Oregon State University
Rather than just waiting patiently for any pollinator that comes their way
to start the next generation of seeds, some plants appear to recognize the best
suitors and 'turn on' to increase the chance of success. These findings stem
from the discovery that the showy red and yellow blooms of Heliconia tortuosa,
an exotic tropical plant, recognize certain hummingbirds by the way the birds
sip the flowers' nectar. The plants respond by allowing pollen to germinate,
ultimately increasing the chances for successful seed formation.
Rather than just waiting patiently for any pollinator that comes their
way to start the next generation of seeds, some plants appear to recognize the
best suitors and "turn on" to increase the chance of success,
according to a new study published this week.
Being picky may
increase access to genetic diversity and thus give the plants a competitive
advantage over their neighbors, but there is a risk, the researchers say. If
the preferred pollinators decline for any reason, the plants may not reproduce
as easily and could decline as well.
These findings stem
from the discovery that the showy red and yellow blooms of Heliconia tortuosa, an
exotic tropical plant, recognize certain hummingbirds by the way the birds sip
the flowers' nectar. The plants respond by allowing pollen to germinate,
ultimately increasing the chances for successful seed formation.
Researchers from
Oregon State University and the Smithsonian Institution announced their results
in this week's issue of the Proceedings
of the National Academy of Sciences, a professional journal.
"To our knowledge, these findings provide the first evidence of pollinator
recognition in plants," they wrote.
Matt Betts, an
associate professor in the Oregon State University College of Forestry is the
lead author. Adam S. Hadley, also at Oregon State, and W. John Kress of the
Smithsonian Institution are co-authors. The National Science Foundation
provided support for the research.
In experiments at the
Las Cruces Biological Station in Costa Rica, Betts and Hadley began by trying
to pollinate Heliconia
plants by hand. Although such methods are commonly used in plant propagation,
the researchers were puzzled by their lack of success. So in an enclosure known
as an aviary, they exposed Heliconia
to six species of hummingbirds and a butterfly. The team discovered that two
types of hummers -- violet sabrewings and green hermits -- achieved more than
80 percent success in fertilizing the plants.
By controlling the
sources of pollen, the researchers excluded the possibility that fertilization
could be explained by specific birds carrying higher quality pollen.
"The ones that
turned it on tended to have long curved bills that could reach the
nectar," said Betts, who works in OSU's Department of Forest Ecosystems
and Society. "The ones that couldn't turn it on had shorter bills and
couldn't get as much nectar."
By modifying their
hand pollinating methods to mimic birds extracting nectar, the researchers were
able to achieve similar success in fertilizing the plants. "That closed
the loop on the mechanism," said Betts.
The two most
effective hummingbird species also shared another characteristic: Compared to
five other species, they tended to travel more widely across the landscape. The
researchers hypothesized that since far-ranging species tend to collect pollen
from more distant plants, the pollen would exhibit more genetic diversity and
enhance the plant's competitive fitness.
Pollen from nearby
plants could come from close relatives and thus have reduced genetic diversity,
the authors wrote.
"The mechanism
may have evolved to enable the plant to sort out pollinators that are likely to
be carrying high-quality pollen from those carrying poor-quality pollen,"
added Betts. "It's a big energy savings. If you bother to make a seed and
fruit every time you get pollen, that's a lot of energy expenditure; you could
be making a seed from your siblings' genes. If you make a seed or fruit only
from distant high-quality pollen, it could be an adaptive advantage."
Examples of
co-evolution of plants and pollinators have been known since Charles Darwin's
day, but the mechanisms that underlie these networks are poorly understood.
It's possible, Betts said, that other examples of pollinator recognition could
occur in tropical forests.
"It is now
well-known that the high cognitive capacity of many vertebrate pollinators
allows them to recognize and specialize on particular flower species," he
and his co-authors wrote. "A growing body of research indicates that
plants may also exhibit complex decision-making behavior."
Betts has conducted
research for six years at Las Cruces. The results of this and other studies
there, he said, suggest that the integrity of these ecosystems could depend on
maintaining corridors to enhance pollinator movement and survival. In some
areas, tropical forests have been broken up into smaller fragments as
development and agriculture have expanded.
"We need to be
more careful in how we manage landscapes in order to maintain the movements and
occurrence of these key species," he said. "We know that if we make
corridors to connect patches, if we have bigger patches of tropical forests,
those species will be maintained, and this plant and its pollinators will do a
lot better."
Journal Reference:
Matthew G. Betts, Adam S. Hadley, W. John Kress. Pollinator recognition by a keystone
tropical plant. Proceedings of the National Academy of
Sciences,
2015; 201419522 DOI: 10.1073/pnas.1419522112
