| Histological changes during gonadal differentiation, development and maturation were observed in Paralichthys olivaceus. Combined with RIA analysis of T and E2 levels in whole body extract of larvae and juvenile, and also in serum, we discussed changes of T and E2 concentrations during gonadal differentiation, development and maturation in endocrine level. Then, by treating undifferentiated normal diploid and gynogenetic fish with high temperature and E2, respectively, we obtained flounder stock with high percentage of male or femal, and studied effects of temperature and exougenous sex steroids estradio-17β(E2) on growth, gonadal differentiation, sex ratios and levels of T and E2. Mechanisms of sex determination and gonadal differentiation in viewpoint of endocrine were also discussed.Histological observation found that when fish were reared at temperature of 18-20℃, gonad remained undifferentiated till the 45th day post hatching (D45). When larvae reached about 38.0±1.7 mm of mean total length (TL) on the D70, ovarian cavity was observed in presumptive ovary. On the D110, when mean total length reached about 86.5±5.9 mm, ovarian cavity grew bigger, and oocytes were firstly detected. Differentiation of testis was firstly observed in juvenile flounder with mean total length of 63.5±3.4 mm on the D90 when spermatogenous cell proliferated rapidly and spermaduct formed. Seminal lobule appeared on the D100 when juveniles reached about 76.0 mm of total length. And when flounders reached total length about 140.0 mm, appearance of spermatocyte indicated the completion of gonadal sex differentiation.Analysis of T and E2 levels in whole body extract from larvae and juvenile, and also in serum in Paralichthys olivaceus, showed that concentrations of T and E2 were higher than those in larvae with the total length of 6 mm. Then, T levels decreased with the process of gonadal differentiation, but E2 levels increased sharply during period of gonadal differentiation and decreased to a lower level thereafter. In male, from tesits stageⅢ, T levels in serum increased with the development of gonad, and finally, it dropped again at the stageⅤafter maturation of testis and spermiation. E2 levels decreased with the development of male gonad from the stageⅡto the stageⅤ, but had no significant difference in general. In female, both T and E2 levels increased gradually with the development of ovary and peaked at the stageⅣ, and then dropped after ovulation. Both levels of T and E2 have an annual cycle. In male, levels of T and E2 increase sharply with the increase of temperature from May, and then it begin to drop in September. In female, E2 increased from March and peaked in June and August, and then dropped, too. As to levels of T, it also had two peaks, one in Feberary and the other in June.Result of temperature treatment showed that there is mechanism of TSD in Paralichthys olivaceus, and ratios of male increased with the increase of temperature. The percentage of male in control, 21℃and 24℃were 51.62%, 60.00% and 73.33%, respectively. And in the groups with high temperature of 28℃, significantly high ratios of male compared to control were observed (87.27%). Analysis of T and E2 levels suggested that there were no obvious difference among control and different temperature treated groups in T levels, but E2 levels in temperature treated groups were lower than that of in control. Especially, E2 levels in high temperature groups of 28℃were significantly lower than that in control groups (P<0.05). By treating gonadal undifferentiated fish with different dose of E2, we found that both mortality and ratios of female were doze-depended. With the increase of dose of E2 in feed, percentage of female was increased in Paralichthys olivaceus. Analysis of T and E2 levels showed that level of T were lower than that in E2 treated group. But as to E2, levels were lower than those in the treatment groups with doze of 0.2 ppm and 2 ppm, but higher than those in the treatment groups with dose of 20 ppm and 100 ppm.In the meantime, we compared gonadal development conditions of gynogenetic flounder. The ratio of female in gynogenetic flounder was 83.33%, and the ratio of XX male in gynogenetic flounder treated with high temperature of 28℃was 91.67%. In the gynogenetic flounders, a certain proportion of individuals had abnormal gonad, some of the gonad grew small and some even lost part of it. Further studies on T and E2 levels in serum showed that gynogenetic individuals had higher T levels and lower E2 levels than those normal diploid groups. And also, higher T level and lower E2 level was observed in flounder with abnormal gonad when compared with control and gynogenetic flounder.
|
PDF Full Text Request