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Seeking the benefits of a parasite

This time of year, when berries fill my thoughts, the early summer blooms that begot them are hazy in my mind. As I pick, I try to remember that my belly and bucket are full thanks to these flowers and their pollinators. And to my surprise, as I learned this summer, perhaps a parasitic plant is also to thank.

Enter woolly lousewort, or Pedicularis lanata. It’s one of �ֻ�ֱ��’s dozen or so native parasitic plants. It reappears from its taproot in the tundra every spring with hairy leaves that huddle together to form a silvery, fluffy cone.

Despite lousewort’s humble emergence, its ecology could prompt scientists to reconsider the role of plant parasites as solely detrimental.

The species is considered a hemiparasite. A true parasitic plant cannot photosynthesize effectively enough to obtain all the nutrients it needs, so it steals food (sugars) from host plants. In contrast, a hemiparasite can photosynthesize to make its own sugars, but does steal water and other nutrients from its host.

As spring moves to summer, louseworts unfurl to reveal finely dissected leaves with wine-colored tips. A tall column of deep pink flowers extends to the sky.

Lousewort’s towering blooms could help produce more berries in the Arctic tundra, according to visiting graduate student Jonathan Carcache.

Many tundra plants have small, unassuming flowers. The delicate pink bells of tundra blueberry bushes hide under larger and more plentiful leaves. Bearberry’s faint yellow blooms blend into the tundra’s early summer palette. Clusters of white lingonberry flowers hug close to the ground.

After spending a couple of summers observing pollinators in �ֻ�ֱ��’s Arctic, Carcache, now a Ph.D. student at Florida Atlantic University, suspected that lousewort’s pillar of vibrant magenta flowers acts as a magnet. It draws in pollinators to both its flowers and those of surrounding plants. As pollination increases, Carcache proposes, so will the number of fruits and seeds. This would offset the cost of lousewort’s parasitism.

“You hear ‘parasite’ and you think, ‘This is bad.’ But it could be offering other resources that we’re not yet envisioning,” Carcache said.

Carcache spent this summer at the �ֻ�ֱ��’ Toolik Field Station as an early-career Tundra awardee with support from the Organization of Biological Field Stations and the Maxwell/Hanrahan Foundation. He sought out lousewort in the foothills of the Brooks Range to test whether the parasite benefits the neighboring plants it targets.

Carcache needed to answer two questions. Did plants near louseworts receive more pollen because they attracted more pollinators? And did that result in more successful fruits and seeds? For this past summer, he focused on Cassiope tetragona, or arctic white heather, a widespread tundra plant. But he hopes to expand to other plants in future work.

To answer the first question, Carcache collected roughly a thousand heather stigmas, the flowers’ female parts that receive pollen. He’ll take microscopic images of them back in his lab in Florida to see if more pollen is received by the plants near lousewort.

But getting more pollen is just one part of the story. Carcache also needed to determine if the costs of being parasitized did not reduce the number of fruits and seeds when flowers were sufficiently pollinated. So, he hand-pollinated a subset of heather flowers to see if they still had more seeds even when parasitized by lousewort.

“I just sat there on my stomach, one by one, pollinating [with a paintbrush],” he said, adding, “I must have pollinated a few hundred flowers, maybe upwards of 400 or 500.”

Carcache’s research fills a gap in the broader conversation in ecology on the nature of symbiosis. Symbiosis, or the close interaction formed between organisms, is often considered either beneficial or harmful. But researchers increasingly find that symbiotic relationships can’t be placed into rigid categories.

“It’s now accepted that other partnerships swing along this pendulum from mutualist to parasite, but there’s nothing talking about parasitic plants,” Carcache said. “It’s a gap that needs to be filled.”

Carcache will spend the winter counting seeds and pollen grains to add up any benefits to lousewort’s parasitism. And I’ll go back to my berries with a little something more to ponder.

Since the late 1970s, the �ֻ�ֱ��' Geophysical Institute has provided the �ֻ�ֱ�� Science Forum column free in cooperation with the UAF research community. Haley Dunleavy is the communications manager at Toolik Field Station.