  We derive homogeneous abundances of Fe, O, Na and alpha-elements from high
resolution FLAMES spectra for 76 red giant stars in NGC 6715 (M 54) and for 25
red giants in the surrounding nucleus of the Sagittarius (Sgr) dwarf galaxy.
Our main findings are that: (i) we confirm that M 54 shows intrinsic
metallicity dispersion, ~0.19 dex r.m.s.; (ii) when the stars of the Sgr
nucleus are included, the metallicity distribution strongly resembles that in
omega Cen; the relative contribution of the most metal-rich stars is however
different in these two objects; (iii) in both GCs there is a very extended Na-O
anticorrelation, signature of different stellar generations born within the
cluster, and (iv) the metal-poor and metal-rich components in M 54 (and omega
Cen) show clearly distinct extension of the Na-O anticorrelation, the most
heavily polluted stars being those of the metal-rich component. We propose a
tentative scenario for cluster formation that could explain these features.
Finally, similarities and differences found in the two most massive GCs in our
Galaxy can be easily explained if they are similar objects (nuclear clusters in
dwarf galaxies) observed at different stages of their dynamical evolution.
