Fig. 1

Neighbor-joining tree showing the phylogenetic relationship among gastropoda MT β-domains (DM) analyzed with MEGA7 software (ver. 7.0.21) (Kumar et al. 2016). Based on the bootstrap tests (1000 replicates), only the clades supported by higher than 50% (condensed tree cutoff value = 50%) are visualized. For MT abbreviations and species, refer to Additional file 1: Table S1. In the NJ tree, two distinct clades, respectively, comprising of β₁- and β₂-domains indicate the presence of two ancestral β-domains in the gastropod lineage (Jenny et al. 2016). Typical arrangement patterns of Cys motifs for gastropod β₁- and β₂-domains are noted at the right side of the tree. Most gastropod MTs are structured as N-terminal β₂-domain linked to C-terminal β₁-domain. However, noticeably, some gastropod MTs are proposed to possess more than two β-domains (likely caused by tandem duplication) based on newly recognized evidences from B. glabrata MT (β₂β₂β₂β_1P-like shape) and L. littorea MT (β₂β₂β₁-structure). Evidence for the β₂-domain duplication from the sequence alignment analysis can also be referred to in Additional file 1: Figure S2