Thursday, August 5, 2010

PS 74-82: Environmental conditions select the life history strategy of Capsella bursa-pastoris (Brassicaceae)

Kyle Harrison, University of Nebraska-Lincoln

Capsella bursa-pastoris (Brassicaceae) is a weedy plant that exhibits a variety of life history strategies, including the fall annual strategy (germinates and flowers in the fall) and winter annual strategy (germinates in the fall and flowers in the spring). In addition, in a Lancaster Co. NE population, C. bursa-pastoris exhibits a twice-flowering life history strategy, germinating and flowering in the fall then flowering again in the spring. What are the conditions favoring the selection for the different life history strategies (i.e. fall, winter, and twice-flowering)? We developed a theoretical model describing the relative conditions for which each life history strategy is selected. Our model predicts: 1.) fall-flowering plants are selected when fall conditions are good, 2.) spring-flowering plants are selected when spring conditions are good, and 3.) the twice-flowering annual is selected when both fall and spring conditions are good. We tested these predictions by sowing seed from mothers with winter annual or twice-flowering life histories and monitoring their rate of germination, flowering, and relative fitness. We monitored these plots through the fall and will continue to monitor them through the spring. 
Our experimental results, thus far, support our first model prediction. As plot quality increased the proportion of plants flowering in the fall increased. In addition, plants that had twice-flowering mothers had a significantly higher probability of flowering in the fall. We will continue our experiment throughout the spring to test the second and third model predictions. In addition, fall and spring data together will allow us to quantify genetic variation of life history strategies. Finally, we will determine if the relative quality of the fall and spring growing seasons affects the expression of life history strategy, conforming to the overall dynamic predicted by our model.