THE EFFECTS OF HOST-PLANT GENOTYPE, HYBRIDIZATION, AND ENVIRONMENT ON GALL-APHID ATTACK AND SURVIVAL IN COTTONWOOD: THE IMPORTANCE OF GENETIC STUDIES AND THE UTILITY OF RFLPS.

Using restriction fragment length polymorphisms (RFLPs) we show how host-plant genotype and hybridization in cotton wood, Populus sp., affects the attack and survival of the gall-forming aphid, Pemphigus betae. Fremont cottonwoods, hybrid ∗∗∗F1's and backcross ∗∗∗1's were found to be highly resistant, while backcross 2's, 3's, 4's and pure narrowleafs ranged from highly resistant to highly susceptible with only a few trees being highly resistant. Results from our genetic analysis also suggest that resistance is likely polygenic, not the result of single gene resistance. In addition, genetic analysis showed that studies based upon leaf morphology alone give an inaccurate assessment of host-plant genotype ∗∗∗and the extent of hybridization, altering previous views of the relationship between plant hybridization and pest attack. Previous studies assumed that narrowleafs were more resistant than backcross genotypes based upon comparisons of overall levels of resistance between the hybrid zone and the "pure" narrowleaf zone. Results from RFLP analyses, however, show that there are no significant differences in the levels of resistance between backcross genotypes (BC2's-4's) and pure narrowleafs. Furthermore, results show that the "pure" narrowleaf zone is in fact a mixture of pure and backcross genotypes, extending the zone of introgression previously reported. Experiments in combination with RFLP analyses suggest that resistance traits are differentially expressed along an environmental gradient partially explaining the previously reported differences in resistance between these two regions. In light of our results it is clear that genetic studies will be necessary to discern the true relationship between hybridization and pest resistance. Until such studies are widely conducted generalizations regarding the effects of hybridization on the structure and dynamics of pest populations will be premature at best. © 1993 The Society for the Study of Evolution.