Presence of an unique IgT on the IGH locus in three-spined stickleback fish (Gasterosteus aculeatus) and the very recent generation of a repertoire of VH genes

This study describes the IGH locus in Gasterosteus aculeatus, with 10 genes encoding three immunoglobulin classes: IgT, IgM and IgD. These genes are organized into a structure with three repeats of IGHT-IGHM-IGHD separated by segments including the VH segments. There was also a fourth IGHT gene. IGHT encodes an antibody with three immunoglobulin domains. Comparative studies indicate it is related to IgT and IgZ and other antibodies located upstream of the IGHM in teleost fish. The IGHM and IGHD are similar to the ones described in teleost. The IGHM has four immunoglobulin domains while the IGHD seven and none is duplicated. The IGH locus of G. aculeatus has 49 VH segments located in four regions. They belonged to four families, whose members show a greater than 92% amino acid identity, indicating that VH families diversified recently. Phylogenetic reconstruction suggests they were originated from four VH segments that must have duplicated with the constant region genes, after that the four VH segments gave rise to the remaining segments. This suggests the presence of an active biological process that generates diversity in VH regions.