Immunoglobulin and T cell receptor genes in Chinese crocodile lizard Shinisaurus crocodilurus

Squamata are reptiles that diverged from mammals 300 million years ago. During this period, the immunoglobulin (IG) and T lymphocyte receptor (TCR) genes evolved parallel to mammals. However, unlike mammals whose IG/TCR locus has retained a constant structure throughout evolution, Squamata have witnessed duplications, losses, and/or gains in the domains of their immunoglobulin genes. The recent genome sequencing of Shinisaurus crocodilurus, a representative species of the oldest reptiles, provides an opportunity to contrast the structure of IG and TCR genes from previously studied Squamata. This study revealed ten immunoglobulin genes: five genes for immunoglobulin M (IgM), two for immunoglobulin D (IgD), one for immunoglobulin D2 (IgD2), and two for immunoglobulin Y (IgY). As in other Squamata, there are genes for the λ light chain (IGLV) but not for the κ chain (IGKV). Here, the data shows that in some IgM genes, the cysteine needed to bind the λ chains does not exist, but we present evidence for possible non-covalent binding to the light chain. With respect to TCR, one gene is detected for the α constant chain (TRAC) and two genes for the β constant chain (TRBC); one of which is located in the locus of the variable regions of the heavy chain. As in the rest of the Squamata, genes for the γ/δ T cell receptor were not found. The V gene repertoire is found to be consistent with all other Squamata with few V genes for beta chain of TCR.