Science News - December 3, 2011

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 male-versus-male competition.

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 feathers dazzle.

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.

Beastly start

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).