  The cluster M54 lies at the centre of the Sagittarius dwarf spheroidal
galaxy, and therefore may be the closest example of a nuclear star cluster.
Either in-situ star formation, inspiralling globular clusters, or a combination
have been invoked to explain the wide variety of stellar sub-populations in
nuclear star clusters. Globular clusters are known to exhibit light element
variations, which can be identified using the photometric construct called a
chromosome map. In this letter, we create chromosome maps for three distinct
age-metallicity sub-populations in the vicinity of M54. We find that the old,
metal-poor population shows the signature of light element variations, while
the young and intermediate-age metal rich populations do not. We conclude that
the nucleus of Sagittarius formed through a combination of in-situ star
formation and globular cluster accretion. This letter demonstrates that
properly constructed chromosome maps of iron-complex globular clusters can
provide insight into the formation locations of the different stellar
populations.
