Complex expression patterns of lymphocyte-specific genes during the development of cartilaginous fish implicate unique lymphoid tissues in generatingan immune repertoire

Cartilaginous fish express canonical B and T cell recognition genes, but th eir lymphoid organs and lymphocyte development have been poorly defined, He re, the expression of Ig, TCR, recombination-activating gene (Rag)-1 and te rminal deoxynucleosidase (TdT) genes has been used to identify roles of var ious lymphoid tissues throughout development in the cartilaginous fish, Raj a eglanteria (clearnose skate). In embryogenesis, Ig and TCR genes are shar ply up-regulated at 8 weeks of development, At this stage TCR and TdT expre ssion is limited to the thymus; later, TCR gene expression appears in perip heral sites in hatchlings and adults, suggesting that the thymus is a sourc e of T cells as in mammals. B cell gene expression indicates more complex r oles for the spleen and two special organs of cartilaginous fish - the Leyd ig and epigonal (gonad-associated) organs, In the adult, the Leydig organ i s the site of the highest IgM and IgX expression. However, the spleen is th e first site of IgM expression, while IgX is expressed first in gonad, live r, Leydig and even thymus, Distinctive spatiotemporal patterns of Ig light chain gene expression also are seen, A subset of Ig genes is pre-rearranged in the germline of the cartilaginous fish, making expression possible with out rearrangement, To assess whether this allows differential developmental regulation, IgM and IgX heavy chain cDNA sequences from specific tissues a nd developmental stages have been compared with known germline-joined genom ic sequences, Both nonproductively rearranged genes and germline-joined gen es are transcribed in the embryo and hatchling, but not in the adult.