Pollination

Why would a flower look and smell like a piece of rotten meat? Traits like color, shape, odor, and size, are used by flowers to attract pollinators. Depending on the pollinator, flowers have evolved the corresponding traits to attract them. Thus, reddish fleshy flowers that smell like rotten meat are pollinated by some types of flies, like blowflies. We describe the pollination types and mechanisms for angiosperms, the flower traits related to each type, and the importance of pollination in the reproduction of flowering plants.

Pollination definition

Plants can reproduce sexually and many can also reproduce asexually. Sexual reproduction occurs through the fusion of two gametes to form a zygote. But adult plants are sessile, meaning they cannot move. How do plants find a “mate” to reproduce sexually? This is the function of pollen. In seed-producing plants (gymnosperms and angiosperms), pollen grains are the reproductive structures that produce the male gametes (sperm). Therefore, pollen must move and reach the female reproductive part of the plant that contains the female gametes (eggs). This is known as pollination.

We will be focusing on angiosperm (flowering plants) pollination. In angiosperms, all sexual reproductive structures are found in the flowers.

Pollination process

Pollen grains need help to move and therefore rely on pollinating agents. These agents can be abiotic (non-living agents, like wind and water) or biotic (living agents, such as insects and birds). The specific traits a flower presents (color, size, shape, scent) are related to the type of pollination agents it relies upon.

Abiotic pollination

About 20% of flowering plants rely on abiotic pollinating agents (98% by wind, and only 2% by water). Flowers that rely on wind for pollination do not need to attract pollinators; thus, their flowers are usually inconspicuous and small, with green or brownish coloration, sometimes lacking petals (Figure 1). For the same reason, they typically do not produce nectar (a solution of water and sugars) and have no scent. The stamens are usually well-exposed to facilitate pollen dispersion and the stigma can be feathery to catch pollen more easily.

Fig. 1 A wind-pollinated grass with a cloud of pollen.

Wind-pollinated plants have developed other traits to facilitate this type of pollination. They produce pollen in abundance (causing pollen clouds), to increase the probability of landing on another individual of the same species. In temperate species, the flowers usually appear early in the spring. This way, new leaves have not yet developed and cannot block pollen movement.

Few angiosperms use water as a pollinating agent. In some aquatic grasses and weeds, the pollen floats on the water until it reaches a flower.

Biotic pollination

Most angiosperms (80%) rely on animal agents for pollination, including a wide variety of insect, bird, and bat species. The most common pollinators are insects, which pollinate about 65% of angiosperms. Flowers attract pollinators with their color, shape, and scent, and reward them most of the time (some attract pollinators but give them no reward!) by offering food in the form of nectar and pollen.

Although most flowers can attract different pollinators, and many pollinators are therefore generalists, some flowers have evolved traits to attract a specific group of pollinators and, sometimes, even a specific pollinator species. Having a specific pollinator can be more efficient than a generalist one because the pollinator transfers the pollen directly to another flower of the same species (instead of visiting several species, wasting some of the pollen). Thus, specialized pollinators can be favored by natural selection. This provokes both the pollinator and the pollinated flower to develop corresponding traits that make the pollination specific to them (like tubular flowers with specific shapes to accommodate birds’ beaks or bats’ tongues). This is called coevolution.

Insect pollinators

Bees are the most common pollinators and most crop plants used for human consumption rely on them for pollination. Flowers pollinated by bees are typically brightly colored (yellow, blue, or purple; bees cannot see red) and have strong fragrances (Fig. 2-3). Besides the color, flowers can have nectar guides to help guide bees directly to the center of the flower. These guides can be small structures in the petal or ultraviolet marks that form a path. Bees can see ultraviolet colors contrasting with the rest of the flower. The pollen attaches to the fuzzy body of the bees.

Fig. 2-3: Insect-pollinated flowers. A bee pollinating an aster flower (left) and a fly on a carrion flower (right).

Butterflies are also common pollinators of brightly colored, sweet-scented day flowers with nectar guides. Moths, on the other hand, pollinate pale or white flowers that are open during the late afternoon or night (see figures 4 and 5).

Although we do not think of flies as pollinators, some species are. These flowers are characterized by an odor of rotten meat and are fleshy colored. Flies mistake them for carrion and approach looking for food or to lay eggs, becoming dusted with the pollen. Some flies also feed on pollen or nectar (flower flies) and are active pollinators.

Bird pollinators

Numerous angiosperms are pollinated by birds. Their flowers are brightly colored (usually yellow or red) and the petals are often fused into a tubular shape to fit the bird’s beak (e.g., hummingbirds, see Figure 4). They are not heavily scented, as birds’ sense of smell is not well developed, but they produce high quantities of nectar that birds need to maintain their metabolic rates. The pollen covers the bird’s head and neck.

Fig. 4 A Ruby-throated hummingbird pollinating a flower.

Bat pollinators

As bats are nocturnal, the flowers they pollinate share some characteristics with moth-pollinated flowers. They are open at night, have a pale or whitish coloration, and are aromatic, commonly with a fruity scent. But because of the large bat head, they are usually large flowers with a wide opening or group of flowers arranged in a wide base. They also produce a lot of nectar. The pollen attaches to the fur in the bat’s head. Bats are especially important pollinators in desserts and the tropics, pollinating plants like agave, guava, and cacti.

Mechanisms of pollination

Pollination can occur by two mechanisms:

• Self-pollination: when the pollen is transferred to the stigma of the same flower or another flower of the same plant. Although this mechanism may decrease genetic diversity, it can be beneficial if pollinators’ abundance decreases.
• Cross-pollination: when the pollen is transferred to the stigma of a flower in another plant of the same species. This mechanism ensures a higher genetic diversity, as the sperm and eggs come from different parents. Many plants only cross-pollinate and avoid self-pollination in different ways. The stamens and carpels can mature at different times (thus pollen release and stigma receptivity are not synchronized), or they can have different lengths or positions (thus the anthers do not touch the stigma), many plants produce unisexual flowers, and female and male flowers can be separated in different plants. In many plants, the pollen cannot germinate if the pistil recognizes that it comes from the same plant (self-incompatible).

Flowers that attract pollinators

Here we give a few examples of native flowers of the United States and the animals they attract (Table 1), for each major group of pollinators:

Table 1: Examples of pollinators and the native flowers they pollinate.

Source: USDA, https://www.fs.fed.us/wildflowers/pollinators/

Examples of other pollinators

We have mentioned several examples of common pollinators and the flowers they pollinate. However, there are some lesser-known pollinators.

• Mammals: lemurs in Madagascar, tropical mammals like bush babies and sugar gliders, rodents, Australian marsupials.
• Invertebrates: ants, true bugs, gnats, and slugs.
• Reptiles: some lizards, geckos, and skinks.

Examples of coevolution between a pollinator species and a corresponding flower

Yucca plants have evolved a close partnership with a group of moths commonly called yucca moths (figure 5). Each plant species has its own moth pollinator species. This is an interesting case because the yucca moths do not feed on the yucca flower’s nectar or pollen.

A female moth collects pollen from a few yucca flowers (ensuring cross-pollination) and then chooses one on which to lay her eggs (inside the ovary of the flower). Then she pollinates the flower depositing the pollen on the stigma. When the moth larvae are born, they feed on some of the yucca seeds. This partnership is so specific that these moths have evolved specialized mouthparts to collect the pollen (and not for eating, adult yucca moths do not eat). Furthermore, natural selection has selected against individuals that lay too many eggs, as the flower would abort and the plant wouldn’t reproduce.

Fig. 5 A yucca moth pollinating a yucca flower.

There is an endemic orchid in Madagascar called Angraecum sesquipedale (Fig. 6). The petals of the orchid form a very long tubular corolla. It is pollinated by the moth Xanthopan morganii praedicta with a proboscis the same length (Fig. 7). When Darwin saw this orchid, he predicted the existence of a moth with a proboscis of equal length, though he never observed it. The moth was discovered 20 years after Darwin’s death.

Fig. 6-7 The orchid Angraecum sesquipedale (left) and its moth pollinator (right).

Importance of pollination and pollinators in plants

Angiosperms, with more than 290,000 species, represent about 90% of all plants. Pollination is a crucial step in the sexual reproduction of flowering plants. After pollination occurs, fertilization takes place, resulting in the production of seeds and fruits that contain the next generation of plants. With 80% of flowering plants relying upon animal pollinators, the maintenance of these symbiotic relationships is critical for plant survival.

Globally, almost 80% of the crops that provide us with all our plant-based food, products, and materials, are pollinated by animals. And this does not consider the ecosystem services provided by other angiosperms not directly used for consumption, but that are part of the ecosystems. These services include oxygen production, carbon dioxide absorption, prevention of soil erosion, cultural significance, maintenance of animal populations, and more. Pollinators ensure the maintenance of plant genetic diversity through cross-pollination, which enhances ecosystem resilience to environmental changes.

Since most groups of plants have coevolved with a specific major group of pollinators (besides the specific pairs of plant species-pollinator) conservation of native plants and pollinators is crucial for ecosystem persistence.