Evidence from Amphioxus for acquisition of alternative mRNA splicing of NCoR corepressor after its duplication and divergence during vertebrate evolution.

NCoR-1 and NCoR-2 are transcriptional corepressors encoded in vertebrates by two interrelated loci and play distinct, though overlapping, roles in development, differentiation, and homeostasis. In contrast NCoR is encoded by a single locus in cephalochordates, urochordates, hemichordates, and echinoderms, with vertebrate NCoR-1 and NCoR-2 thought to be the products of a gene duplication originating near the beginning of vertebrate evolution. The structures, molecular properties, and functions of extant NCoR-1 and NCoR-2 are each substantially further diversified by alternative mRNA splicing; however it is unresolved as to whether the alternative-splicing observed in current day vertebrates reflects patterns present in the ancestral common gene or instead arose after the NCoR duplication event. This manuscript reports that Amphioxus, a cephalochordate considered representative of the organisms that gave rise to the vertebrate lineage, lacks the alternative NCoR splicing events characteristic of vertebrates. This, together with prior taxonomic comparisons, suggests that the patterns of corepressor splicing found in existing vertebrates arose exclusively after the NCoR duplication event. Further, given that alternative-splicing of NCoR-1 and NCoR-2 appears to have arisen by a mix of convergent and divergent evolution, it is likely that both common and distinct selective pressures were operative on these corepressor paralogs after their divergence.