Mass-metallicity relation. LEWIS data points are colour-coded as in Fig. 1, while other coloured data points correspond to literature samples: UDGs in clusters (light green circles) and in low-density environments (dark green diamonds) from Ferré-Mateu et al. (2023), UDGs in low-density environments (violet circles) from Buzzo et al. (2024a), and UDGs and nearly UDGs in high-density environments (violet diamonds) from Buzzo et al. (2024b). The solid black line represents the mass-metallicity relation found for dwarf galaxies, assuming a constant α-enhancement (Kirby et al. 2013), and the dashed grey lines represent the 1σ and 2σ confidence regions of the relation. The yellow line represents the evolving mass-metallicity relationship at redshift z ∼ 2.2 from Ma et al. (2016).