Why does the immune system of Atlantic cod lack MHC II?

• DOI: 10.1002/bies.201200005
• Publication Date: 2012-04-13T09:15:52Z
• Authors: Bastiaan Star; Sissel Jentoft
• Keywords: 0303 health sciences; Insights & Perspectives; Bacteria; Genes, MHC Class II; Genetic Drift; Toll-Like Receptors; Adaptive Immunity; Environment; Biota; 3. Good health; Cold Temperature; Evolution, Molecular; 03 medical and health sciences; Gadus morhua; Species Specificity; Host-Pathogen Interactions; Animals; Selection, Genetic

MHC II, a major feature of the adaptive immune system, is lacking in Atlantic cod, and there are different scenarios (metabolic cost hypothesis or functional shift hypothesis) that might explain this loss. The lack II coincides with an increased number genes for I Toll-like receptors (TLRs). MHC, histocompatibility complex; TLR, receptor. Basic features vertebrate system have been conserved since their emergence ancestral lineage lead to all jawed vertebrates 1, 2. cod (Gadus morhua) first reported lost one these features, antigen presenting complex (MHC) 3. This responsible classical response against bacterial parasitic infections through activation CD4+ T cells other vertebrates, defects functionality general considered severe immunodeficiency death. Therefore, evolutionary understanding phenomenon could uncover novel immunological elucidate basic selective mechanisms affecting systems. context which unusual evolved unclear, though two may its existence. scenario hypothesizes not crucial defense teleosts, loss has predominantly driven by genetic drift, rather than specific biological environmental factors. second somewhere past function directional selection. An important assumption such selection some factors process, contrast whereby no association expected. Within scenario, we here consider alternative hypotheses separately combination II. First, metabolic suggests despite being value, benefit was environments sufficient compensate costs expressing system. Thus, would favor mutations disrupt expression resulting eventual underlying function. Second, components either belonging innate provided more flexible strategy compared then given better adaptation certain challenges making obsolete exposed drift. It also be further evolution required dismantling mechanistic reasons thus caused pressure presence. Below assess likelihood each using available empirical data elaborate on investigations aimed separate them. drift appears implausible from mammalian perspective. However, fundamental differences genomic organization found teleost lineages. For example, mammals, linkage exists teleosts between 4, absence facilitate independent Moreover, show greater structural diversity while restricted, example limited antibody repertoire 5. Because matures relatively late ontogeny, initially rely responses during extrauterine development 6. therefore critical easily If critical, expected wide variety ecological relationships cod. Despite reports systems sea horse pipefish, widespread however, does appear prevalent among 7, 8. pure insufficient regard architecture fundamentally different. proposes can independently changes whereas expand as prerequisite Can find evidence gene hypothesis? Interestingly, several families both expansions (TLRs) 3, indicative enhanced (Fig. 1). expansion TLRs particularly intriguing. High copy numbers distant lineages invertebrates (sea urchin, 222 TLRs) chordates (amphioxus, 77 TLRs), discussed relation specialized species 9-11. positive receptor families. These symbolized extensive tree topology (red) (shades green). While still unknown, highlights possible existence trade-off capabilities stronger may, extent, reduced dependence 12. Functional studies specificity support view. Initially, were poor 13 subject debate 14. production detected recent experiments 15, vaccination proved effective measurable 16, suggesting provide protection. indication particular experienced hypothesis. Nevertheless, after lost, merely incidentally coincide phenomenon. They direct rejects difficult distinguish retrospectively if they affected simultaneously. predominant force exist absent, coincided Following similar reasoning, alternate occurred before loss, addition stages disarray. In fact, phenomena observed genome tightly coupled. assumptions predict distinct repertoires Each assumes definitive factors, whether environmental, associated distribution. current setting, habitats identifiable. Indeed, lives identified presumably functionality. characteristics shared within gadoid burbot (Lota lota) fresh water lacks same set involved expressed transcripts findings indicate peculiar far back setting reflect originally evolved. belongs family Gadidae, order Gadiformes fossil early 65 million years BP 17. (i.e. Macrouridae, Bathygadidae, Muraenolepdidae) inside contain deep-sea populate bathypelagic bathybentic regions ocean 18. gadoids habitat, environment cold, microbial community mesopelagic epipelagic environments. characterized larger proportion archaea lower abundance increasing depth 19. Both temperature composition drivers 5, 20-22. Temperature strongly influences rates poikilothermic organisms low temperatures modulate so parameters become weaker 23. characteristic promoted relative strong component component, How investigate validity associate scenario? Of course, it optimal obtain thorough divergent Such achievable near future, those without commercial value. predicts distributions teleosts. factor coinciding TLRs, expect analogous adapted cold environment, Antarctic ice fishes (Notothenioidei). interesting comparison: A high load 24, 25. sequence analysis fish immunoglobulins shown unexpected long hinge peptide complete yet obtained species. By characterizing range environments, infer likely. will illuminate strength potential like putative architecture, full comparative multiple rapidly becoming economical, considering ongoing reduction sequencing costs. central question immunology remains host-microbial co-evolution most evolution. Host-commensal recently received substantial attention, e.g. commensal loads intestines absent 26. host-commensal associations extensively regulated 27 relevant interactions ecologically comparable So far, intestinal 28, 29, but metagenomic natural conditions done. core gut zebrafish, rearing 30. observation unique composed bacteria employ biochemical signaling cascades idea role immunity. Here, determined responsible, scenarios. doubt, provides window opportunity studying questions our We thank group at Centre Evolutionary Ecological Synthesis (CEES) discussions ideas presented here. paper benefited significantly comments suggestions Stig W. Omholt Kjetill S. Jakobsen.