Sex determination in fish is complex because the differentiation level of sex chromosomes is low. Both genetic and environmental factors may have influenced on the sex of fishes. Natural hybridization and genetic introgression in fish have also been concerned by ichthyologists and ecologists. Using such cytogenetic approaches as interspecific hybridization, gynogenesis, artificial sex reversal, together with the cloning of Sox genes and detection of microsatellite polymorphisms, this dissertation focuses on the possible chromosomal mechanisms for sex determination and natural hybridization in large scale loach, Paramisgurnus dabryanus, in which a female heterogamete sex determination was previously proposed, and also on sex determination of rare minnow Gobiocypris rarus, a potential Chinese model fish, in order to provide new evidence for further understanding and exploring the genomic characters of the two small experimental species native to China. The results would be useful for effective protection and utilization of the natural resources of fish.
Two major achievements were obtained in this dissertation. First, some genomic characters were studied in large scale loach in the following aspects: 1) Genetic structure of seven populations (six from the Yangtze River system and one from the Pearl River system) of large scale loach was studied at six microsatellite loci, and the results showed that the populations exhibited low genetic diversity but significant genetic differentiation. The differentiation may be still at the level of population or geographical differentiation. 2) Artificial hybridization between large scale loach and its close-related species, pond loach M. anguillcaudatus, was performed as the positive references in the studies of their natural hybridization. The hybrids were triploids with female sterile, and the sex ratios of the reciprocal hybrids were significantly biased. Through the investigation on some wild samples at three microsatellite loci, evidence for rare hybridization between the two loach species was obtained from those individuals with ambiguous morphology. 3) Meiotic and mitotic gynogenesis were successfully induced in large scale loach by using heterogeneous sperms of red crucian carp and temperature shock, and the results showed that cold shocks could not stably induce mitotic gynogenesis as effectively as heat shocks. By scoring sex ratios of gynogenetic F1 and F2 progenies, a female heterogamete sex determination in large scale loach was generally confirmed, although temperature effects may not be excluded. 4) Eighteen Sox genes were cloned and characterized from the genomic DNA of large scale loach, and six of them were found to be orthologous to those of mammals and have two or more copies. With the preliminary RT-PCR analyses combined with sequencing Sox genes expressed in the gonad tissues of two loach species and their reciprocal hybrids, the results indicated that Sox9 and Sox8 would be important in sex differentiation of large scale loach and its hybrids with pond loach.
Second, meiotic gynogenesis was induced in rare minnow, and some of the meiogens were subjected to artificial sex reversal by feeding androgens. F2 progenies were obtained by self-mating among the sex-reversed progenies. Based on the results from scoring sex ratios in the F1 and F2 progenies, a complex mechanism of sex determination in rare minnow was proposed, and factors on autosomal chromosomes might have influenced on its sex in this study.