Many flowers are either male or female. In some species, though, hermaphroditic flowers arose that have both sexual parts; once they appear, males and females begin to vanish, because they can't spread their genes as quickly. And yet in a few of these species, female flowers have strategies to survive alongside the hermaphrodites. Now botanists have discovered a situation in which the females owe their saving grace, oddly enough, to a weevil.
Only 2% of plant species have both females and hermaphrodites. The rare checker mallow Sidalcea hendersonii, which ranges from British Columbia to Oregon, has a particularly high percentage of females: almost 40% of the population. The puzzle is why. The seeds produced by female plants don't seem to survive better than those produced by hermaphrodites, says Fred Ganders, a botanist at the University of British Columbia in Vancouver.
He and his colleague Melanie Marshall wondered whether the weevils that eat checker mallow seeds might be responsible, so they studied the pattern of weevil predation at six field sites for 2 years. At the five sites where the weevil Anthonomus melancholicus was present, they consumed 39% of the seeds on hermaphroditic plants, compared with just 6% on the females, the researchers report in the August issue of the American Journal of Botany. The weevil's culpability was confirmed by the sixth site, where this weevil species was not present and other predators showed no such preferences. The females manage to survive, despite their less prolific reproduction, because the weevils spare them. The researchers are not sure what accounts for the bugs' taste for hermaphrodites, but suspect that they may be attracted to the pollen.
The study is the first to show that predators can change the proportion of the sexes among plants, says evolutionary ecologist Tia-Lynn Ashman at the University of Pittsburgh. She believes that the phenomenon is more widespread than scientists might think. "The fact that it hasn't been investigated," she says, "is probably more of an oversight than a result of it not going on."