasps...

... show mutualism despite conflict yuccas and yucca moths

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Responses

  • grant
    Why is the dispersal of pollen in many plants considered to be mutualism?
    7 years ago
  • hyiab
    How are mulberry seeds dispersed?
    12 months ago

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