Supplementary Table 1 . The SEPP1 Sec content, selenium requirements, and the biomarkers, statistical methods and the selenium species used to assess the selenium requirements of species included in this study
Abbreviations; Sec, selenocysteine, SEPP1, Selenoprotein P; TXNRD, thioredoxin reductase; GPX, glutathione peroxidase; BLR, Broken line regression; Na2SeO4, sodium selenate; Na2SeO3, sodium selenite; Se-yeast, selenoyeast; NaHSeO3, sodium hydride selenite; SeMet, selenomethionine. * Methods utilised to analyse tissue GPX activity are unable to distinguish between isoforms, so are listed as total GPX activity. However, in mammals GPX1 is responsible for the majority of total GPX activity (Brigelius-Flohe et al., 2002) . 1The authors of the guinea pig study state a Se requirement of 0.08 mg Se/kg DM, which includes a safety margin above the 0.06 mg Se/kg DM predicted with BLR. 2Data from actively growing juvenile animals was utilised in preference to adults 3Sec content of these species were based on closely related species (Gibel carp and loach are both cyprinids, as are common carp (Cyprinus carpo) and zebrafish which both have SEPP1 (SEPP1a) with 17 Sec residues) or on salt water fish (Both green spotted pufferfish (Tetraodon nigroviridis) and fugu (Takifugu rubripes) have SEPP1 with 17 Sec residues). Irrespective of this, the range of Sec residues found in fish SEPP is small, being 15 to 17 (Lobanov et al., 2008) .
Supplementary Table 2 . The amino acid sequences of SEPP1 (aka SEPP1a) in vertebrate species included in this study and closely related species (fish)
The total Sec (U) and the Sec content upstream and including the APOER2 binding site (E-CQC----A; shaded in yellow) within the C-terminal domain (SEPP1←APOER2), and the region downstream of the APOER2 binding site (SEPP1APOER2→) are also shown. *Sequence obtained at ensemble.org then searched for Sec and SECIS elements using http://seblastian.crg.es/ **Sequences obtained from Lobanov et al. (2008). Abbreviations; SEPP1←APOER2, Sec residues in the C-terminal domain of full length SEPP1in and upstream of the APOER2 binding site (E-CQC----A; in fish may range between E-CQC--A to E-CQC-----A); SEPP1APOER2→, Sec residues in full length SEPP1upstream of the APOER2 binding site.
Supp. Fig. 1. The relationship between the selenocysteine content within specific domains of selenoprotein P and selenium requirements
The solid lines with the solid circles (●) is the best fit model for the number of Sec residues found upstream and including the APOER2 binding site in the C-terminal of SEPP1 versus the selenium requirements (mg Se/kg DM) in mammals and bony fish. The broken lines represents the same data modelled with an additional five bony fish species with known Se requirement levels (○), but unannotated genomes as described in Fig. 2. The solid line is linear, R2 = 0.82, y = 1 + 35x, while the dashed line is 5PL asymmetric sigmoidal, R2 = 0.92, y = = -6.54 + (17.5/((1+10^((-1.75538-X)×5.851))2.999^10)). X axis is log transformed.