On the other hand, recent molecular studies have begun to reveal the genetic basis of defense variation in model species . However, identifying candidate genes involved in ecologically relevant phenotypic variation remains a challenging task for wild species. Furthermore, the ecological function of polymorphic genes in natural conditions is SCH727965 poorly understood . In VE-821 particular, little is known about how genetic polymorphism is maintained within a natural population, because most of relevant studies took a species-wide sampling strategy . As each natural population has unique history of selection and demography, investigating how genetic polymorphism is maintained at the single population level is the first step towards understanding evolutionary processes that have shaped genetic and phenotypic variation. In the present study, we examined fitness consequences and genetic basis of within-population variation in trichome production in Arabidopsis halleri subsp. gemmifera . In one natural population in Japan, nearly half of the plants develop trichomes on the surface of their leaves and flowering stems , whereas the other half completely lack trichomes . This system provides a unique opportunity to study the genetic basis and ecological consequences of distinct morphological variation for the following reasons. First, previous studies in various species have shown that trichomes represent a defense trait against insect herbivores , including in A. thaliana , A. lyrata and other Brassicaceae species . However, the detailed knowledge of ecological factors that allow the coexistence of distinct trichome phenotypes within a population is still limited. In particular, relative fitness for different phenotypes needs to be examined across multiple environmental conditions. Second, we are able to adopt a candidate gene approach because A. halleri subsp. gemmifera is a close relative of the model plant species A. thaliana . The molecular genetics of trichome development in A. thaliana is well understood, and genes involved in trichome production have been identified . As a candidate gene, we focused on a homologue of GLABROUS1 , which encodes a MYB-family transcription factor involved in the initiation of trichome development in A. thaliana .