Evolutionary scenarios for the convergent loss of Sowah coding exons near Irx genes in chordates. (A) A retroposition event to other parts of the genome (indicated as a solid black arrow, r-Sowah) allows the original, Irx-linked Sowah to lose the coding sequences (black bars), while retaining the functional noncoding regions (red). A similar event occurred at the base of the vertebrates. (B) A polyploidization (a WGD) creates redundancy of Sowah genes. Therefore, some Sowah genes can lose their coding sequences. This event was observed in teleosts, after the third round of WGD. (C) Gene redundancy is acquired by tandem duplication of Irx and Sowah, as reported for amphioxus. Subsequently, one of the Sowah copies loses its coding sequences, whereas the functional noncoding regions are maintained.
