Improved Phytate Phosphorus Utilization by Japanese Medaka Transgenic for the Aspergillus niger Phytase Gene

The inefficient digestion of phytate phosphorus by fish has created environmental concerns associated with phosphorus pollution from aquaculture production facilities. To further complicate this situation, phytate is known to chelate minerals and proteins, making them nutritionally unavailable. The enzyme phytase degrades phytate into inorganic phosphorus, which can be directly utilized by fish. As a model to examine the feasibility and efficacy of producing fish capable of degrading phytate, Japanese medaka (Oryzias latipes) transgenic for an Aspergillus niger phytase gene were produced and their ability to utilize phytate phosphorus tested. Cell culture techniques, including transfection, RT-PCR, Northern blot, Western blot, and enzyme activity analysis demonstrated that the protein was expressed, active, and secreted. Survival of transgenic fish was significantly greater on all examined diets than their nontransgenic siblings and up to six-fold higher on a diet with phytate as the main phosphorus source. Similar results were obtained with nontransgenic fish when fed the same diet supplemented with phytase, suggesting that phytase, whether ingested or produced by the fish, is effective in degrading phytate and overcoming many of the known antinutritional factors.