The sigma light chain locus is located in the centromeric portion of chr1p (609.1 - 609.2 Mbp). (A) Gene density plot of chr1p where the IGS and the putative surrogate light chain (SLC) loci are encoded (box). Dark blue colors indicate low gene density. (B) Zoom of the whole putative SLC locus (67.5 - 68.6 Mbp). Non-Ig genes (black) in proximal flank DNAJC18 and distal flank TM4ESF5. Zoomed view of IGLC genes (blue). (C) Zoomed view (68.08 - 68.29 Mbp) of the putative SLC gene. (D) Spleen RNA-seq coverage histogram. (E) Zoom of the whole IGS locus (601 - 616 Mbp). Non-Ig genes (black) in proximal flank RAB11B gene and distal flank PRPF3. (F) Zoomed view of IGSC genes (blue), IGSV genes (red) and IGSJ (yellow) (609.15 - 610 Mbp) (G) Spleen RNA-seq coverage histogram. (H) Schematic representation of Immunoglobulin sigma locus in A. mexicanum genome (v6) (chr1p:609 - 610 Mbp), and X. tropicalis (chr1: 82.1 - 82.22 Mbp). Color code as in A. X. tropicalis displays the canonical architecture, in which the V, J and C clusters are in the same orientation. Note that in A. mexicanum, IGSV gene orientation in axolotl is intercalated, which is atypical. Non-Ig genes in the downstream flank are different in A. mexicanum and at least the PRPF3, RPRD2 and TARS3 genes are located in chr8 106.9 – 107.1 Mbp in X. tropicalis. Also, non Ig genes in the upstream flank correspond to the downstream flank in X. tropicalis. It is possible that this chromosomal configuration is correct, however it is also possible that the IGS locus may be inverted so that the downstream flank is syntenic with X. tropicalis. Not on scale.