Hybridization is an important evolutionary force and allows for the opportunity to study the genetic basis of complex traits in long-lived organisms, such as forest trees.  Previous work in the genus Populus has shown that genetic variation across a hybridizing system can structure arthropod communities, affect ecosystem processes such as litter decomposition and influence avian herbivory through tri-trophic interactions.  These studies propose that gene introgression is a potential process contributing to variation in community and ecosystem phenotypes but few have demonstrated how specific genetic material affects these patterns.  Here we examined how gene introgression between Fremont cottonwood (Populus fremontii) and narrowleaf cottonwood (P. angustifolia affects patterns of tree susceptibility to the keystone herbivore, Pemphigus betae.  In a genome wide survey of gene introgression, 322 diagnostic Fremont cottonwood AFLP markers were identified from 15 primer combinations.   

Results/Conclusions

Using a maximum likelihood method for determining marker introgression, we identified 32 diagnostic Fremont markers that introgressed at significantly higher than expected frequencies under neutral conditions.  To determine if gene introgression affects host plant susceptibility to P. betae we used association-mapping techniques for structured populations.  After controlling for populations genetic structure (hybrid index) and multiple test, four unlinked introgressing markers were identified as contributing to P. betae susceptibility.  Individually, these markers explained 7% to10% of the variation in tree susceptibility.  Together these markers displayed an additive effect on susceptibility and explained 21.8% of the variation in narrowleaf cottonwood susceptibility.  Our data demonstrate that introgression of genetic material from Fremont cottonwood can have pronounced affects on narrowleaf cottonwood susceptibility to P. betae.  These genetic findings are important because tree susceptibility to P. betae is a complex trait that has been shown to increase species diversity in riparian communities and affect ecosystem processes such as litter decomposition. Our findings also argue that AFLP markers are a useful tool for investigating the links between genetic introgression and tree susceptibility to key community members.