Effects of habitat heterogeneity on {\it Impatiens capensis\/} reproduction, genetic and phenotypic variation

Date of Completion

January 1996


Biology, Botany|Biology, Ecology|Biology, Genetics




The effects of natural light variation on the reproduction and on the distribution of both polygenic and phenotypic variability were investigated in an understory herbaceous species, Impatiens capensis, displaying an environmentally dependent mixed mating system. Previous greenhouse studies have shown that all individuals of I. capensis reproduce through selfing (cleistogamous flowers) regardless of light regime, but only those receiving enough light undergo outcrossing (chasmogamous flowers). The first objective was to study the impact of a continuous range of natural light regimes on I. capensis breeding strategy. I found that, first, the outcrossing rate does not vary continuously with light availability, but there is threshold lower limit to the amount of light necessary to induce chasmogamy. Second that light has a direct effect not only on chasmogamous, but also on cleistogamous flower production. However light has an even stronger impact on plant size which controls the production of both flower types. Therefore the reproductive mode of I. capensis is dependent on light availability primarily through its effects on plant size and only marginally through its direct impact on flower production. The second objective was to determine if populations growing in different mean light habitats, differ in the amount and/or distribution of their polygenic variation. Polygenic variation exists at a fine scale, among families within populations for a wide variety of morphological characters, regardless of population light regime. Among all the characters measured, only adult size traits showed evidence of genetic differentiation among populations, while no differences for reproductive traits were detected. This lack of correspondence between variation for adult size and reproductive traits suggests that adaptation to local environmental conditions results in different vegetative responses characterized by similar reproductive performances. Light environment also affects the distribution of phenotypic variation. However, fluctuations in light availability in highly shaded areas do not affect the expression of morphological variation, presumably because this variation in light values is outside I. capensis' range of light receptiveness. ^