| Silver carp (Hypophthalmichthys molitrix) is one of the main-farmed freshwater fish and possess a large proportion in freshwater aquaculture in China, due to the advantages of its short food chain, easily rearing and low cost during farming. At the present time, because of the lack of techniques in seed selection and seed protection, the natural resources of silver carp were destroyed, and the propagations in close relatives lead to the germplasm degeneration. Researchers in the Yangtze River Fisheries Research Institute apply themselves to the seed selection and breeding of gynogenetic silver carp all the times, and have preliminarily established two artificial gynogenetic strains at present based on molecular marker techniques. But the success in inducing gynogenesis is only the first step to achieve breeding selection of good strain in silver carp, we must still solve the problems in reproducing offspring and keeping good characters of parents. During the period from May to December in 2002, histological method was used to study the early gonadal differentiation of silver carp, in order to hold its rule and provide basis for artificial sex reversal. During the period from June 2002 to December 2003, experiments were done to induce sex reversal in order to probe into the feasibility in sex reversal and reproducing offspring in silver carp. During the period from December 2002 to April 2003, RAPD technique was used to analyze genetic diversity of inbreeding F1 progeny in two artificial gynogenetic strains of silver carp, in order to test the stability of their genetic characters. The main results obtained are as follows:1. Gonadal differentiation of inbreeding F1 in gynogenetic strain I of silver carp appeared at anatomical level after hatching for 35 days, and ovary differentiated first, with the emergency of ovarian cavity as a marker. The cross section of ovary was strip-shaped, and that of spermary was ellipse. After hatching for three months, the position of main blood vessel in the gonad could also be used as an anatomical marker for early gonadal differentiation of silver carp, of which the main blood vessel of the ovary closed with center of ventral side and that of the spermary closed with dorsal side. It was difficult to distinguish oogonium from spermatogonium according to the shape and size of gonium under light microscope. After five months, the ovary started to form a spawn-stored plate, and existed in stage I. The ovary of seven months had formed a full spawn-stored plate, and existed in a transition period from stage I to stage II. After hatching for sixty days, a spermaduct rudiment appeared near the stalk portion of the spermary. The spermary of seven months existed in stage I, and the cross section was oviform.2. Sex reversal experiment was performed among silver carp fries of 14 days. After the fries were reared with different feed containing 30mg, 15mg and 5mg 17a-methyltestostercne per kilogram for 43 days respectively, they were shifted into pond for normal rearing till their gonads developed to stage II. Morphological methods were used to identify individual sex, and the effects of different dosages of male hormones on sex reversal of silver carp were compared. The results showed that percents of male individuals in 15mg dosage group, 5mg dosage group and blank control group were 68.8%, 54.8% and 43.5% respectively; but that of 30mg dosage group was only 35.3%, which was lower than that of control group on the contrary. Therefore, appropriate amount of 17a-methyltestostercne in the feed might help to increase the percent of male individuals in silver carps.3. Genetic distance, genetic similarity and population genetic diversity of inbreeding F1 progeny in two artificial gynogenetic strains of Silver Carp {Hypophthalmichthys Molitrix) were analyzed with the methods of RAPD, with common carp and common silver carp as control groups. The results indicate that mean genetic similarities among individuals in inbreeding F1 progeny of GSC I (Gynogenetic Silver Carp,... |
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