| C. gigas grows faster than the native Zhe oyster and Suminoe oyster, so it has been preferentially adopted for farming, and become the dominant farmed oyster in China. Because of the high fecundity of oyster species, relatively few parents are required to generate the numbers of offspring needed. So, there were concerns that genetic variability might have been lost during more than 20 years of artificial cultivation. Crossbreeding has been widely applied for the genetic improvement of animal and plants. Hybrid crosses may produce different traits compared with parental stocks. Shull (1952) defined the term heterosis as the interpretation of increased vigor, size, fruitfulness, speed of development, resistance to disease and to insect pests, or to climatic rigors of any kind, manifested by crossbred organisms as compared with corresponding inbreds. In molluscs, many reports exist on the application of heterosis in the genetic improvement of molluscan brood stocks.To determine if heterosis exists and can be used for genetic improvement in the Pacific oyster, interspecific hybridization among the Pacific oyster (C. gigas), the Iwagaki oyster (C. nippona), and the Kumamoto oyster (C. sikamea), and the intraspecific hybridization of different geographical populations of China, Korea and Japan were carried out. Cytological process of fertilization was observed, and growth, survival and heterosis were discussed. The main results are listed as follows:1. A reciprocal hybrid crosses between C. gigas and C. nippona was carried out. Meiosis and early development were observed under a fluorescence microscope with DAPI stain. The results obtained in this study demonstrated that the eggs of C. nippona initiated the resumption of meiotic maturation, following the incorporation of C. gigas sperm into the egg cytoplasm of C. nippona. During meiotic process of the maternal chromosomes, sperm nuclei enlarged at two distinct phases. Sperm nuclei underwent a rapid, initial enlargement during metaphase I, but condensed slightly during polar body formation, then re-enlarged during female pronuclear development. Similar phenomenon was also seen in the hybrid cross between female C. gigas and male C. nippona. Asynchronism of egg development was remarkable in the two hybrid crosses. The developmental progress of the cross C. nippona♀×C. gigas♂was delayed compared with that of the cross C. gigas♀×C. nippona♂.2. Interspecific hybridization was investigated using two cupped oysters C. gigas and C. sikamea. We observed nuclear behavior during meiosis and early development under a fluorescence microscope with DAPI stain. The results obtained in this study demonstrated that the eggs of C. gigas initiated the resumption of meiotic maturation, following the incorporation of C. sikamea sperm into the egg cytoplasm of C. gigas. During meiotic process of the maternal chromosomes, sperm nuclei enlarged at two distinct phases. Sperm nuclei underwent a rapid, initial enlargement during metaphase I, but condensed slightly during polar body formation, then re-enlarged during female pronuclear development. Similar phenomenon was also seen in the hybrid cross between female C. sikamea and male C. gigas. Asynchronism of egg development was remarkable in the two reciprocal hybrid crosses. In the hybrid cross between female C. gigas and male C. sikamea, after the incorporation of C. sikamea sperm into the egg cytoplasm of C. gigas, most eggs did not resume the meiotic maturation. Growth and survival rates at larval, juvenile and adult stages were compared between two reciprocal hybrid crosses (C. gigas♀×C. sikamea♂and C. sikamea♀×C. gigas♂) and two parental groups (C. gigas♀×C. gigas♂and C. sikamea♀×C. sikamea♂), respectively. The results indicated that the larval growth of hybrid crosses were inferior to those of parental groups, and the larval survival rate of C. gigas♀×C. gigas♂and C. sikamea♀×C. gigas♂crosses were superior to those of C. gigas♀×C. sikamea♂and C. sikamea♀×C. sikamea♂crosses. The growth and survival performance at juvenile and adult stages of hybrid cross was inferior to the parental groups, showing no heterosis in the characteristics of shell height, shell length and survival for the hybrid crosses between C. gigas and C. sikamea.3. A complete diallel cross between two Pacific oyster populations, one from China and the other from Korea, was carried out. The fertilization rate, hatching success, growth, survival rate, and heterosis were compared between two reciprocal hybrid crosses (C♀×K♂, CK and K♀×C♂, KC) and two parental groups (C♀×C♂, CC and K♀×K♂, KK). The results obtained in this study demonstrated that fertilization rate of hybrid crosses was between that of parental groups, and CK cross had top hatching success while KC cross had the lowest. Shell height and shell length of KC cross were superior to those of the other three groups in larval, juvenile and adult stage, followed by CK cross. The heterosis of the hybrid crosses (KC, CK) in shell height and shell length was obvious in larval, juvenile and adult stage. The heterosis for survival existed in larval stage of KC cross, but turned to be negative in juvenile and adult stage. In contrast, CK cross had negative heterosis for survival in larval stage, but positive in juvenile and adult stage. Results indicate that the hybridization between the Pacific oysters, one from China population and another from Korea population, may be a promising way for genetic improvement of existing Pacific oyster brood stocks in China.4. Intraspecific hybridization was investigated using two stocks of the Pacific oyster, China population (C) and Japan population (J). The fertilization rate, larval survival rate, growth and heterosis were compared between two reciprocal hybrid crosses (C♀×J♂, CJ and J♀×C♂, JC) and two parental groups (C♀×C♂, CC and J♀×J♂, JJ). The results indicated that the fertilization rate of parental groups was between that of hybrid crosses, and there was no significant difference between two reciprocal hybrid crosses and two parental groups. JJ group had top hatching success, followed by JC cross and CC group, and CJ cross was lowest. No significant difference was observed among the four groups with the same egg origin. Shell height and shell length of JJ group were superior to those of the other three groups in larval, juvenile and adult stage, followed by JC cross, and the heterosis of JC cross in shell height, shell length and survival was obvious. In contrast, the CJ cross did not show positive heterosis in all performance during the whole experiment. The different heterosis between the two reciprocal hybrid crosses might be related to the maternal effects, heredity and environment. |
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