If plants had a perspective, they would probably think of pollinators as more than just extra-friendly house guests. That is, plants would be more likely to view pollinators as the mutual friend who likes to set up blind dates. Bees might limit pollen to its use as a protein source for the hive, and birds might devour the flesh of a fruit and eliminate the seed as waste. However, many flowering plants, as Bug Girl pointed out in a post in honor of National Pollinator Week, have evolved alongside these pollinators for only one purpose: reproduction.
“Sure, you can toss your pollen out on the wind and hope it lands in the right place. And for a lot of plants, evergreens in particular, this works just fine,” she wrote. “That methodology results in a lot of wasted gametes (plant sperm) though, so for nearly all flowering plants, insects or other pollinators are needed for plant nookie.”
Sometimes the pollinator-plant relationship is mutualistic, and in many cases, one species or another is dependent upon the other for its survival. Take the agave plant. Probably the most well-known species is the blue agave plant (Agave tequilana), the nectar of which is used as a granular sugar substitute and to make tequila (one of the “finer” products of pollination, along with chocolate and coffee, mentioned by Bug Girl ). Leptonycteris nivalis, known as the greater long-nosed bat or Mexican long-nosed bat, and the lesser long-nosed bat (Leptonycteris curasoae), are the primary pollinators of this economically and ecologically valuable plant.
This agave-bat relationship is mutually beneficial. The bats, hovering in place like a hummingbird, use their long muzzles to feed on the high-fructose nectar of the agave. At the same time, the plants’ pollen collects on the bats’ fur. The bats then travel from plant to plant, spreading pollen as they drink from the nectar-filled stalks that bloom each night across the southwestern U.S. and Mexico.
The bats also migrate based on the blooming time of these plants. They arrive in Texas—particularly in Big Bend National Park, where a single colony resides in the Chisos Mountains—shortly after agave plants, such as the century plant (Agave havardiana), begin to bloom. Unfortunately, the lesser long-nosed bat and the Mexican long-nosed bat are endangered—and as their numbers decline, agave plant reproduction becomes more limited. A little farther north, however, some species of agave plants—those that are not harvested for tequila— have evolved to attract both bats and moths to serve as pollinators. Agave plants have several ways of advertising their nectar: the scent, the color of the flower and the shape, or morphology, of the structure holding the nectar. As described by Jeffrey Riffell from the University of Arizona and colleagues in a 2008 Communicative and Integrative Biology article, bat-pollinated agave plants have flowers that “are dull yellow or cream-colored, emit a strong fetid or pungent scent, and have hexose- (fructose and glucose) dominant nectar.”(Note: it is this fructose-dominant nectar that is so attractive to humans as well.)
Agave plants pollinated by hawkmoths, which also feed at night, “are white and highly reflective, emit a strong pleasant scent, and have sucrose-dominant nectar.” These differences, explained the researchers, can be linked to the distribution of the plants. That is, areas visited by migrating bats would likely have features adapted to attract these animals; areas that are only intermittently visited by migrating bats would probably need to rely on other pollinators, such as moths. However, some species of agave—such as Agave chrysantha— are adapted to attract both bats and moths, suggested Riffell and colleagues. In the article, the researchers explained that Agave chrysantha produces a large amount of nectar, emits odors known to attract diverse insect species and has a stalk that allows insects, bats and birds to access the nectar, which has the insect-attractive high sucrose content. A scent profile also “revealed low levels of sulfur compounds—a common marker of bat-adapted flowers,” wrote the authors. This scent-morphology combination allowed the plants to attract multiple types of pollinators in order to increase the chances of reproduction.
But reproduction of flowering plants does not end with pollinators. Bats, moths, bees and others may transport pollen from the stamen to the egg-producing pistil, but there is still work involved on the part of plants. Pollen grains contain two sperm cells and a pollen tube—this tube connects the pollen grain to the egg, creating a pathway for the sperm. According to a press release on a Proceedings of the National Academy of Sciences article published this week, “competition to grow the fastest pollen tube is the vegetative equivalent of male-male combat in the animal kingdom, a measure of fitness of the father plant that is one of several factors that drives flowering plant evolution.”
As described by Julien Bachelier from the University of Colorado, Boulder and William Friedman from Harvard University, flowering plants did not always depend on male competition—eggs in ancient flowering plants showed similar competition as do present-day sperm. The researchers attributed this shift toward male competition among flowering plants to the rise of pollinators. That is, instead of multiple eggs competing for only a few sperm, pollinators’ frequent visitations showered pollen grains on females, making female competition less useful.
Perhaps this adaptation, and coevolution with pollinators, helps to explain why one plant, such as a bat-pollinated agave plant, could be so similar and yet so different from, say, a moth-pollinated relative. An increase in pollinators is linked to a rise in flowering plant diversity.
Photo Credit: Al_HikesAZ