  We present reliable measurements of the metallicity distribution function
(MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf
spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of
candidate M giant members of the Sgr system. The Sgr MDF is found to evolve
significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr
leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal
stripping) age. This is direct evidence that there can be significant chemical
differences between current dSph satellites and the bulk of the stars they have
contributed to the halo. Our results suggest that Sgr experienced a significant
change in binding energy over the past several Gyr, which has substantially
decreased its tidal boundary across a radial range over which there must have
been a significant metallicity gradient in the progenitor galaxy. By accounting
for MDF variation along the debris arms, we approximate the MDF Sgr would have
had several Gyr ago. We also analyze the MDF of a moving group of M giants we
previously discovered towards the North Galactic Cap having opposite radial
velocities to the infalling Sgr leading arm stars there and propose that most
of these represent Sgr trailing arm stars overlapping the Sgr leading arm in
this part of the sky. If so, these trailing arm stars further demonstrate the
strong MDF evolution within the Sgr stream.
