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 , \sim 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 .