We hypothesize that an AgR precursor was present in the PIC prior to the first round of whole-genome duplication (1R). Subsequently, MHC class I/II arose on the chr6/19 precursor after 1R. We anticipate that the RAG transposon insertion had not occurred until after the after the second round of genome duplication (2R), separated the VJ exon into separate exons only in genes on the chr19 precursor. Hallmark genes are indicated in red and psmb genes in light blue. MHC class I/II, AgR, complement, and NK receptors are shown in green, blue, orange, and purple, respectively.