Characterizing the genetic variation in seven species of deciduous native azaleas and identifying the mechanism of azalea lacebug resistance in deciduous azalea
Chappell, Matthew Randolph
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Despite the ecologic and economic importance of native deciduous azaleas (Rhododendron spp. section Pentanthera), our understanding of interspecific variation of North American deciduous azalea species is limited. Furthermore, little is known concerning intraspecific or interpopulation genetic variation. The present study addresses questions of genetic diversity through the use of amplified fragment length polymorphism (AFLP) analysis. Twenty-five populations of seven species of native azalea were analyzed using three primer pairs that amplified a total of 417 bands. Based on analysis of molecular variance (AMOVA) and estimates of Nei’s coefficients of gene diversity (HS, HT, and GST), the majority of variation in deciduous azalea occurs within populations. Both among species and among population variation was low, likely the effect of common ancestry as well as frequent introgression among members (and populations) of section Pentanthera. The majority of populations were grouped into species based on Nei’s unbiased genetic distances viewed as a UPGMA phenogram. The significance of these results is discussed in relation to breeding in section Pentanthera. In addition to the lack of information concerning genetic variation in North American native azaleas, little is known concerning the insect-plant interaction between the primary azalea pest in the United States, azalea lace bug (ALB) (Stephanitis pyrioides Scott), and deciduous azalea. Azaleas are largely resistant to predation by insects, with the exception of ALB. Within deciduous azalea (Rhododendron section Pentanthera) varying levels of resistance to ALB is observed with a continuous distribution from susceptible to highly resistant. In this study, epicuticular leaf wax from two ALB resistant [R. canescens Michaux and R. periclymenoides (Michaux) Shinners] and two ALB susceptible (‘Buttercup’ and ‘My Mary’) deciduous azalea genotypes was extracted and re-applied to fresh azalea foliage. Leaf wax extracted from ALB resistant genotypes and applied to ALB susceptible genotypes conferred a high level of resistance to ALB feeding and oviposition. Conversely, leaf wax extracted from ALB susceptible genotypes and applied to ALB resistant genotypes conferred susceptibility to ALB resistant genotypes. The results indicate that leaf wax serves as a primary mechanism of resistance of deciduous azalea to ALB.