Genetic polymorphism in defense traits has been reported on diverse plant taxa. In this study, we tested a hypothesis that the negative frequency-dependent selection (NFDS) maintains defense polymorphism. In NFDS, rare morphs become more adaptive than majorities. To test this hypothesis, we focused on genetic dimorphism of trichome-producing (hairy) and non-trichome (glabrous) morphs in Arabidopsis halleri.

First, our field census examined the relationships among leaf damage, trichome production and morph frequency of plants growing within a 1m² patch. The leaf damage of one morph was negatively correlated with the relative frequency of the other morph growing in the close proximity. This suggests NFDS acts on the trichome dimorphism in the field.

Second, we performed a choice experiment to assess the feeding preference of the major herbivore Phaedon brassicae under different frequencies of the hairy and glabrous leaf diets. P. brassicae avoided the hairy leaves under the glabrous-abundant (hairy: glabrous = 1:3 in total 4 leaf discs) and equal (2:2) conditions. This result indicates that the leaf beetle foraged on the hairy leaves in negative-frequency dependent manner.

In summary, both our field and experimental results suggested the hypothesis that NFDS maintains anti-herbivore defense polymorphism in plant populations.