博士 / 國立臺灣大學 / 漁業科學研究所 / 90 / Abstract
Milkfish is an economically important culture teleost and its industry has played a significant role in the fisheries development in Taiwan. However, milkfish is known to be a tropical teleost and less tolerant to the cold temperatures suitable for culture.
Mass mortality of these cultured fish has often caused tremendous loss for the milkfish culture industry. Whenever a severe cold current approaches in the winter season. In order to comprehend the differences on cold tolerability between stenothermic teleost (i.e., milkfish) and eurythermic teleost (i.e., grass carp), the physiological responses, energy-metabolism processes and membrane lipid structure of these species under cold shock were investigated.
Temperature is one of the most important environmental factors influencing the growth and survival of poikilothermal teleosts. In order to maintain physiological homeostasis of the organisms in response to severe environmental temperatures, every teleostean species has developed its own specific adaptive mechanisms to cope with temperature fluctuations, including behavioral responses and physiological adaptations. For the physiological responses to cold shock at 15 °C, remarkable hyperglycemic responses in milkfish were detected, increasing rapidly from the initial level of 85±4.88 to 458.2±14.57 mg/dl within 24 hours, but less significant glycemic responses in grass carp were observed, increasing only from 62.69 mg/dl to 127.33 mg/dl. Since glycemic response is one of the reliable indicators for acute stress responses. Differential responsiveness of fish with varying cold tolerability was evidently demonstrated. Besides, the significant elevation of plasma lactate contents in milkfish were detected on days 1 and 2, followed by a rapid decline from day 3 and thereafter, in which the plasma lactate maintained at level insignificantly different from that of day 0. By contrast, the plasma lactate showed a gradually and steadily increase in grass carp. Plasma lipid contents in milkfish increased from 44.8±2.48 to 191±9.08 mg dl-1 over 5 days period, followed by a decreasing trend down to 125.6±6.55 mg dl-1 on day 7. In grass carp, continuous elevation of plasma lipid was observed, all significantly higher than the initial level.
An important strategy for homeoviscous adaptation to low temperature in poikilotherms is to maintain the homeoviscous fluidity of biological membranes through increased proportion of unsaturated fatty acids by way of desaturation of membrane lipids. In the biosynthesis of polyunsaturated fatty acids and the maintenance of the homeoviscous fluidity of biological membranes, stearoyl-CoA desaturase (EC 1.14.99.5) is known to be the key enzyme that catalyzes the double-bond formation in fatty acids. The present studies showed that liver microsome stearoyl-CoA desaturase activities in milkfish under the cold were markedly elevated, all significantly higher than those at 25°C, but in grass carp, the changes in the activities were insignificant in the initial phase of cold acclimation, but increased dramatically towards the late period of the process. Moreover, the proportion of unsaturated fatty acids in milkfish dramatically increased to the peak on day 3, followed by a decreasing trend, and the proportion of saturated fatty acid decreased steadily to the lowest followed by an increasing trend. However, the proportions of fatty acids in grass carp, neither saturated nor unsaturated, did not change significantly before day 7 but increased unsaturated fatty acids and decreased saturated fatty acid were observed on days 14 and 21 only.
The stearoyl-CoA desaturase cDNA in milkfish and grass carp was cloned by RT-PCR and RACE, and further compared with the stearoyl-CoA desaturase in cold-tolerant teleosts, common carp and grass carp. Nucleotide sequence analysis revealed that the cDNA clone has a 972-bp open reading frame encoding 323 amino acid residues. Alignments of the deduced amino acid sequence showed that the milkfish stearoyl-CoA desaturase shares 79% and 75% identity with common carp and grass carp, and 63%-64% with other vertebrates such as sheep, hamsters, rats, mice, and humans. Like common carp and grass carp, the deduced amino acid sequence in milkfish conserves well three histidine cluster motifs (one HXXXXH and two HXXHH) that are essential for catalysis of stearoyl-CoA desaturase activity. However, RT-PCR analysis showed that stearoyl-CoA desaturase expression in milkfish is detected in the tissues of liver, muscle, kidney, brain, and gill, and the expression sites were found more diversed in milkfish than those in common carp and grass carp. Phylogenic relationships among the deduced stearoyl-CoA desaturase amino acid sequence in milkfish and those in other vertebrates showed that the milkfish stearoyl-CoA desaturase amino acid sequence is phylogenetically closer to those of common carp and grass carp than to other higher vertebrates.
In summary, physiological responses to cold stress is evidently different in stenotherms and eurythems. The degree of the physiological response also varies with the fish tolerability to the temperature changes. Enhancement of the cold tolerance of fish for improved survival in severe cold environments is perhaps possible by using physiological and genetic engineering manipulations.
| Identifer | oai:union.ndltd.org:TW/090NTU00451002 |
| Date | January 2002 |
| Creators | Hsieh Shu-Ling, 謝淑玲 |
| Contributors | Kuo ching-Ming, Liao Wen-Liang, 郭欽明, 廖文亮 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 155 |
Page generated in 0.0198 seconds