Threshold Size for Vernalization in Senecio jacobaea: Genetic Variation and Response to Artificial Selection

Abstract

1. Flowering in Senecio jacobaea is dependent on plant size at the time of vernalization and at the time of photoinduction in spring. We present data on genetic variation in the threshold size for vernalization in this species. 2. Field measurements revealed ample phenotypic variation in threshold size for flowering in a natural sand dune population of S. jacobaea. The parental generation of the artificial selection experiment, carried out in a climate chamber, showed that the genetic variation for this character was also very high. 3. In the artificial selection experiment offspring from different selection lines were compared. In the parental generation 56% of the largest plants did not flower. Large plants that remained vegetative in the parental generation did not flower after a second period of growth and subsequent vernalization. A selection line for ability to flower under the conditions provided was established by selecting the largest flowering plants. One generation of selection decreased the non-flowering fraction to 20%, compared to 38% in the control group, which was a second sample from the same seed pool as the parental generation. 4. In a second line, selection took place on low threshold sizes for vernalization, by selecting the smallest flowering plants in the parental generation. In this group the non-flowering fraction also decreased, and in addition the median threshold size was much lower than in the line selected for flowering ability only, indicating that selection on low threshold size was successful. 5. Significant differences in threshold size were found among five clonal families grown in a controlled environment. The within-clone flowering percentage was always below 100% for large plants. 6. The results show that threshold size for vernalization in S. jacobaea has a genetic component, and may be subject to natural selection.