Abstract
BackgroundColumnaris causes severe mortalities among many different wild and cultured freshwater fish species, but understanding of host resistance is lacking. Catfish, the primary aquaculture species in the United States, serves as a great model for the analysis of host resistance against columnaris disease. Channel catfish in general is highly resistant to the disease while blue catfish is highly susceptible. F2 generation of hybrids can be produced where phenotypes and genotypes are segregating, providing a useful system for QTL analysis. To identify genes associated with columnaris resistance, we performed a genome-wide association study (GWAS) using the catfish 250 K SNP array with 340 backcross progenies derived from crossing female channel catfish (Ictalurus punctatus) with male F1 hybrid catfish (female channel catfish I. punctatus × male blue catfish I. furcatus).ResultsA genomic region on linkage group 7 was found to be significantly associated with columnaris resistance. Within this region, five have known functions in immunity, including pik3r3b, cyld-like, adcyap1r1, adcyap1r1-like, and mast2. In addition, 3 additional suggestively associated QTL regions were identified on linkage groups 7, 12, and 14. The resistant genotypes on the QTLs of linkage groups 7 and 12 were found to be homozygous with both alleles being derived from channel catfish. The paralogs of the candidate genes in the suggestively associated QTL of linkage group 12 were found on the QTLs of linkage group 7. Many candidate genes on the four associated regions are involved in PI3K pathway that is known to be required by many bacteria for efficient entry into the host.ConclusionThe GWAS revealed four QTLs associated with columnaris resistance in catfish. Strikingly, the candidate genes may be arranged as functional hubs; the candidate genes within the associated QTLs on linkage groups 7 and 12 are not only co-localized, but also functionally related, with many of them being involved in the PI3K signal transduction pathway, suggesting its importance for columnaris resistance.
Highlights
Columnaris causes severe mortalities among many different wild and cultured freshwater fish species, but understanding of host resistance is lacking
The causative SNP could be within any one of the 10 genes found in the 620 Kb region, the fact that PI3K pathway was reported to be involved in immunity and resistance makes them interesting as candidate genes [28]
On linkage groups 7 and 12, the resistant genotypes were homozygous with both alleles from channel catfish
Summary
Columnaris causes severe mortalities among many different wild and cultured freshwater fish species, but understanding of host resistance is lacking. The primary aquaculture species in the United States, serves as a great model for the analysis of host resistance against columnaris disease. Flavobacterium columnare, a Gram-negative bacterium, is the causative agent of columnaris disease, which is very common in wild and cultured freshwater fish worldwide [1]. This pathogen can infect a variety of fish species through mucosal attachment points on the gill and skin, causing external erosion and necrosis [2]. Columnaris disease is considered as one of the most important diseases in the catfish industry, causing
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