Selection and adaptation to novel environmental conditions in introduced genotypes of the annual plant Brassica rapa

Background/Question/Methods

Understanding the selective pressures and evolutionary responses of introduced species is crucial to making informed decisions on biodiversity management. While there is some evidence for rapid evolution occurring in invasive species, we currently know relatively little about the process by which introduced species adapt to novel conditions in their new habitat. To examine evolution following introduction, we collected seeds of the annual plant Brassica rapa L. (Brassicaceae) from southern California and planted them in the field in New York.  Brassica rapa is an appropriate model organism to study evolution following introduction since there is documented evidence of rapid evolution to changing climatic conditions and the genome has recently been sequenced, allowing investigation of the genetic basis of adaptation. We collected data on climatic and soil conditions and collected tissue samples for analysis of water use efficiency in both locations.  For the plants collected from seed in California and grown in New York, we measured plant growth rate, size, and reproductive output.  We predicted that water and nutrients would be more abundant in New York than California, resulting in positive selection for increased plant size, more rapid growth rate, and decreased water use efficiency. 

Results/Conclusions

We found substantial differences in environmental conditions between the southern California collection location and the New York planting location.  Both average growing season precipitation and total soil nitrogen concentration were over 200% greater in New York than California (p<0.001).  Plants grown in New York had lower water use efficiency (more negative δ13C [t=4.08, p<0.001]) compared to those in California, as expected under conditions of reduced water limitation in New York. Selection analysis on the first generation of B. rapa plants grown in New York showed positive linear multivariate phenotypic selection gradients for larger overall size (increased stem diameter [β=0.688] and number of leaves [β=0.913]) as well as decreased leaf size (decreased length of longest leaf [β=-0.277]). These results suggest that under conditions of greater resource abundance, there is selection for larger size and decreased water use efficiency, and we would predict evolutionary change in this direction.  This prediction will be tested using the ‘resurrection’ approach of ancestor-descendent comparisons, which will directly examine if adaptive evolution has occurred.