Abstract
Vertebrates have evolved a complex immune system required for the identification of and coordinated response to harmful pathogens. Migratory species spend periods of their life-cycle in more than one environment, and their immune system consequently faces a greater diversity of pathogens residing in different environments. In facultatively anadromous salmonids, individuals may spend parts of their life-cycle in freshwater and marine environments. For species such as the brown trout Salmo trutta, sexes differ in their life-histories with females more likely to migrate to sea while males are more likely to stay and complete their life-cycle in their natal river. Salmonids have also undergone a lineage-specific whole genome duplication event, which may provide novel immune innovations but our current understanding of the differences in salmonid immune expression between the sexes is limited. We characterized the brown trout immune gene repertoire, identifying a number of canonical immune genes in non-salmonid teleosts to be duplicated in S. trutta, with genes involved in innate and adaptive immunity. Through genome-wide transcriptional profiling (“RNA-seq”) of male and female livers to investigate sex differences in gene expression amplitude and alternative splicing, we identified immune genes as being generally male-biased in expression. Our study provides important insights into the evolutionary consequences of whole genome duplication events on the salmonid immune gene repertoire and how the sexes differ in constitutive immune expression.
Highlights
Species that migrate face a range of challenges
Through comparison with annotated immune genes in zebrafish, mouse and human, we identified 2,275 putative homologs encoded by the brown trout genome (Supplemental Information Table 1)
Due to the salmonidspecific whole genome duplication events (WGD) event, the number of genes with putative roles in the immune system were elevated in salmonids in comparison to non-salmonid fishes (Figure 2) with the analysis by OrthoFinder indicating an additional 1,132 homologous sequences based on sequence similarity alone
Summary
Species that migrate face a range of challenges. First, the physical act of migration can be metabolically and energetically demanding, resulting in trade-offs with other metabolically intensive physiological processes, such as immunity, when resources are limiting [1,2,3]. Migratory species move through different environments and may be exposed to different pathogens and parasites [4, 5]. Immune Repertoire of Brown Trout the challenges imposed by living in, and moving between, different osmotic environments with different pathogen and parasite communities. A sophisticated immune system has evolved that performs two vital functions: 1) the recognition and distinction of invasive pathogenic organisms from normal cells (“self”), and 2) coordinating an appropriate response through triggering pathways responsible for the synthesis of effector molecules that directly or indirectly reduce or remove the pathogenic threat [6, 7]. Aside from detection of non-selfpathogenic organisms, the immune system functions in the removal of abnormal cells and provides an important role in reducing the development and onset of disease
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