| Research Abstract |
Phosphate transporters (NaPi) are membrane proteins, controlling phosphorus (P) absorption in the intestine. NaPi is found in many species, and its gene expression responds rapidly to P-status of the organism. Thus, by measuring NaPi expression, P-status of organisms could be sensibly diagnosed. This research aimed at reducing dietary P input to the minimum required by fish by using this P-biomarker, and thereby reducing P'excretion to the environment. Previously, differentially expressed genes under dietary P restriction were screened in trout, and NaPi, among other genes, was upregulated by several folds. Subsequently, similar responses were confirmed. However, before this study, there was no research in fish regarding NaPi response to dietary P except trout. Studying the NaPi response requires information on the base sequence. However, before this study, only four NaPi sequences were known in fishes. In this study, NaPi sequences were newly isolated from 18 fish species and 3 mollus
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can species. Isolating NaPi sequences from crustacean species, however, was unsuccessful despite several attempts. Among 22 fishes whose NaPi sequences were identified, taxonomically distant 3 species, i.e., tilapia, crucian carp and trout were chosen to examine tissue distribution of NaPi. Ayu was originally used as experimental fish to examine NaPi expression; however, all fish died due to disease outbreak before completion of study. The experiment was repeated, but to no avail. When tilapia and crucian carp were fed with low-P diet, NaPi expression was upregulated in crucian carp as expected; however, tilapia did not become P deficient and the NaPi expression remained unaltered. Chronic dietary P restriction in trout did not significantly alter expressions of hepatic genes related to lipid metabolism. However, as expected, degree of P deficiency and body fat content were significantly correlated to one another, and body fat content correlated significantly to expressions of some genes. Less
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