|Other Abstract||Gibel carp (Carassius auratus gibelio) is a triploid fish producing both gynogenetically and gonochoristically in China. Previous studies on it mainly focused on several limited clones identified twenty years ago in populations from Shuangfeng reservoir, Fangzheng County, northeast China. Although gibel carp has been preserved and popularized for years, and several triploid endemic populations of gibel carp have been also reported in China, little is known about patterns of distribution in natural water areas, the status of genetic diversity in cultivated and wild populations, and phylogenetic relationships among populations. Moreover, studies on reproductive behaviour have been never carried out in gibel carp from China. In this dissertation, these questions were addressed by field sampling across geographical regions, flow cytometry to detect ploidy level, combination of multiple molecular markers, as well as design of reproductive behaviour. Furthermore, several issues on evolutionary potential of gibel carp were discussed, including characteristic of geographical distribution, coexistence among different clones and with the sexual relatives, and mechanisms responsible for the origins of clonal and genetic diversity.
This dissertation has been divided into four parts, as below:
Firstly, fourteen wild populations of genus Carassius were collected from Xinjiang Province, northeast China and the middle reaches of the Yangtze River. After all the 459 samples were identified as C. aurassius based on morphological characters, their ploidy level was successfully determined by flow cytometry. Based on the result of ploidy analysis and field investigations, we found that triploid gibel carp was far more widely distributed than previously imagined, because it occured in all the sampling sites, most where gibel carp had never been reported; its closely related sexual, diploid C.a.auratus were found existing only in the middle reaches of the Yangtze River. In combination with previous, these results indicated that gibel carp shows a classic pattern of "geographical parthenogenesis", this phenomenon reported in C. auratus for the first time. Because of a few triploid fertile males (2.5 -16.7%) found in natural populations, here we propose the "occasional sex" hypothesis, holding that occasional sexual reproduction in gynogenetic gibel carp will produce a few males and thus partly compensates for the twofold disadvantage of sex, contributing to coexistence of gynogenetic triploid and sexual diploid C. auratus. Female-biased sex ratio (1 : 2 - 1 : 8) was observed in most of diploid populations, implying that the sexual form may mainly produce the female more than the male and thus the twofold cost of sex decreases. This process also promotes the coexistence of gynogenetic and sexual forms, supporting the "female-biased sex allocation" hypothesis. Besides the significance for evolutionary ecology of unsexual vertebrates, triploid gibel carp with diverse morphological characteristics discovered in different regions will be likely to provide new materials for selective breeding of excellent clones.
Secondly, clonal diversity of gibel carp was examined in nine different geographical populations by using serum transferrin marker. Ten clones were clearly distinguished from 50 samples collected from Heilongjiang Province, further revealed by analysis of RAPD marker; eight, eight and fourteen clones were clearly identified in 51 samples from Henan Province, 133 from Xinjiang Province and 33 from Hubei Province, respectively. Therefore a total of 29 clones were detected in 267 samples in the nine populations, among which 28 were first found. This supports the notion that unisexual vertebrate populations are composed of numerous coexisting clonal lineages. Even, the level of clonal diversity in two national stock hatcheries was higher than that in some of wild populations, suggesting that they still preserve abundant clonal diversity. Sex and recombination may provide the principal source of clonal variation. Besides, other factors such as mutation, introgression of paternal DNA, polyploidy, natural selection, hybridization and migration also contribute to clonal diversity in gibel carp. The wide geographical distribution of clone A probably support the general-purpose genotype hypothesis, but we also can not exclude the possibility that clone A found in different sites is the one escaped from cultured ponds to natural rivers and lakes owing to the impacts of human activities and natural disasters such as flood. The frozen niche variation hypothesis is very helpful to explain coexistence of multiple clones, but further ecological studies are required to reveal it. The alternative reproduction modes of gynogenesis and sexual reproduction favor origin, maintenance and long-term evolution of clonal divisity. In some instances, females of gibel carp are likely to be independent of sperm of its closely related sexual males but select that of other fishes, thus avoiding competition with sexual hosts as well as facilitating colonization of new habitats. This change will greatly improve evolutionary potential of gibel carp clones. Furthermore, plenty of clones found will help to further selective breeding of gibel carp.
Thirdly, to obtain more knowledge on genetic diversity and genealogical relationship within gibel carp, sixty-four samples representing all known clones from four hatcheries (FZ, QH, WH and PZ) were selected for complete mitochondrial DNA (mtDNA) control region (920 bp) sequencing. Genetic diversity varied remarkably among the hatcheries, with FZ and QH demonstrating high diversity and WH and PZ showing no variation. Sequence analyses indicate that FZ and QH might represent two distinct matrilineal sources. One of QH samples carried the haplotype shared by a most widely cultivated FZ clone, implying the probability the clone escaped from cultivated ponds to QH, whereas four samples in FZ clustered within the lineage formed mainly by QH samples, most likely reflecting historical gene flow from QH to FZ. Our data reveal that clones in WH originated from FZ, consistent with their introduction history, and support the hypothesis that gibel carp in PZ was actually domesticated from individuals of the FZ clone widely cultivated.
Finally, after courtship behaviors in gibel carp were observed and documented outdoors, courtship communication were summarized. Synchronization of female producing eggs and males producing sperms will help to improve reproductive success. In a group mating experiment, females mate with males randomly. The following group and individual choice experiments indicated that in simulative natural conditions, females of gibel carp prefered conspecific males over males of carp (Cyprinus carpio), but occasionally the females demonstrated a preference for males of carp over conspecific. These flexible reproductive behaviors will facilitate origin of clonal variation and improving abilities of competition with closely related sexual species. If the results in this study are applied to natural conditions, much data are requred to test them.|