Genetic analysis of Cryptocaryon irritans field outbreak in a breeding population of turbot (Scophthalmus maximus)

Abstract

Cryptocaryonosis, caused by ciliate parasite Cryptocaryon irritans, is one of the diseases that cause large economic losses in turbot (Scophthalmus maximus). Genetic improvement of disease resistance represents one strategy for controlling infectious diseases in farmed fish. However, knowledge of whether genetic variation exists for C. irritans resistance is needed in order to determine the feasibility of including this trait into the breeding goal. In the current study, genetic parameters for host resistance to C. irritans were estimated by analyzing the data from a natural field outbreak in a turbot breeding population. As comparison, survival trait under rearing condition was also evaluated. Based on data collected before and during C. irritans outbreak, survival and resistance to C. irritans were both defined as binary traits and analyzed using four different cross-sectional models: Linear animal model (LAM), threshold (probit) animal model (TAM), linear sire-dam model (LSDM) and threshold (probit) sire-dam model (LAM). Through 9 months of normal cultivation, survival rate was 88% (1321 in 1500), and 65% (854 in 1321) individuals died during C. irritans outbreak. Significant genetic variation for resistance to C. irritans was found, and heritability estimates using LAM, LSDM and LAM were very close, ranging from 0.29 to 0.31, while that using TAM was 0.12 ± 0.03. Correlations among family EBVs ranking based on different models were high (0.879–0.998). Heritability estimates of survival were all far lower, ranging from 0.04 to 0.08, and there was no evidence of a genetic correlation between resistance to C. irritans and survival under rearing condition (0.23 ± 0.27). These results suggest that selective breeding to increase resistance and reduce mortalities due to C. irritans is a feasible and promising approach