Joined: May 2002
here's an extremely important article from nature immunology:
Nat Immunol 2002 Dec;3(12):1200-7
Identification of diversified genes that contain immunoglobulin-like variable regions in a protochordate.
Cannon JP, Haire RN, Litman GW.
 Immunology Program, H. Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Avenue, Tampa, FL 33612, USA.  All Children's Hospital, 801 Sixth Street South, St. Petersburg, FL 33701, USA.
The evolutionary origin of adaptive immune receptors is not understood below the phylogenetic level of the jawed vertebrates. We describe here a strategy for the selective cloning of cDNAs encoding secreted or transmembrane proteins that uses a bacterial plasmid (Amptrap) with a defective beta-lactamase gene. This method requires knowledge of only a single target motif that corresponds to as few as three amino acids; it was validated with major histocompatibility complex genes from a cartilaginous fish. Using this approach, we identified families of genes encoding secreted proteins with two diversified immunoglobulin-like variable (V) domains and a chitin-binding domain in amphioxus, a protochordate. Thus, multigenic families encoding diversified V regions exist in a species lacking an adaptive immune response.
link to article
incidently, i did a blast search of one of these genes on the recently-sequenced ciona intestinalis (tunicate) genome, and got a hit.