Plant sex just got a little more interesting. By switching mating systems, casting off the normal, play-it-safe requirement to cross breed and adopting the ability to self-fertilize, plants are more likely to evolve the ability to ramp up chemical defenses when critters are eating them.
In a new PNAS Early Edition paper Cornell ecologists Stuart Campbell and Andre Kessler propose that the ability of nightshades to fight herbivores while being munched upon is driven by shifts in the plant’s mating strategy. This provides a new view of how plants evolve defense strategies, they write.
Generally, plants protect themselves through two means: weaponry, either chemical or physical, that is maintained all the time, as a constant guard; or they can deploy nasty chemicals when sensing an
attack. Botanists believe that, in general, plants living in varied environments, where the threat of herbivore attack is unpredictable, favor the latter strategy, termed “inducible.”
“This theory of becoming inducible when you escape enemies is an old one,” says Campbell. “What we have shown is that it may work as a result of the mating systems.”
Plants in the nightshade family (Solanaceae), including potatoes, tobacco, chilies, tomatoes, and Belladonna, are chemical-producing all-stars. They bring us many of our most loved scents and tastes, as well as some of our most notorious poisons. Ancestrally nightshades cannot self-fertilize–the gametes will not fuse. However, they have, over 60 separate times, lost this self incompatibility, giving researchers like Campbell and Kessler the chance to study what this mating shift has meant evolutionarily and ecologically.
“It was entirely possible that we would find nothing at all,” writes Campbell. “We found the strength of the effect to be somewhat surprising, particularly given the diversity of species.”
They found a strong link between mating strategy and a plant’s ability to respond quickly to herbivores. Across the 56 species tested, self-compatible plants were 63% more responsive to the hungry tobacco hornworms applied by researchers, as measured by a bioassay. Campbell believes this association exists in other plant families too, beyond the nightshades.
“[It] is a very good family to look at for a number of logistical reasons,” he says. “But many families exhibit similar variation in both reproduction and defense, so I would expect the pattern to hold generally.”
Campbell and Kessler believe this is because self-fertilizing plants are able to live on the fringes of habitats, where pollinators and other mates are in short supply. In more isolated habitats they are better able to escape their herbivorous enemies and favor the on-call mode of protection instead of the “constant guard.”
As far as mating strategies go, sexual reproduction is considered king by most species. Outcrossing normally gives species the most genetic variety for the next generation, and the best chance of survival. But in plants, at least, this doesn’t always seem to be the case. And in the future, Campbell says, as habitats become increasingly degraded, Campbell says, it’s possible that plants able to inbreed will have the upper hand.