Our results suggest, however, that balancing selection at the single population level, even if it occurred, may not have been a dominant process in shaping trichome variation. A caveat is that we do not know how non-selective processes such as demographic history and gene flow have affected trichome Axitinib variation in the study population. Under any assumption on population history, however, the lack of polymorphism in the glabrous allele of GL1 cannot be explained if balancing selection within the population has played a major role in shaping trichome variation. Even if balancing selection occurred in the past, its influence for increasing polymorphism of GL1 must have been overwhelmed by other processes that diminished the polymorphism of the glabrous allele. Our results are in contrast with a study on trichome variation in A. lyrata in which a fitness advantage of trichome production was found . However, a number of studies on variation in a defense trait against herbivores and pathogens have shown that the benefit of the trait was not clear, whereas the cost of defense was significant. The cost of trichome production was also found in A. kamchatica, in which glabrous plants produced more fruits than did hairy plants . In natural populations of D. wrightii, in which strongly defended sticky plants and weakly defended velvety plants coexist, a higher cost of sticky trichomes was found, but there was no evidence for balancing selection acting on the two phenotypes . Similarly, a cost of a resistance gene to a pathogenic Navitoclax infection was found but no significant benefit of the gene was detected in Ipomoea purpurea . Recent studies on plant resistance genes have also found patterns of genetic polymorphism that cannot be explained by either positive selection alone or balancing selection alone . Furthermore, a meta-analysis of phenotypic selection studies unveiled the temporary dynamic nature of selection acting in the wild . Thus, the simple form of balancing selection at the single population level may not be a general process for the maintenance of defense variation. For trichome variation in A. halleri subsp. gemmifera, we found that trichomes serve as a defense against larvae of the butterfly P. napi .