Population differences in defense trait expression in Passiflora incarnata

Background/Question/Methods

Most plants exhibit a range of defenses against insect herbivores, but the determinants of this production are still poorly understood.   Numerous studies have demonstrated local adaptation of species among populations in plant defense production.  One way to better understand plant defenses is to utilize population variation to investigate how varied investment in herbivore defenses impacts plant fitness.

Passiflora incarnata (‘maypop’) is a perennial vine native to the southeast United States that produces both direct, physical defenses (leaf toughness and trichomes) and indirect defenses (extrafloral nectar in a defense mutualism with ants).  To look for differences in expression of these defense traits, we used two years of common garden experiments with plants originating from two different populations.  We calculated the heritability of these traits to determine the environmental versus genetic determinants of trait values and looked at genetic correlations between traits.  We also investigated if investment in defense traits lead to changes in herbivore damage between populations.  

Results/Conclusions

We found differences in defense trait expression and herbivore preference between plants from southern (Florida) and northern (North Carolina) populations.  Plants from Florida invested more in both direct and indirect defenses.  For both populations, trichomes featured higher levels of heritability than the other measured traits, indicating greater genetic control over their expression.  Genetic correlations between traits were low, indicating low levels of linkages between different traits.  Despite differing in defense production, plants from both populations experienced similar levels of damage from herbivores.  These population differences in defense trait values can be utilized to further investigate the impacts of plant defense traits on plant and herbivore fitness.