Pollen plants dupe tiny allies to out-manoeuvre team seed
SCIENCE by Gregory Beatty
Insects, Flowers & Food
Royal Saskatchewan Museum
Plants first emerged on land 450 million years ago. Green algae was an early precursor, and from there ferns and gymnosperms (seed-producers such as conifers) evolved. To pollinate and reproduce, they relied on the wind. And they were highly successful — until angiosperms arrived around 150 million years ago.
“The story of angiosperms vs. gymnosperms from the Cretaceous onward is the story of flowering plants displacing conifers,” says Ryan McKellar, Curator of Paleontology at the Royal Saskatchewan Museum. “Nowadays, conifer forests are usually in places angiosperms can’t survive — high elevation, extreme cold, poor soil.”
Saskatchewan is a perfect example, he adds.
“One hundred million years ago it was mostly redwood-type trees along the edge of a shallow seaway with angiosperms a small shrubby under-layer. Now, conifers have been displaced to Cypress Hills and other select areas. Pretty much everything else is angiosperms.”
What gave angiosperms the edge? That’s the subject of a new RSM exhibit McKellar co-curated with Cory Sheffield (curator of invertebrate zoology) and Evelyn Siegfried (curator of aboriginal studies).
And if you haven’t guessed the answer yet, it’s there in the title.
Instead of relying on the random (and somewhat incestuous) practice of airborne pollination, angiosperms enlisted insects, birds, bats and other animals to deliver pollen for them.
“The insects went there because they wanted food,” says Sheffield. “Even today, the act of pollination is something no insect sets out to do. When a bee visits a flower it’s not to assist the plant. It’s there to get food for its offspring, or for itself if it’s nectar. However, the plants were able to take advantage of the insects visiting them.”
“Take advantage” is an understatement. As the exhibit outlines through displays of prehistoric insects trapped in amber, magnified pollen grains, a hands-on pollination station and more, angiosperms, over tens of millions of years, developed sophisticated mechanisms to attract pollinators.
“It’s a question of offering rewards,” says McKellar. “You’ve got insects feeding on flower tissues, or the pollen itself. By offering other rewards such as nectar, resins, oils and waxes, flowering plants were able to attract additional pollinators. They’d get coated in pollen, then would move to another flower, or from one reproductive site to the next.”
Most conifers, conversely, have male cones at the tree top which shower female cones below with pollen. Unless you’ve got a hellacious wind, that restricts pollination to a small area. It’s a relatively random process too, whereas angiosperms, through the rewards they developed — plus the evolution of precise characteristics tied to blossom size, shape, colour, scent, UV markings, and even landing areas for larger pollinators such as butterflies — were able to pollinate over a wider area more efficiently.
“Lots of flowers have the ability to trick insects,” says Sheffield. “There’s some that smell really bad that mimic carrion, and they actually trap flies inside overnight. To get out they have to crawl through a small passage and their bodies become covered in pollen. When the flies are tricked into visiting other flowers, they deliver the pollen.”
Some orchids are even more devious, he adds. “They don’t produce any floral rewards. Instead, they mimic the appearance or scent of a female wasp or bee to attract the male and trick it into copulating with the flower to pick up pollen. The males probably leave a bit embarrassed, but they do it again, and that’s how the pollen gets spread.”
So yes, the insects do perform a valuable service for the plants. But they also get rewarded for their labour.
And that’s been of great benefit to them.
“If you think of your typical conifer, with needles and such, there aren’t the same soft plant parts that are easy to consume,” says McKellar. “So in the Cretaceous you’re seeing a transition from conifer-dominated forests to a higher concentration of angiosperms. At the same time, you’re seeing insects that either feed on plant tissues or pollen, be they pollinators or herbivores, really taking off in diversity and abundance.”
As angiosperms (and insects) colonized different ecosystems around the world, they developed distinct physiological characteristics that enhanced their survival and spawned new species. At 268,000 and one million respectively, the number of angiosperm and insect species now dwarf those of gymnosperms (1,021) and mammals (5,487).
Which isn’t to say mammals, such as us, haven’t benefited from pollination as well. Before European settlement, Siegfried shows in her curatorial contribution, First Nations people relied on flowering fruit plants such as saskatoons, blueberries and cranberries to supplement their diet.
Today, it’s estimated that every third bite of food we consume is due to pollination. But despite its long history of success, pollination is still a delicate process, and there’s growing evidence that industrial-scale agriculture, urban sprawl and other types of habitat modification/destruction are screwing things up.
Colony Collapse Disorder, which is devastating bee populations, is a high-profile example. Sheffield points to bee management as a big problem there.
“In the U.S., you have large colonies that move from Florida to Maine to California to pollinate different crops,” says Sheffield.
“They may already be stressed from parasitic mites, then we further stress them by moving them long distances,” he says. “There’s also chemical use, and in some cases crop plants may not be as nutritious for bees as they could be. Each stressor may not be the nail in the coffin, but when they act together [you get CCD].”
In Saskatchewan, Sheffield adds, he often consults with growers.
“I might say, ‘Okay, we know bumblebees like your haskap. But it flowers early in the spring, and bees need food plants all season long.’ So we work with growers to ensure they make lots of good food plants available.
“From my perspective as a researcher, we have to start thinking of this as a two-way street,” Sheffield says. “It’s not just what we can get from pollinators, but what can we do to ensure pollinators continue to be there for us.”