The authors gratefully acknowledge the support of the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Alberta Agricultural Research Institute in the form of research grants to CPC, and the technical assistance of Charlotte Yates.

21kDa family of inhibitors from tomato has diverged to an extent that it is not easily detected outside of the Solanaceae (Van der Hoeven et al. 2002; Barta et al. 2002). Tiffin and Gaut (2001) investigated wipl, a wound inducible Bowman-Birk PI in Zea mays, and found evidence for an accelerated evolutionary rate of this PI in Zea relative to other monocotyle-doneous genera. Likewise, rapid evolution of animal PIs was deduced from the existence of hypervariable reactive centers of homologous proteins isolated from related species (Hill and Hastie 1987; Laskowski et al. 1987). In addition, positive selection acts on several plant resistance and defense-related genes causing rapid evolution of these genes (reviewed by Stahl and Bishop 2000). The availability of Populus genome sequences will allow such evolutionary analyses to be expanded to other defense related genes. Ultimately, our aim is to understand the extent to which defense responses vary among Populus species and genotypes, in the context of their ecological adaptations.

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