Barriers to Gene Flow across a Light Gradient in Cardamine cordifolia.

Abstract

Ecotypic variation can be driven by a disproportionate balance between selective pressures and gene flow. If the strength of selection is weak relative to the level of gene flow, genetic variation will likely be homogenized among populations. However, in the absence or severe limitation of gene flow, divergent selection pressures are more likely to result in adaptive evolution to local environmental conditions. Both biotic and abiotic factors can limit gene flow, and these barriers can highly influence the exhibited traits in plants along a habitat gradient. At the Rocky Mountain Biological Laboratory, a native mustard Cardamine cordifolia has exhibited habitat specific differentiation across a light quality gradient. Flowering phenology was examined in Cardamine cordifolia in open sun, intermediate shade and deep shade habitats to determine if a separation of first, peak and last flowering time within the different environments was a mechanism for pre-zygotic isolation. I hypothesized that C. cordifolia found in open sun habitats will have an earlier flowering phenology in comparison to C. cordifolia found in deep shade habitats. Using two sites, a highly significant difference in first, peak and last flowering time was observed in the open sun and deep shade habitats. This gap in flowering phenology demonstrates a limited ability for open sun and deep shade plants to outcross in the short alpine growing season. Without gene flow the extremes of the light gradient are able to adapt locally in response to unique, divergent selective pressures.

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