Cutting edge: recruitment of the ancestral fyn gene during emergence of the adaptive immune system.

The adaptive immune system (AIS) is characterized by the MHC molecules and the rearranging Ag receptors, and was established in a common ancestor of jawed vertebrates. Fyn, a Src-family tyrosine kinases, is important for normal development and function of T lymphocytes and neuronal cells. Indeed, as the result of an alternative splicing of a distinct exon 7, fyn encodes for two isoforms, FynT in T lymphocytes and FynB in the brain. How this alternative splicing of fyn transcripts has emerged and evolved in relation to the setting of the AIS remains to be established. In this study, we show that exon capture in a vertebrate ancestor by the fynT-like gene has yielded a novel fyn-encoded isoform, fynB. Unexpectedly, the newly established AIS recruited the ancestral Fyn isoform, FynT, whereas the CNS expresses the most recent one, FynB. These results shed new light on the emergence of the AIS.