Atlantic herring show record-setting diversity in key immune genes

Atlantic herring appear to carry an unusually broad genetic toolkit for recognizing disease-causing organisms, according to research reported by Phys.org from a study published in Science Advances. The study examined genes in the major histocompatibility complex, or MHC, a crucial part of the vertebrate immune system. MHC molecules display fragments of viruses, bacteria, parasites and other foreign proteins to immune cells, helping the body identify potential threats and respond to them. Researchers led by the Texas A&M College of Veterinary Medicine and Biomedical Sciences found especially high diversity in the herring’s MHC class II genes. The study reports that this level of diversity exceeds what has been recorded in any other vertebrate studied so far, including humans. To investigate the complex gene region, the team sequenced the complete genomes of 14 Atlantic herring collected from three locations across the species’ range. The researchers used long-read sequencing, which reads extended stretches of DNA and can help distinguish closely related genetic sequences that are difficult to separate with shorter reads. The variation came from two sources. The sections of MHC molecules that bind and present foreign proteins differed substantially among individual fish. In addition, the number of MHC genes varied from one herring to another, producing different combinations of immune-related genes. Together, those differences give individual fish distinct immune profiles and create a wide range of pathogen-recognition capabilities across the population. Atlantic herring commonly travel in schools that can contain hundreds of millions or even billions of fish, conditions that also allow viruses, bacteria and parasites to spread readily. The researchers believe the gene diversity reflects natural selection over millions of years as herring faced changing pathogen threats. However, the study does not establish whether similarly extreme variation occurs in other highly abundant species or across vertebrates more broadly. The team said applying long-read sequencing to other species could help determine whether the herring’s exceptional MHC diversity is unusual or part of a more widespread evolutionary pattern.