Antti Kause

Selection against early maturity in large rainbow trout Oncorhynchus mykiss: the quantitative genetics of sexual dimorphism and genotype-by-environment interactions

Abstract In rainbow trout Oncorhynchus mykiss , females typically mature at a later age than males, and males mature either early or late. When farmed rainbow trout is produced to obtain large fillets of high quality, especially early maturation of males reduces economical benefits. To examine whether it is possible to change the degree of this sexual dimorphism using selective breeding, we estimated sex-specific heritabilities, genetic correlations between sexes, and cross-environment correlations between two production environments for age at maturity and body weight. Extensive data set including individuals in five generations measured in brackish and fresh water in a split-family design was utilised to estimate the genetic parameters. The timing of maturity ( h 2 =0.12–0.56) and body weight ( h 2 =0.20–0.27) exhibited significant heritabilities, allowing genetic changes in trait means in response to selection. Genetic change in sexual dimorphism of body weight is, however, strongly constrained, as shown by the high genetic correlations between sexes ( r A ≥0.95). For timing of maturity, the heritabilities of males and females differed, but the genetic correlation between sexes was high and positive ( r A =0.70). Although sexual dimorphism of age at maturity is less strongly contained than that of body weights, we conclude that no rapid genetic changes can be achieved in the sexual dimorphism of age at maturity through selective breeding. However, breeders may delay the timing of maturity in both sexes to avoid the drawbacks associated with the early maturation. Cross-environment correlations for the timing of maturity and body weight were highly positive ( r A ≥0.61), indicating weak genotype-by-environment interactions and allowing parallel genetic changes in two environments where rainbow trout is farmed.

Coupling body weight and its composition: a quantitative genetic analysis in rainbow trout

Abstract We examined whether selection for rapid growth rate in rainbow trout, Oncorhynchus mykiss , will potentially lead to correlated genetic responses in body composition. The potential for correlated responses was evaluated by estimating heritabilities and genetic correlations for body weight and traits characterising body composition (condition factor, wasted biomass, fillet weight, three measures of fat, flesh redness, percent fillet protein, ash and water) at the age of 3 years in a half/full-sib mating design. Percent abdominal fat from body weight, percent fillet protein, ash and water were the only traits to display heritabilities close to zero ( h 2 =0.02–0.06). The other body composition traits and body weight displayed moderate levels of genetic variation ( h 2 =0.11–0.45), making direct and correlated genetic responses to selection possible. Genetic correlations of body weight with percent fillet fat, protein, ash and water and flesh redness were low ( r A =−0.12–0.36), indicating that the quality of fillets is not strongly changed by the selection for rapid growth rate. In contrast, the weight of abdominal fat and waste will be increased as a correlated response, because due to a part–whole relationship, the traits showed strong positive genetic correlations with the body weight ( r A =0.63–0.98). Moreover, we found large differences between sexes especially in the traits that are closely related to body size (i.e., condition factor, weight of waste and abdominal fat) and, to a lesser extent, in the traits related to fillet quality. However, the differences between sexes in body composition could be mainly explained by the difference between sexes in body size. In conclusion, the genetic properties of body weight and body composition traits of the rainbow trout population studied do not impose strong constraints on the progress of the breeding programme.

Recording strategies and selection potential of feed intake measured using the X-ray method in rainbow trout.

This study examines the way long-term feed intake should be recorded accurately for selective breeding purposes, and estimates selection potential in feed intake using the X-ray method to record individual daily feed intake in rainbow trout (Oncorhynchus mykiss). The analysis showed that the point estimates of daily feed intake displayed low repeatabilities (r = 0.09–0.32). This indicates that a minimum of three repeated records were needed to accurately record average feed intake at a fixed age. To effectively breed for feed intake over the whole growing period, it is necessary to determine average feed intake at different ages, since there were only moderate phenotypic and genetic correlations between average daily feed intake recorded at 140 g, 750 g and 2000 g wet mass. Heritability for average daily feed intake was low (average h2 = 0.10), indicating that modest genetic changes can be obtained in response to selection. It was concluded that selection to genetically change long-term feed intake can be successful, yet repeated observations at several life stages are needed to ensure the accuracy of feed intake estimates and the efficiency of selection.

Changes in the expression of genetic characteristics across cohorts in skeletal deformations of farmed salmonids

Genetic analysis of disorder incidence in farmed animals is challenged by two factors. Disorders in different cohorts and environments could be caused by different factors, leading to changes in heritability and to less than unity genetic correlations across cohorts. Moreover, due to computational limitations, liability scale heritabilities at very low incidence may differ from those estimated at higher incidence. We tested whether these two dilemmas occur in skeletal deformations of farmed salmonids using multigeneration data from the Finnish rainbow trout breeding programme and previous salmonid studies. The results showed that heritability was close to zero in cohorts in which management practices maintained incidence at a low level. When there was a management failure and incidence was unusually high, heritability was elevated. This may be due to computational limitations at very low incidence and/or because deformations are induced by different factors in different cohorts. Most genetic correlations between deformations recorded in different generations were weakly to strongly positive. However, also negative correlations between generations were present, showing that high liability at one time can be genetically connected to low liability at another time. The results emphasise that genetic architecture of binary traits can be influenced by trait expression.

Genetics of Ascites Resistance and Tolerance in Chicken: A Random Regression Approach

Resistance and tolerance are two complementary mechanisms to reduce the detrimental effects of parasites, pathogens, and production diseases on host performance. Using body weight and ascites data on domesticated chicken Gallus gallus domesticus, we demonstrate the use of random regression animal model and covariance functions to estimate genetic parameters for ascites resistance and tolerance and illustrate the way individual variation in resistance and tolerance induce both genotype re-ranking and changes in variation of host performance along increasing ascites severity. Tolerance to ascites displayed significant genetic variance, with the estimated breeding values of tolerance slope ranging from strongly negative (very sensitive genotype) to weakly negative (less sensitive). Resistance to ascites had heritability of 0.34. Both traits are hence expected to respond to selection. The two complementary defense strategies, tolerance and resistance, were genetically independent. Ascites induced changes to the correlations between ascites resistance and body weight, with the genetic correlations being weak when birds were ascites-free but moderately negative when both healthy and affected birds were present. This likely results because ascites reduces growth, and thus high ascites incidence is genetically related to low adult body weight. Although ascites induced elevated phenotypic and genetic variances in body weight of affected birds, heritability displayed negligible changes across healthy and affected birds. Ascites induced moderate genotype re-ranking in body weight, with the genetic correlation of healthy birds with mildly affected birds being unity but with severely affected birds 0.45. This study demonstrates a novel approach for exploring genetics of defense traits and their impact on genotype-by-environment interactions.

Genetic improvement of feed conversion ratio via indirect selection against lipid deposition in farmed rainbow trout (Oncorhynchus mykiss Walbaum)

In farmed fish, selective breeding for feed conversion ratio (FCR) may be possible via indirectly selecting for easily-measured indicator traits correlated with FCR. We tested the hypothesis that rainbow trout with low lipid% have genetically better FCR, and that lipid% may be genetically related to retention efficiency of macronutrients, making lipid% a useful indicator trait. A quantitative genetic analysis was used to quantify the benefit of replacing feed intake in a selection index with one of three lipid traits: body lipid%, muscle lipid% or viscera% weight of total body weight (reflecting visceral lipid). The index theory calculations showed that simultaneous selection for weight gain and against feed intake (direct selection to improve FCR) increased the expected genetic response in FCR by 1·50-fold compared with the sole selection for growth. Replacing feed intake in the selection index with body lipid%, muscle lipid% or viscera% increased genetic response in FCR by 1·29-, 1·49- and 1·02-fold, respectively, compared with the sole selection for growth. Consequently, indirect selection for weight gain and against muscle lipid% was almost as effective as direct selection for FCR. Fish with genetically low body and muscle lipid% were more efficient in turning ingested protein into protein weight gain. Both physiological and genetic mechanisms promote the hypothesis that low-lipid% fish are more efficient. These results highlight that in breeding programmes of rainbow trout, control of lipid deposition improves not only FCR but also protein-retention efficiency. This improves resource efficiency of aquaculture and reduces nutrient load to the environment.