Searching for signs of picky, competitive
mating in a whole other kingdom
By Susan Milius
The term “bar fight” does not actually appear in Saila Varis’ recent paper in the journal Trees or in her Ph.D. dissertation on the Scots pine. But she’s a good
sport about discussing whether her
research suggests that tree pollen grains
have their own versions of nose-punch-ing brawls over female favor.
After all, pollen grains from genetically different trees of the same species appear to be able to sabotage each
other’s race to a mate, says Varis, of the
Finnish Forest Research Institute near
Helsinki. Though it is not exactly like
a bar fight, she says, there are hints of
Plant pollen may be basically microscopic dust, but as far as evolutionary
biology goes it can be as male as any
swaggering pool-hall hound with smooth
moves and high hopes for the night. Pollen grains competing for access to the
alluring green nubbins of female tissue in
a pine tree add to growing evidence that
a quirky evolutionary force known from
animals, called sexual selection, may also
show up in plants.
This evolutionary force is not the familiar survival of the fittest, but rather a sort
of survival of the sexiest. For no matter
how vulnerable a feather display or antler
challenges, that it might in theory live
forever — yet it could still end up as an
evolutionary dud. An organism not only
has to survive, but it also has to make
babies. Otherwise it’s as much of a dead
end as a weakling culled from the gene
pool at birth.
Darwin acknowledged that survival was not the whole story. In the
same chapter of The Origin of Species
that describes natural selection, he
introduced sexual selection, which he
describes as depending on “a struggle
between the individuals of one sex, generally the males, for the possession of
the other sex.” He observed that reproductive success doesn’t depend just
on robust vigor; rooster spurs and stag
antlers trounce rivals, and extravagant
As the idea of sexual selection developed, theorists predicted that these
special devices for winning a mate could
drain resources that would otherwise
go to the business of survival.
Darwin’s ideas on sexual selection
could make sense of why the males
and females of a species look different.
Applying the principle to plants jolted
some researchers because many plants
grow as hermaphrodites, not as boys and
girls with different features.
“There has of course been some reluctance to accept that sexual selection can
operate in hermaphrodites, but I think
the evidence is now clear that it can,”
says plant biologist Spencer Barrett of
the University of Toronto.
rack may make its bearer, the features
can count as an evolutionary advantage if
they boost the organism’s number of offspring. Biologists have plenty of examples
of such sexy evolution among animals.
Finding examples among plants has been
a trickier business, though.
What’s a loud, visible chair-smashing
fight between animals could in plants
be a silent molecular conflict of pollen
grains, a seepage of compounds or the
activation of genes. Scientists are now
developing ways to scrutinize these
hidden interactions to look at basic
questions of plant sexual selection.
Evolutionary biologists want to know
whether individual plants can exercise
some choice among possible mates, and
whether those mates vie among themselves. Though the means, the scale and
the potential for lethal spurs or extreme
lipstick are quite different among plants,
the pressures driving flirting and fighting may be the same.
Even though “survival of the fittest” has
become the bumper sticker explanation for how natural selection shapes
life, the phrase omits a crucial point: An
aardvark or a zebra could grow so robust,
so attuned to its local environmental
Male fights Pollen grains from the Scots pine (right) may compete to fertilize
a female sex cell. When pitted against another strain, pollen dubbed P595 faced
a 46 percentage point drop in germination rate (left graph). That wimpy showing
may explain why a third strain, E88, held its own against P595 (right graph).
SOURCE: S. VARIS E T AL/ TREES 2010
Competitive failure of P595
Competitive success of E88
All alone With competitor
December 3, 2011 | SCIENCE NEWS | 17