Figure 5.
Alternative SPS1/2 transcript isoforms sorted by retrotransposition within ascidians. This figure expands the section for tunicates in Figure 3. At the root of ascidians, the ancestral SPS2-Sec gene acquired a novel SPS-Gly transcript isoform through alternative exon usage at the 5′ end (AE). Then, at the root of the ascidian lineage, Styelidae and Pyuridae, the SPS-Sec transcript of this dual SPS1/SPS2 gene (SPS-ae) retrotransposed to the genome creating a novel SPS2-Sec gene (GDR). This presumably triggered the loss of Sec from the parental gene, which, because both the SECIS and the UGA containing exon degenerated (SL), specialized only in the production of SPS1-Gly.
