Immunoglobin D (IgD) is present in all jawed vertebrates, but its existence in acipenseriformes has previously not been proven. The acipenseriformes are fishes with a cartilaginous endoskeleton which are evolutionarily placed between elasmobranchs and teleost fish. The discovery of the IgD encoding gene in the Siberian Sturgeon (Acipenser baerii) bridges the gap in immunoglobin evolution between elasmobranchs and teleosts.
A gene encoding immunoglobin W (IgW) has been identified in sharks, whilst the IgD encoding gene has been proved to be present in teleost fish. As IgW has previously been believed to be a homolog of IgD, this would indicate that the IgD found in teleost fish and mammals - including humans - can be traced back to the early evolution of sharks. However, without evidence of similar IgD genes in the intermediate phase between elasmobranchs and teleost fish, the evolutionary pathway could not be assured.
The researchers used polymerase chain reaction (PCR) and high throughput sequencing to identify the IgD gene in the acipenseriform fish A. baerii. The researchers found for the first time that the IgD encoding gene was expressed in the Siberian Sturgeon and furthermore that the gene shared features with that of teleost fish.
Immunoglobins (or antibodies) are key components of the vertebrate immune system and being able to trace the lineage of our own immune system back to sharks provides evidence that sharks have been an important evolutionary template for our own species as well as all fish and mammals which express this antibody.
Lin Zhu, Zhenxin Yan, Mengyang Feng, Dezhi Peng, Ying Guo, Xiaoxiang Hu, Liming Ren, Yi Sun. (2014) Identification of sturgeon IgD bridges the evolutionary gap between elasmobranchs and teleosts. Developmental & Comparative Immunology. 42(2):138
