Abstract
Oysters represent a significant molluscan taxon that is widely distributed in world oceans and is the leading molluscan species by quantity produced. By now, oysters have been cultured on all continents, excluding Antarctica. Among all oysters, Crassostrea gigas, also known as the Pacific oyster, is the most economically important in aquaculture around the world. It is native to China, Japan and Korea, but has been introduced to many countries of the world, because of its potential for rapid growth and tolerance of a wide range of environment conditions. The global aquaculture production of C. gigas continues to expand year by year. In China, C. gigas is also one of the dominant species of marine culture. Like many other oysters, diploid C. gigashave an inferior taste and low meat quality during the reproductive seasons (spring and summer) each year, which can be attributed to the sharp decrease of glycogen content during gametogenesis. At the same time, it will also be accompanied by decreased stress resistance and slower growth rate during spawning. Therefore, reproductive control has been an important research field in genetic breeding of oysters. At present, artificial induction of triploid oysters is the main way to control the fecundity of C. gigas. There are two primary methods to produce triploid C. gigas: by inhibiting polar body formation after fertilization through chemical induction and crossing tetraploid oysters with diploid ones. Chemical induction is not reliable in producing a hundred percent triploids, while crossing tetraploids with diploids can reach very close to pure triploids. Furthermore, to avoid chemical toxicity, triploid C. gigas have been mostly produced by crossing tetraploid males and diploid females in recent years. Triploid oysters have advantage in terms of fast growth, delicious meat, and high survival rate during reproductive seasons, due to the poorly developed gonad, which has improved marketability relative to diploid in the reproductive seasons. Triploid C. gigas has entered into commercial farming in many countries, including China and America. Although triploid C. gigas has generally been considered to be sterile, some triploid C. gigas exhibit the same fecundity as diploids. Two types of gametogenic pattern have been reported in triploid C. gigas: fertile and sterile types. Most triploid oysters cannot produce or produce a very small number of gametes, which are referred to as being sterile (3nβ) and as genetically confined. Nevertheless, about 25% of triploid oysters can produce a significant number of mature gametes, referred to as non-sterile or fertile triploids (3nα). It has been reported that gametes of triploid oysters can reach functional maturity and can produce some viable progeny. The existence of triploid oysters with different fertility provides important materials for researchers to carry out studies on the gonadal development of polyploid molluscs. The development of gonad in molluscs is closely related to the changes in the biochemical composition of the edible part. Under the condition of sufficient food, molluscs can regulate the stored glycogen, protein and fat to provide energy for gametogenesis. The gametogenesis of oysters requires stored glycogen to provide energy. Though the contents of biochemical components of triploid C. gigas have been reported in the previous studies, the authors did not specifically distinguish between fertile and infertile triploid oysters. Whether there are differences in the content of biochemical components during gonadal development of triploid oysters with different fertility has not been reported yet. In addition, glycogen content is an important indicator of the high quality of triploid oysters and the main molecular contributor to flavor quality in oysters, and whether it changes with fertility is also an important issue for researchers. Therefore, in the present study, in order to clarify the relationship between gonadal development and the changes of nutritional components in different fertility triploid C. gigas, we analyzed the main nutritional components (glycogen, total protein and total fat content) in the gonadal development of sterile and fertile triploid C. gigas, and compared them with diploid C. gigas. According to the near-infrared reflectance spectroscopy (NIR) analysis model, Fourier transform NIR spectrometer was used to analyze the optical density of the transmitted or reflected light of the sample. The content of different substances has a linear relationship with the absorption peaks of multiple different wavelength points in the near-infrared region. According to this theory, the glycogen content, total protein content and total fat content of each sample was tested. The results showed that the content of glycogen in gonad-visceral mass, adductor muscle and mantle of triploid C. gigas was significantly higher than that of diploid C. gigas in the same period, and the content of total protein in gonad-visceral mass and adductor muscle of triploid C. gigas was significantly lower than that of diploid C. gigas in the same period. During gonadal development, the glycogen content in gonad-visceral mass of fertile triploid C. gigas decreased by 31.88%, that of diploids decreased by 82.41%, and that of sterile triploid C. gigas decreased by 0.55%, which was closely related to the energy supply of glycogen for gametogenesis. In addition, the content of glycogen, total protein and total fat in gonad-visceral mass of sterile triploid C. gigas did not fluctuate significantly in the reproductive seasons, while the trend of nutrient content of fertile triploid C. gigas was similar to that of diploid because of the development of gonad to a certain extent. The results showed that there were significant differences in nutritional components between sterile and fertile triploid C. gigas during the reproductive seasons, and the glycogen quality of sterile triploids was better than that of fertile triploids. It will be interesting to address the questions why there are two different types of gametogenic pattern in triploid C. gigas and what controls the changes of biochemical components in future. To our knowledge, this is the first demonstration of nutritional components in two different types of triploid C. gigas. The findings in the present study not only are important for promoting the application of triploid C. gigas in the industry, but also provide important information for reproductive control in the breeding of C. gigas.
DownLoad: