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Title: Assessment of genetic correlation between bacterial cold water disease resistance and spleen index in a domesticated population of rainbow trout: Identification of QTL on chromosome Omy19

item Wiens, Gregory - Greg
item Vallejo, Roger
item Leeds, Timothy - Tim
item Palti, Yniv
item HADIDI, SIMA - The Biomedical Research Centre
item Liu, Sixin
item Evenhuis, Jason
item Welch, Timothy - Tim
item Rexroad, Caird

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2013
Publication Date: 10/9/2013
Citation: Wiens, G.D., Vallejo, R.L., Leeds, T.D., Palti, Y., Hadidi, S., Liu, S., Evenhuis, J., Welch, T.J., Rexroad Iii, C.E. 2013. Assessment of genetic correlation between bacterial cold water disease resistance and spleen index in a domesticated population of rainbow trout: Identification of QTL on chromosome Omy19. PLoS One. 8(10): e75749: DOI: 10.1371/journal.pone.0075749.

Interpretive Summary: Significant loss of rainbow trout in US aquaculture is due to bacterial cold water disease. Bacterial cold water disease is caused by infection with the bacterium named Flavobacterium psychrophilum. At the NCCCWA, we have been selectively breeding rainbow trout since 2005 for increased resistance to the bacterial cold water pathogen. In conjunction with our breeding efforts, we have been investigating the mechanism(s) of how fish are disease resistant. In a previous study, we reported a correlation between disease resistance and larger spleen size in rainbow trout. The spleen is an important immune organ but it is not known how size relates to resistance. In this paper, we measured whether there are common genes that influence both spleen size and disease resistance. To determine this, we measured both traits in two breeding populations of rainbow trout that spawn on either even or odd-number years, and we collected data over a 5 year period. Our analyses indicate that spleen size is a highly heritable trait and there is a significant genetic correlation with disease resistance in the even-year spawning population of rainbow trout. In one family, we mapped regions in the trout genome that determine spleen size and found links to chromosomes 19, 16 and 5. We also mapped disease resistance to chromosome 19. The association of both spleen size and disease resistance within a 26 cM region on chromosome 19 may explain the genetic correlation between the two traits. This is the first study to identify a genetic link between a physical trait (spleen size) and specific disease resistance in fish. These findings increase our knowledge of disease resistance mechanisms and may be useful in future selective breeding efforts.

Technical Abstract: Selective breeding of animals for increased disease resistance is an effective strategy to reduce mortality in aquaculture. However, implementation of selective breeding programs is limited by an incomplete understanding of host resistance traits. We previously reported results of a rainbow trout selection program that demonstrated increased survival following challenge with Flavobacterium psychrophilum, the causative agent of bacterial cold water disease (BCWD). Mechanistic study of disease resistance identified a positive phenotypic correlation between post-challenge survival and spleen somatic-index (SI). Herein, we investigated the hypothesis of a genetic correlation between the two traits influenced by colocalizing QTL. We evaluated the inheritance and calculated the genetic correlation in five year-classes of odd- and even-year breeding lines. A total of 322 pedigreed families (n=25,369 fish) were measured for disease resistance, and 251 families (n=5,645 fish) were evaluated for SI. Spleen index was moderately heritable in both even-year (h2= 0.56 plus/minus 0.18) and odd-year (h2= 0.60 plus/minus 0.15) lines. A significant genetic correlation between SI and BCWD resistance was observed in the even-year line (rg=0.45 plus/minus 0.20, P=0.03) but not in the odd-year line (rg=0.16 plus/minus 0.12, P=0.19). Complex segregation analyses of the even-year line provided evidence of genes with major effect on SI, and a genome scan of a single family, 2008132, detected three significant QTL on chromosomes Omy19, 16 and 5, in addition to ten suggestive QTL. A separate chromosome scan for disease resistance in family 2008132 identified a significant BCWD QTL on Omy19 that was associated with time to death and percent survival. In family 2008132, Omy19 microsatellite alleles that associated with higher disease resistance also associated with increased spleen size raising the hypothesis that closely linked QTL contribute to the correlation between these traits. To our knowledge, this is the first estimation of spleen size heritability and evidence for genetic linkage with specific disease resistance in a teleost fish.