Early Evolution of Immunoglobulin Genes

Considerable progress has been made in the characterizationof immunoglobulin genes from several lower vertebrate taxa.Isolation and identification of immunoglobulin genes in phylogeneticallyprimitive species is based predominantly on heterologous crosshybridization.The unit, clustered organization of heavy chain segmental elementsobserved in the germline of the horned shark (Hinds and Litman,1986) has also been found in another elasmobranch. Studies todetermine whether the clustered organization is universal throughoutthe entire cartilaginous fish assemblage are ongoing. In contrast,the ray-finned (bony)fishes appear to possess a mammalian-typeheavy chain gene organization. Additionally, immunoglobulingenes are being characterized in two relict fish species whoseexact systematic relationships are unknown. Isolation of putativeimmunoglobulin genes from the phylogenetically- ancient hagfishis being attempted using a PCR-based approach. Other ongoingor future research efforts involve characterization of lowervertebrate light chain genes, heavy chain isotype evolution,and the divergence of the immunoglobulins and T-cell antigenreceptors