Conserved linkage among sea urchin homologs of genes encoded in the vertebrate MHC region

Abstract

The echinoderms, along with their sistergroup the hemichordates, form an outgroup to the chordates within the deuterostomes. As such, they offer a potentially valuable source of information from which to infer the primitive state of chordate chromosomal organization. As part of an ongoing Sea Urchin Genome Project we have constructed Bacterial Artificial Chromosome (BAC) and P1 Artificial Chromosome (PAC) libraries from the purple sea urchin (Strongylocentrotus purpuratus). We have isolated a number of sea urchin genes that have vertebrate homologs encoded in the MHC locus. BAC and PAC clones that contain these genes have been identified and are being characterized. Two complement factor genes, an S. purpuratus C3/4/5 homolog and a Bf/C2 homolog, are both encoded within a single 140 kb BAC clone insert. We have also found convincing evidence for linkage between S. purpuratus homologs of Notch and a PBX homeodomain transcription factor, vertebrate homologs of which are encoded in close proximity within the MHC class III region. These observations suggest a remarkable conservation of large scale synteny within the deuterostomes, and may lead to the identification of divergent sea urchin relatives of vertebrate class I or II MHC genes.