QTL mapping in a foundation tree (Populus) reveals a potential relationship between introgression, ecologically significant traits, and community phenotypes

Thirty years of research on a cottonwood hybrid zone (Populus fremontii x P. angustifolia) has shown that genes in a foundation tree have ‘community phenotypes’ represented by predictable effects on dependent species and communities.  A gene ‘filter’ bounds the hybrid zone at its upper end, allowing few alleles from the low elevation species (P. fremontii) to introgress beyond the hybrid zone.  Introgression likely has important ecological consequences for both the recipient species (here P. angustifolia) and its’ dependent community. In this study we employ QTL data in order to better understand the genetic basis of both community phenotypes and introgression, as well as their potential relationships.  Using a linkage data from a controlled cross of naturally occurring parents, we have identified QTL for traits for chemical and phenological phenotypes that have extended effects on the community.  Preliminary data from association studies suggest markers linked to some QTL are under selection in the natural population.  Populus fremontii alleles associated with increases in the defensive chemical salicortin introgress beyond the hybrid zone, and may confer an adaptive advantage to recipient P. angustifolia.  In contrast, markers associated with early leaf flush appear to be negatively selected at higher elevations, possibly as a response to late frosts.  Phenology is also known to affect arthropod herbivores, and preliminary results suggest a second leaf flush QTL may be associated with abundance of the keystone leaf-galling aphid Pemphigus betae.  These results suggest potential relationships between leaf flush and introgression, with implications for community-wide effects.