Loss or gain of DNA methylation can affect gene expression and is sometimes transmitted across generations. Such epigenetic alterations are thus a possible source of heritable phenotypic variation in the absence of DNA sequence change. However, attempts to assess the prevalence of stable epigenetic variation in natural and experimental populations and to quantify its impact on complex traits have been hampered by the confounding effects of DNA sequence polymorphisms. To overcome this problem as much as possible, two parents with little DNA sequence differences, but contrasting DNA methylation profiles, were used to derive a panel of epigenetic Recombinant Inbred Lines (epiRILs) in the reference plant Arabidopsis thaliana. The epiRILs showed variation and high heritability for flowering time and plant height (∼30%), as well as stable inheritance of multiple parental DNA methylation variants (epialleles) over at least eight generations. These findings provide a first rationale to identify epiallelic variants that contribute to heritable variation in complex traits using linkage or association studies. More generally, the demonstration that numerous epialleles across the genome can be stable over many generations in the absence of selection or extensive DNA sequence variation highlights the need to integrate epigenetic information into population genetics studies.

Maize plants (Zea mays) attacked by caterpillars release a mixture of odorous compounds that attract parasitic wasps, natural enemies of the herbivores. We assessed the genetic variability of these induced volatile emissions among 31 maize inbred lines representing a broad range of genetic diversity used by breeders in Europe and North America. Odors were collected from young plants that had been induced by injecting them with caterpillar regurgitant. Significant variation among lines was found for all 23 volatile compounds included in the analysis: the lines differed enormously in the total amount of volatiles emitted and showed highly variable odor profiles distinctive of each genotype. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which may share common metabolic pathways. European and American lines belonging to established heterotic groups were loosely separated from each other, with the most clear-cut difference in the typical release of (E)-b-caryophyllene by European lines. There was no correlation between the distances among the lines based on their odor profiles and their respective genetic distances previously assessed by neutral RFLP markers. This most comprehensive study to date on intraspecific variation in induced odor emission by maize plants provides a further example of the remarkably high genetic diversity conserved within this important crop plant. A better understanding of the genetic control of induced odor emissions may help in the development of maize varieties particularly attractive to parasitoids and other biological control agents and perhaps more repellent for herbivores.The release of odorous compounds by plants in response to herbivore attack and the subsequent use of these odorous signals by natural enemies to locate the herbivores is a widespread phenomenon observed in various tritrophic systems, e.g. predatory mites and spider mites on Lima bean (Dicke and Sabelis, 1988), parasitic wasps and lepidopteran caterpillars on cabbage (Mattiacci et al., 1994) and cotton (Loughrin et al., 1995), and anthocorid predators and psyllids on pear trees (Scutareanu et al., 1997). We are currently studying this kind of interaction in a system that comprises maize (Zea mays), folivorous caterpillars in the genus Spodoptera, and the associated endoparasitic wasps (Turlings et al., 1990). Odor release by maize plants is a rapid systemic response (Turlings and Tumlinson, 1992) induced by elicitors present in the oral secretions of the herbivores, such as volicitin, a compound isolated from regurgitant of Spodoptera exigua Hü bner caterpillars (Alborn et al., 1997;Turlings et al., 2000).Plants belonging to various species and families differ in their odor profiles (Turlings et al., 1993b). Nonetheless, the odor patterns of different plant taxa exhibit considerable overlap, and certain compounds like the terpenoids linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, and (E)-b-ocimene seem to be qu...

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