without having looked at the images:
check your color space settings in nuke's read nodes.
if your maya renders are true linear set it to linear in Nuke as well, I think the default for tiffs is sRGB which would cause such a descrepancy.
also be aware of the fact that by default nuke is applying an sRGB look up to the viewer (check the settings pannel's LUT tab). Depending on your color workflow you may not want this.
cheers,
frank