Abstract
The African lungfish, Protopterus annectens, can undergo aestivation during drought. Aestivation has three phases: induction, maintenance and arousal. The objective of this study was to examine the differential gene expression in the liver of P. annectens after 6 months (the maintenance phase) of aestivation as compared with the freshwater control, or after 1 day of arousal from 6 months aestivation as compared with 6 months of aestivation using suppression subtractive hybridization. During the maintenance phase of aestivation, the mRNA expression of argininosuccinate synthetase 1 and carbamoyl phosphate synthetase III were up-regulated, indicating an increase in the ornithine-urea cycle capacity to detoxify ammonia to urea. There was also an increase in the expression of betaine homocysteine-S-transferase 1 which could reduce and prevent the accumulation of hepatic homocysteine. On the other hand, the down-regulation of superoxide dismutase 1 expression could signify a decrease in ROS production during the maintenance phase of aestivation. In addition, the maintenance phase was marked by decreases in expressions of genes related to blood coagulation, complement fixation and iron and copper metabolism, which could be strategies used to prevent thrombosis and to conserve energy. Unlike the maintenance phase of aestivation, there were increases in expressions of genes related to nitrogen, carbohydrate and lipid metabolism and fatty acid transport after 1 day of arousal from 6 months aestivation. There were also up-regulation in expressions of genes that were involved in the electron transport system and ATP synthesis, indicating a greater demand for metabolic energy during arousal. Overall, our results signify the importance of sustaining a low rate of waste production and conservation of energy store during the maintenance phase, and the dependence on internal energy store for repair and structural modification during the arousal phase, of aestivation in the liver of P. annectens.
Highlights
Lungfishes are an archaic group of Sarcopterygian fishes characterized by the possession of a lung opening off the ventral side of the oesophagus
Six months of aestivation led to changes in gene expression related to nitrogen metabolism, oxidative defense, blood coagulation, complement fixation, iron and copper metabolism, and protein synthesis in liver of P. annectens
These results indicate that sustaining a low rate of waste production and conservation of energy store were essential to the maintenance phase of aestivation
Summary
Lungfishes are an archaic group of Sarcopterygian fishes characterized by the possession of a lung opening off the ventral side of the oesophagus. They hold an important position in the evolutionary tree with regard to water-land transition, during which many important physiological and biochemical adaptations occurred (e.g. air-breathing, urea synthesis, redirection of blood flow, heart partitioning). These adaptations supposedly facilitated the migration of fishes to terrestrial environments, leading to the evolution of tetrapods. African lungfishes can aestivate in subterranean mud cocoons for ~4 years [1], which could be the longest aestivation period known for vertebrates
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