Why do some plants defend themselves from insect attacks better than others? New evidence shows that the difference might be due to whether they're getting any plant love.
In research published in Proceedings of the National Academy of Sciences, scientists from North Carolina State University and Duke University discovered that sexually produced evening primrose plants withstand attacks from plant-eaters like caterpillars better than plant relatives that reproduce by themselves.
The findings are important steps to learning more about how plants have evolved defenses against insect herbivores, says Dr. Marc Johnson, assistant professor of plant biology at NC State and the lead author of the research paper.
"The variation in sexual reproduction has a large impact on the ability of plants to evolve defenses against herbivores," Johnson says.
In the study, the researchers performed both lab and field experiments on evening primrose (Onagraceae) plants, a plant family that has 259 different species - 85 percent of which reproduce sexually with the remainder reproducing asexually - to gauge the effects of plant sex on defense mechanisms. The researchers found that so-called generalist herbivores - those that eat a variety of plants - preferred to feed on the asexual species and lived longer while doing so.
The results were a bit different for so-called "specialist" plant-eaters, however. Those insects that prefer just one kind of food were more apt to munch on sexually reproduced species of plant. This most likely occurs, Johnson says, because specialized plant-eaters evolve alongside their hosts and have found ways to co-opt plant defenses. Instead of being deterred by certain chemical compounds produced as defenses by the plant, the specialized plant-eaters are attracted to them.
Johnson says the nuanced results make sense.
"Sex shuffles up genes and allows individual plants to get rid of bad genes and keep good ones," he said. "That helps them evolve defenses against generalist herbivores. Though there are short-term benefits to asexual reproduction - populations can grow more rapidly and propagate even when pollination is not possible - losing sex puts plants at a long-term disadvantage.
"In the end, asexual reproduction appears to be an evolutionary dead-end."
The research was funded by NC State, the Natural Sciences and Engineering Research Council of Canada, the National Science Foundation, Duke University and the National Institutes of Health. Johnson's co-authors from Duke University are Dr. Mark D. Rausher, professor of biology, and Dr. Stacey D. Smith, a post-doctoral researcher in biology.
Note: An abstract of the paper follows.
"Plant Sex and the Evolution of Plant Defenses Against Herbivores"
Authors: Marc T.J. Johnson, North Carolina State University; Stacey D. Smith and Mark D. Rausher, Duke University
Published: The week of July 13, 2009, in Proceedings of the National Academy of Sciences
Abstract: Despite the importance of plant-herbivore interactions to the ecology and evolution of terrestrial ecosystems, the evolutionary factors contributing to variation in plant defenses against herbivores remain unresolved. We used a comparative phylogenetic approach to examine a previously untested hypothesis (Recombination-Mating System Hypothesis) that posits that reduced sexual reproduction limits adaptive evolution of plant defenses against arthropod herbivores. To test this hypothesis we focused on the evening primrose family (Onagraceae), which includes both sexual and functionally asexual species. Ancestral state reconstructions on a 5-gene phylogeny of the family revealed between 18 and 21 independent transitions between sexual and asexual reproduction. Based on these analyses, we examined susceptibility to herbivores on 32 plant species representing 15 independent transitions. Generalist caterpillars consumed 32% more leaf tissue, gained 13% greater mass, and experienced 21% higher survival on functionally asexual than on sexual plant species. Survival of a generalist feeding mite was 19% higher on asexual species. In a field experiment, generalist herbivores consumed 64% more leaf tissue on asexual species. By contrast, a specialist beetle fed more on sexual than asexual species, suggesting that a tradeoff exists between the evolution of defense to generalist and specialist herbivores. Measures of putative plant defense traits indicate that both secondary compounds and physical leaf characteristics may mediate this tradeoff. These results support the Recombination-Mating System Hypothesis and suggest that variation in sexual reproduction among plant species may play an important, yet overlooked, role in shaping the macroevolution of plant defenses against arthropod herbivores.