Herbivores, pollinators and selection on flowering time in tetraploid and octoploid Cardamine pratensis
Repeated polyploidization events are thought to be among the most important causes of sympatric speciation throughout evolutionary time. Changes in phenology and trait expressions that arise in polyploids have been shown to affect plant interactions with insect herbivores and pollinators. If these interactions are changed, selection subsequent to the polyploidization event could contribute to further divergence, or increased similarity, of cytotypes. This study was conducted to investigate plant interactions with insect herbivores and pollinators and patterns of selection in tetraploid and octoploid Cardamine pratensis L. (Brassicaceae), and to answer the following questions: i) Is there phenotypic selection on flowering phenology and number of flowers? ii) Is pollen limitation or herbivory most important for variation in reproductive output? iii) During what stages of ovary and ovule development do variation in fitness mainly arise? iv) Can the intensity of interactions be linked to phenology and number of flowers, suggesting that selection is mediated by pollen limitation or herbivory? v) Do intensity of interactions, and of interaction-mediated selection, differ between ploidy levels?
From a combination of an observational study of herbivory and a hand pollination experiment, selection was found for earlier flowering in both subspecies. Intensity of herbivory was an important determinant of plant reproductive success, while hand pollination had little effect. Positive effects of flowering time on intensity of herbivory suggest that selection for earlier flowering was mediated by herbivores.
Tetraploids initiated flowering later and suffered from more intense herbivory than did octoploids. The direction of selection was similar in both subspecies. However, differences strength of selection and intensity of herbivory suggest that there is a possibility of selection for their further divergence.
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