The two main scale patterns of common carp are scaly and mirror phenotype. However, the pleiotropic effects of genes for mirror phenotype significantly reduce mirror carp’s performance (growth, survival) compared to scaly ones. Therefore, the Amur mirror carp (AMC) , a new Czech mirror carp breed established in 2014 , has been genetically approved to overcome this problem . The breeding approach to establish the AMC was based upon the introgression of Amur wild carp - AC (Cyprinus rubrofuscus ) genes into the Hungarian synthetic mirror carp (HSM; Cyprinus carpio) to improve the non-specific resistance of mirror carp stocks. Since then, AMC has become the primary mirror carp used in Czechia’s hybridization and selection program. Despite its better performance, genes of AC significantly affect the general appearance of AMC, whose scales are distributed diversely over the body and a high ratio of AMC individuals do not display a typical and demanded mirror phenotype. Therefore, t his study aimed to evaluate the possibility of improving unfavourable scaly patterns in AMC by selective breeding.
The experimental stock of AMC established out of 20 dams and 40 sires was reared communally until market size (three years) under semi-intensive pond conditions. After the second summer, the stock was harvested (n = 1,864), and each fish was photographed to further determine scaly patterns and body area. To measure the number and total area of scales and body area without head and fins, two dedicated deep learning models were trained based on a cellpose 2.0 fine-tuning cyto2 model. The resulting images were quantified using an ImageJ macro. The scaly pattern phenotypes were scaliness (Scale_Bin: 0 = standard mirror, 1 = other), total scale area (TotalSC_A), relative scale area to body area (Rel_SC_A) and scale number (Scale_NO). Fish were also phenotyped at market size ( n = 1,572) for survival (Surv), body weight (BW), Fulton’s condition factor (FC), muscle fat content (Fat) and fillet yields (Res_Fill). Heritability and genetic and phenotypic correlations were estimated with DMU software using an animal model and pedigree record.
The heritability estimates of scaly pattern phenotypes were very high (0.54 – 0.79), suggesting a great potential to change the unfavourable scaly patterns of AMC to a typical mirror phenotype using a selection program (Table 1). Moreover, high genetic correlations among the scaly pattern phenotypes show a reduction of technical issues among the different ways to phenotype the scaly pattern in AMC. Hence, the easiest selection based only on the scaly appearance of fish (Scale_Bin) might be applicable in a breeding program. The genetic correlations between scaly pattern phenotypes and performance traits were, in many cases, insignificant . Only the correlations with BW (-0.28 – -0.29), FC (-0.27 – -0.33) and Fat (0.23 – 0.29) were found at the edge of significance, but they were low. Thus, including the scaly phenotype trait in the AMC breeding program should not affect the effort for further genetic improvement of the new and prospective mirror carp breed.