| Chinese giant salamander(Andrias davidianus), also known as "baby-fish", which belonged to Caudata, Cryptobrachidae, Andrias Tschudi, is the largest living amphibian in the world, and also our country's rare and endangered endemic species. Due to their low dispersal ability, high dependence on water environment, and poor performance in overcoming various barriers, the range of Caudata's distribution reflects their geological age and evolutionary history to some extent, that is, the species which occurs most widely may be the most ancient. Thus, Chinese giant salamander, the most widely distributed in the living amphibians with tails, used to be located in the central and south China across different climate zones and water areas, showing their primitive taxonomy and long evolutionary history. In addition, Chinese giant salamander is regarded as a model system for the research on the evolutionary rate and pattern of newly-arising vertebrate anatomic structures, and of significant scientific value for the study on phylogeny of terrestrial tetrapoda. This paper provide comprehensive evidence about the evolutionary status of Chinese giant salamander in Caudata from the perspective of morphological anatomy, cytological biology, molecular biology and so on, and the major results are as follows:1. To know more exactly about the phylogenetic position in Caudata of Chinese giant salamander, further research was carried out on the embryonic development and morphological anatomy about skeletal system, respiratory system, cardiovascular system, and reproductive system, which was compared with systematic characteristics of Caudata in previous studies. Chinese giant salamander possessed many distinctive morphological structures, as well as some evolutionary trace. For instance, primitive characters of skeletal systems were observed, likeω-shaped vomerine tooth, completely reserved Maxwell cartilage, biconcave vertebra, and almost undifferentiated sacral vertebra. In addition, pectoral girdle and pelvic girdle were mostly consist of gristle, and the joint of iliac bone and sacral rib wasn't fixed which were connected through the cartilage, resulting in the limited support to body of posterior limbs. Furthermore, appendage bone of Chinese giant salamander also exhibited primitiveness of transitional type. Posterior limbs with webbed-toes, of which body position resembled that of fish, stretched backward and adapted to moving about in the water. Four limbs of Chinese giant salamander were used more in water, rather than in land. No obvious pharyngeal was found in Chinese giant salamander, and then respiratory passage, namely short laryngo-tracheal chamber and esophagus intersected at oral cavity. Differentiation between larynx and trachea wasn't clear, and the back of laryngo-tracheal chamber interconnected with lung sac directly. Air was pressed into pulmonary during inspiration by buccal pump, while squeezed out during expiration by the contraction of abdominal wall muscle. The existence of pulmonary would gain buoyant force, while too much buoyant force prove to be a problem for the living in aquatic environment. Pulmonary of Chinese giant salamander was partially simplified, however, gas exchange could be strengthened through wrinkled surface of skin and the movement of body back and forth. Body cavity with no division, defective rid, pulmonary without highly specialized muscle tissue, all contributed to the fact that respiratory function of Chinese giant salamander was fairly original.Individual development of Chinese giant salamander experienced two stages:larval stage with gill and lung-breathing adult stage. The structural succession of its cardiovascular replayed the evolutionary process from hydrophilous to terricolous. The adult cardiac atrium Mediastinum occurred in was divided by interatrial septum with some holes in it, causing the mix of blood. Only one heart chamber was observed in Chinese giant salamander, however, trabecula existed on the inner wall of heart chamber, most stretching back and forth and capable of reducing the interfusion of blood more or less. Longitudinal carinate spiral valve in the arterial cone could approximately distinguish two kind of blood with different oxygen content, and distributed them into pulmonary circulation and systemic circulation, separately. The adults had four pairs of arterial arcades the same as fish, and also retained carotid duct, Botalli ductus, posterior cardinal vein, and vena epigastrica.The anterior portion of the kidney was replaced by testis linked with urinary system by mesonephric duct, which served as both seminal duct and urine duct. Despite that the great majority of amphibians with tails conducted in vivo fertilization, Chinese giant salamanders was in vitro fertilization as well as some ancient species of amphibians (Hynobiidae and Sirenidae). In addition, the way of cleavage of Chinese giant salamanders differed with other amphibians, but presented similarity with that of Acipenseridae in ray-finned fishes. 2. Combining previous data, genome size, chromosome number and karyotype, and microstructure and ultrastructure of gonads were studied to determine the phylogenetic position of Chinese giant salamanders.Genome size (C value) was measured using the flow cytometer. The number and karyotype of chromosomes were examined, and the result indicated that the chromosome number of Chinese giant salamanders was 2n=60. The whole chromosome set was composed of 15 pairs of large chromosomes and 15 pairs of microchromosomes, respectively, Of which, large chromosomes contained 3 pairs of metacentric,3 pairs of submetacentric and 9 pairs of telocentric chromosomes, and microchromosomes included 9 pairs of metacentric and submetacentric chromosomes and 6 pairs of telocentric chromosomes.Spermatogenic cells of Chinese giant salamanders at different stages developed synchronously and joined together into a mass, resembling the arrangement of seminiferous epithelium of seminiferous tubule in Osteichthyes, but showing great difference from the normal alignment of spermatogenic cells, that is, spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and spermatzoa lying from base layer to lumen. Testis of Chinese giant salamanders proved to be a microphyll type, comparatively original.The spermatzoon of Chinese giant salamanders consisted of head, neck piece, and tail. Compared to other amphibians, the spermatzoon of Chinese giant salamanders lacked of acrosomal barb, mitochondria in the tail absent, axial fiber cylinder-shaped. In all the Caudata fertilized in vivo, the generation of sperm was related to cloacal gland, correspondingly, the neck of sperm was long. In contrast, there was no cloacal gland in the species fertilized in vitro, and neck of sperm was short. Chinese giant salamanders conducted in vitro fertilization, neck of sperm was short, but had cloacal gland. All the evidence suggested that Chinese giant salamanders had stayed at the intermediate stage from in vivo fertilization to in vitro fertilization.3. Isoenzymes and mitochondrion genome of Chinese giant salamander were analyzed using molecular biology methods in this paper. To know more about the genetic characteristics of Chinese giant salamander, study on three isoenzymes (LDH, EST, SOD) in six tissues was conducted, which could serve as genetic markers of biochemical characteristics. At the same time, the development of LDH was also discussed in the population of amphibians.Based on analysis and comparison of complete sequences of mitochondrion genome derived from Genebank of Caudata and some typical vertebrates, phylogenetic development of Chinese giant salamander was investigated by means of molecular biology experimental methods, under the guidance of cladistics and molecular evolution biology. In addition, phylogenetic tree was constructed to elaborate the issue of species classification, and inquire further into the origin of Chinese giant salamander and its evolutionary status in amphibians.
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