Effect of breeding density on the malformation of olive flounder(Paralichthys olivaceus) and transcriptome analysis of deformed ovary

Abstract

Breeding density is one of important factors affecting the growth of olive flounder(Paralichthys olivaceus). Higher density breeding of olive flounder in a commercial scale has been suggested to be hindered by a lower growth and, malformations caused by breeding density stress. In this study, growth and deformity rates of olive flounder were compared upon growing at different densities. The experiment was carried out with stocking densities of 800(LD), 1500(MD), and 4000(HD) fish/m2 for 40 days (1.49 ± 0.02 cm in total length). Analysis of the growth of fish indicates a lower growth rate of fish grown at higher density as compared to those of LD and MD. RNA was isolated from fish to analyze the expression level of genes associated with stress and deformity by using qRT-PCR. The result showed that the expressing level of stress-related genes including HSP70 and GPX was higher in fish grown at higher density and in deformed fishes as compared to those of fish grown at lower density. Expression levels of some immune-related genes including PIR and IgM in fish grown at higher density was higher than those in fish grown at lower density, although the expression level of TLR gene showed an opposite trend. Transcriptome analysis of normal and deformed ovaries was carried out to examine the genes associated with deformity in fish. The result showed a higher level of expression of the gene encoding heme binding protein in deformed ovaries in comparison to the level in normal ovaries. However, but the level of the gene encoding integral component of membrane in the cellular component was lower than those of normal ovaries. While the gene encoding perfornin showed a lower level of expression in malformed ovaries that of the normal ovaries, the gene encoding MYL showed an opposite expression profiles in transcriptome analysis and qRT-PCR verification experiments. Comparison of the sex conversion rates in fishes grown under different densities showed levels of 14.29% at LD and 30.43% at HD. The result showed a higher level of sex-conversion rate in fish grown at higher density.