A survey of 39 discrete populations of Turnera ulmifolia on Jamaica reveals extensive phenotypic variation for cyanogenesis among populations. The variation is quantitative and appears to be the result of differences in the quantity of cyanogenic glycoside possessed by plants. Controlled crosses and greenhouse studies show that there is a genetic basis to the variation with between-family variance accounting for more than 80 per cent of the variation in one population. Seedlings have significantly higher levels of cyanogenesis than mature plants in largely acyanogenic populations but this age-specific variation is absent in predominately cyanogenic populations. We identify potential selective agents that might account for the spatial distribution of cyanogenesis on the island and discuss the geographical pattern with respect to population elevation, precipitation regimes and the distribution of herbivores. This investigation provides the first detailed study of the ecological genetics of cyanogenesis in natural populations of a tropical plant.
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