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We examine the metallicity trends in the Milky Way (MW) bulge - using APOGEEDR13 data - and explore their origin by comparing two N-body models of isolatedgalaxies which develop a bar and a boxy/peanut (b/p) bulge. Both models havebeen proposed as scenarios for reconciling a disc origin of the MW bulge with anegative vertical metallicity gradient. The first model is a superposition ofco-spatial, i.e. overlapping, disc populations with different scaleheights,kinematics and metallicities. In this model the thick, metal-poor, andcentrally concentrated disc populations contribute significantly to the stellarmass budget in the inner galaxy. The second model is a single disc with aninitial steep radial metallicity gradient, which is mapped by the bar into theb/p bulge in such a way that the vertical metallicity gradient of the MW bulgeis reproduced -- as shown already in previous works in the literature. However,as we show here, the latter model does not reproduce the positive longitudinalmetallicity gradient of the inner disc, nor the metal-poor innermost regions ofthe Bulge seen in the data. On the other hand, the model with co-spatial thinand thick disc populations reproduces all the aforementioned trends. Wetherefore see that it is possible to reconcile a (primarily) disc origin forthe MW bulge with the observed trends in metallicity by mapping the inner thinand thick discs of the MW into a b/p. For this scenario to reproduce theobservations, the $\alpha$-enhanced, metal-poor, thick disc populations musthave a significant mass contribution in the inner regions -- as has beensuggested for the Milky Way.

What the Milky Way bulge reveals about the initial metallicity gradients in the disc