Molecular evolution of the wound-induced serine protease inhibitor wip1 inZea and related genera

Plant defense mechanisms have been the subject of intensive investigation. However, little is known about their long-term evolutionary dynamics. We in vestigated the molecular diversity of a wound-induced serine protease inhib itor, wip1, in the genus Zea, as well as the divergence of wip1 among four genera, Zea, Tripsacum, Sorghum, and Oryza, in order to gain insight into t he long-term evolution of plant defense. The specific objectives of this st udy were to determine (1) whether wip1 has a history of positive or balanci ng selection, as has been shown for genes involved in plant defense against pathogens, and (2) if the evolutionary histories of wip1 inhibitory loops, which come into closest contact with proteases, differ from the evolutiona ry history of other parts of this gene. The Zea polymorphism data are consi stent with a neutral evolutionary history. In contrast, relative-rate tests suggest a nonneutral evolutionary history. This inconsistency may indicate that selection acting on wip1 is episodic or that wip1 evolves in response to selection favoring novel alleles. We also detected significant heteroge neity in the evolutionary rates of the two inhibitory loops of wip1-one inh ibitory loop is highly conserved, whereas the second has diverged rapidly. Because these two inhibitory loops are predicted to have very similar bioch emical functions, the significantly different evolutionary histories sugges t that these loops have different ecological functions.