Ratio of the fraction of stars forming at low metallicity (Z_O/H <  Z_O/H⊙) in the non-universal and universal IMF scenarios (hatched filled area) and the ratio of the number of stars forming in the two IMF scenarios (filled area), as a function of redshift. The solid lines correspond to low-Z extreme f_SFR(Z, z) distributions and the dashed lines to high-Z extreme f_SFR(Z, z) distributions. The thick lines correspond to the IGIMF3 model, while the thin lines show the IGIMF2 model (lower than the corresponding IGIMF3 lines). The coloured areas correspond to stars with initial masses in the range of 3–8 M_⊙, which is the mass range relevant for the formation of the progenitors of SNeIa. The grey areas in the background correspond to the WD progenitors with initial masses in the range of 1–8 M_⊙ (same as in Figs. 5 and 6).