メダカの卵巣発達に及ぼす光の強さの影響

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(1)Title. メダカの卵巣発達に及ぼす光の強さの影響. Author(s). 吉岡, 寛. Citation. 北海道教育大学紀要. 第二部. B, 生物学,地学,農学編, 26(1): 43-49. Issue Date. 1975-08. URL. http://s-ir.sap.hokkyodai.ac.jp/dspace/handle/123456789/6331. Rights. Hokkaido University of Education.

(2) Journal of Hokkaido University of Education (Section II B ) Vol. 26, No. 1 August 1975. -lfci®iS^t-*^SS (^ 2 g|S B ) ^ 26 ^ ^ 1 ^ Bgffl 50 -^ 8 fl. Effects of Light Intensity on the Ovarian Development of the Medaka Oryzias Latipes. Hiroshi YOSHIOKA Biological Laboratory, Hakodate College, Hokkaido University of Education, 040 Hakodate. ^ IN 5^ ^ •: ^^^ ^^(DM^^ZA^'tTfcc7)^^ n 1>UJ t/^ypTRSt.Hit^-^Xtd. ^ 7Ll/^3B c? v/tfy. W^H^meWfs.^^S.. In vertebrates light intensity seems to play a minor role in the stimulation of their gonads as long as it reaches above the threshold value. As for fishes, a considerable number of studies on. the effects of light upon their reproduction have been published hitherto. However, most of them have been concerned with the effects of photoperiod (Harrington, 1959, etc.). The paper by Shiraishi (1965) is the only one which is concerned with light intensity, but it aims at knowing briefly the effective lower limit of light intensity for gonadal maturation. On the other hand, several interesting papers have been reported on this problem in birds. (Bissonnette, 1931; Rollo and Domm, 1943; Bartholomew, 1949; Kirkpatrick, 1955; Farner, 1959; Tienhoven, 1961; Wilson et al., 1964). From the reported results, however, it is impossible to draw a definite conclusion. Thus the knowledge accumulated up to date is meagre for a complete. understanding of the effects of light intensity on gonadal development. The present study was undertaken to ctify the threshold intensity being effective in inducing gonadal development and the relationship between the light intensity and the ovarian development of the medaka.. The author wishes to express his hearty thanks to Prof. Kiichiro Yamamoto of Hokkaido University for his kind guidance and advice in preparing the manuscript. The author is also very grateful to Prof. Rinjiro Ohno for his critical reading of the manuscript. Materials and Methods Materials used for the experiments were Oryzias latipes of. a wild type, which were collected. from an irrigation pond of Yunokawa Hot Springs, in the suburbs of Hakodate City. The present experiment was carried out during the period from Jan. 5, 1972 till Feb. 17, 1972. Six groups, each consisting of 20 males and 20 females, were kept in separate aquaria containing about 40 litres of water. The groups were subjected to the long day-length of 15-hour light per day. The. intensities of light were 360 lux (Group A) ,160 lux (Group B), 50 lux (Group C), 10 lux (Group D) and 5 lux (Group E). Another group was kept under natural daylight (Group F). The long day-. (43).

(3) 44. H. YOSHIOKA. length and the light intensities metioned above were provided by fluorescent lamps. The fish were fed on living freshwater oligochaetes or commercial food "Tetramin" and the water temperatures of the aquaria ranged from 18 to 21°C. At the beginning of the experiment and at intervals of one week during the experiment, 3 fishes were sampled from each group to examine the changes in their ovaries. The body weight and ovary weight of the fishes were also recorded. For histological observations, their gonads were fixed with Allen-Bouin's solution. They were cut at 8-10 micra in thickness by the usual. paraffin method and stained with Delafield's haemotoxylin and eosin. Results 1. Changes in ovary weight Ovary weight is represented by gonadosomatic indices (GSI), percentages of the ovary weight to the body weight (Text-fig. 1). At the beginning of each experiment the mean values of gonadosomatic indices (GSI) were about 9.0 for all of the groups to be experimented. Two weeks later, no significant difference in the mean values of GSI was found among Group A, B, C and F. The values in these groups tended to increase slowly but steadily and arrived at a range of 10.2 to 15.0 after 2 weeks. On the other hand, the values in Group D and E showed little increase during. that period.. 3C. a-—. --® 360 lux. ®-- -. -- ® 160 lux. 0-. 00-. @. (Group. (Group •o 50 lux (Group. -—o 10 lux. (Group. •o 51ux. (Group. ® Natun. A) B). 0. D) E). .•—-'/. daylight (Group F. > /^:^. I /''/" ~'~/. /,' /. /. //,. //7. /°^. ^L .0. .9-;.. :^. ^. '. <-"0. /. /'. /. .'. /~. /.-'. V. >/. •. @. -7^. .0.. ///. 2(. 1C. -. —- ®. /:"'. /. /. /. /. ^°. '°^-'. ../ •o. -.0-. o - 0- - '. - - 0. 0. 0. ~ 0- '. 0)2345 Week Text-fig. 1. Changes of gonadosomatic index (GSI) of the medaka at various light intensities. By 3 weeks of experiment, a significant difference was found among Groups A, B, C and F.. The ovaries of the fish in Groups A, B and F gained their weight promptly and their GSI values attained an average of 25.4 for Group A, 23.6 for Group B and 24.5 for Group F. On the contrary,. (44).

(4) Effects of Light Intensity on Ovary of the Medaka 45. the ovaries of the fish in Group C developed very little during this period and the mean GSI value of the fish was about the same as that of the preceding week.. The ovaries of the fish in Group D continued to develop slowly, and the GSI value of the fish became near that of Group C. The average values were 12.7 for Group C and 11.3 for Group D,. but that of Group E changed very littll during this period. Four weeks later, however, the ovaries of the fish in Group C gained weight abruptly and the. mean GSI value in the fish reached about the same value as those of Groups A, B and F in the preceding week. The GSI value of Group C was 24.6, which was twice as much as that of the same Group the preceding week, while the mean GSI values of Groups A, B and F were almost the same as those seen the preceding week. A slow increase in ovary weight was also found in Group D. On the contrary, a slight decrease in ovary weight was found in Group E. Five weeks later, the mean GSI values in all groups except for Group D were almost the same as those of the preceding week. Group D showed a distinguishable increase in GSI which attained 24.0. Few changes were observed in the mean value of Group E.. From the above findings it can be reasonably concluded that the ovaries of the medaka can respond to the light above 10 lux. The response of ovaries appears to increase with the increment of light intensity up to 50 lux, but no further increase in the response of ovaries was seen under. the light intensity above 160 lux. At the intensities of 10 and 50 lux, accordingly, the development of ovaries showed two- and one- week delays, respectively, as compared with the fish kept. under the light intensities above 160 lux. 2. Size com.posit.ion in oocytes During the course of the experiment, 3 fishes from each group were sampled on Jan. 5, Jan. 12, Jan. 19, Jan. 26, Feb. 2 and Feb. 9, and the oocyte composition in each ovary was examined. The largest 50 oocytes in each ovary were chosen for measuring the diameter under a microscope.. The frequency distribution of the diameter is shown in Text-fig. 2. At the beginning of the experimenit, the oocytes of the initial control were less than 0.45 mm in diameter. Histological observations revealed that many yolkless oocytes were found in the ovary of initial control in. addition to a small number of oocytes with yolk vesicles (Fig. 1 ). On Jan. 26, 3 weeks after the fish had been exposed to experimental light conditions, the oocytes of the fish of Groups A , Band F increased in size varying from 0.40 to 0.90 mm in diameter. The large yolk-laden oocytes of about 0.80 mm in diameter were undoubtedly laid within a few days (Figs. 2—4). On the other hand, no marked increase in the size of oocytes was. found in the fish of Groups C, D and E. The diameter of oocytes ranged from 0.30 to 0.50 mm in Group D and from 0.30 to 0.45 mm in Group E. However, the ovaries of Groups C and D contained. yolk-laden oocytes at the yolk vesicle stage (Figs. 5 and 6). The ovary of Group E showed an appearance similar to that of the initial control (Fig. 7). Investigations on the ovaries obtained during the 4th week revealed that the size composition of oocytes in Groups A, B and F was similar to that seen the preceding week. The size of oocytes in Group C reached about the same values as those of Groups A, B and F the preceding week. A lot of large yolk-laden oocytes were commonly seen in the ovaries of Group C. Although the. (45).

(5) 46. 401 20| 40|. x. H. YOSHIOKA. Initial control Jan. 5. Natural daylight (Group F: Jan. 26. 20| 40| 20| 401 20| 40. 360 lux (Group A Jan. 2B. 160 lux (Group B) Jan. 26. 50 lux ( Group C] Feb. 2. 20 10 lux (Group D I. 40. Feb. 9. 20 5 lux (Group E ). 40. Feb. 9. 20. 0.3 0.5 0.7 0.9 0.3 0.5 0.7 0.9 0.3 0.5 0.7 0.9 Diameter of oocytes ( mm) Text-fig. 2. The frequency distribution of the largest 50 oocytes diameters of ovaries of the fish exposed to light of various insitiensities.. oocytes in the ovaries of the fish of Group D became large to some extent, they did not develop. as those in Group C by the same time (Fig. 9). No difference in the histological appearance of the ovaries between those of the initial control and those of Group E (Fig. 10) was seen. On Feb. 9, 5 weeks after the start of the experiment, a marked increase in oocyte size was found in the fish of Group D. A lot of large yolk-laden oocytes of 0.45-0.90 mm in diameter were commonly seen in the ovaries of the fish (Fig. 11). No marked increase in the size of oocytes was. found in the fish of Group E by the end of the experiment (Fig. 12). From the above results, therefore, it may be concluded that, at the light intensities between. 10 and 50 lux, the development of oocytes tends to increase in proportion to the increment of light intensities, but it seems to exhibit a similar increase in the case of the light intensity above 160 lux.. (46).

(6) Effects of Light Intensity on Ovary of the Medaka. LU. s Q.. I: :tU. a.. (47). 47.

(7) 48. H.. YOSHIOKA Explanation of Plates All figures are photomicrographs of the sections of the medaka.. PLATE I. Fig.1. The ovary of a fish obtained on Jan. 5 (initial control). Many yolkless oocytes smaller than 0.45 mm in diameter are found. X 28. Figs. 2—4. The ovaries of fish in Group F (Fig. 2), Group A (Fig. 3) and Group B (Fig. 4) obtained on Jan. 26. Large yolk-laden oocytes are found. X 28. Fig. 5. The ovary of a fish in Group C obtained on Jan. 26. X 28. Fig. 6. The ovary of a fish in Groug D obtained on Jan. 26. X 28. Fig. 7. The ovary of a fish in Group E obtained on Jan. 26. X 28. Pig. 8. The ovary of a fish in Group C obtained on Feb. 2. A few large oocytes with yolk mass are found, ym.; yolk mass. X 28.. PLATE II. Fig.9 The ovary of a fish in Group D obtained on Feb. 2. The oocytes with yolk globule begin to appear in the ovary. yg.; yolk globule. X'28. Fig.10. The ovary of a fish in Group E obtained on Feb. 2. X 28. Fig.ll The ovary of a fish in Group D obtained on Feb. 9. X28. Fig.12 The ovary of a fish in Group E obtained on Feb. 9. X 28 .. Discussion. Although a considerable number of studies concerning the effects of light on the reproduction of fishes have been performed in recent years, there are few papers which ascertain the corre-. lation between light intensity and gonadal development. The study of Shiraishi (1965) in Plecoglossus alt.ivelis may be the only one which dealt with the effects of light intensity on fish reproduction, but it cleared only the lower limit of light intensity being effective in inducing gonadal development. According to his study, the gonadal maturation of the fish is affected by the light of about 0.1-0.2 lux and a retardation of gonadal maturation is taking place in the fish exposed to the light of a long day-length at the intensity of about 0.1—0.2 lux. The present. findings made it clear that, in the medaka, the threshold intensity of light effective in the promotion of ovarian development may be around the level between 5 and 10 lux. The value of. the critical light intensity may be different in various species and the difference in the value may be due mainly to the difference of environments of their habitat. In addition, it is very interesting to note that the ovarian response of the medaka tends to increase with increasing light intensities in the range between 10 and 50 lux. The development. of ovaries was delayed two weeks at 10 lux and one week at 50 lux as compared with that of the fish under natural daylight. Similar results were obtained in a small number of birds (Bissonnette, 1931; Bartholomew, 1949). Wilson et n.l. (1964), in chickens, found a graded response of sexual maturity at low light. intensities (0 to 74 lux) and Rollo and Domm (1943), in the finch, found a similar phenomenon. As for fish, the presensiment is the first study on the effects of light intensity on sexual maturity, and the data dealing with the light intensity being effective for gonadal maturation are too meagre to draw a definite conclusion. Therefore, there remain many problems to be clarified. in this field of study especially in view of light energetics.. (48).

(8) Effects of Light Intensity on Ovary of the Medaka 49. Summary. The writer carried out experimental studies on the effects of light intensity upon the ovarian development of the medaka, Oryzias latipes during winter months in 1972. The results obtained from the experiment are summarized as follows :. 1 . The threshold intensity of light effective for ovarian development must be lower than 10 lux.. 2 . The ovarian response tends to increase in proportion to the increment of light intensities in the range between 10 and 50 lux. 3 . It seems to exhibit a similar response in the case of light intensities above 160 lux.. References Bartholomew, G. A., 1949 The effect of light intensity and day length on reproduction in the English sparrow. Bull. Mus. Camp. Zool. Harvard Coll. 101 : 433-476. Bissonnette, T. H., 1931 Studies on the sexual cycle in birds. 5. Effect of light of different intensities upon the testis activity of the European starting (Stiiruns vnlgan's). Physiol. Zool. 4 : 542—574. Farner, D.S., 1959 Photoperiodic control of annual gonadal cycle in birds. In R. B. Withrow(ed) :Photoperiodism and related Phenomena in Plants and Animals. Amer. Assoc. Adv. Sci., Washington, 717—750 Harrington, R. W, Jr.,1959 Effects of four combinations of temperature and daylength on the ovogenetic cycle of a low-latitude fish, Fnnillilns con/lneiifns. Zoologicca N. Y. 44 : 149—168, Kirkpatrick, C.M., 1955 Factors in photoperiodism of bob-white quail. Physiol. Zool. 28 : 255—264. Rollo, M. and Dornm, L. V., 1943 Light requirement of the weaver finch. 1. Light period and intensity. Auk. 60 : 357-367. Shiraishi, Y., 1965 The influence of photoperiodicity on the maturation of Ayu-Fish.Plecoglossns altiuelis. S.The limit of the intensity of light. Bull. Freshwater Fish Res. Lab. 15 : 70—76. (in Japanese). van Tienhoven, Y., 1961 Endocrinology of reproduction in birds. In W. C. Young (ed): Sex and Internal Secretions. Williams & Wilkins Co., Baltimore, 2 : 1088-1169. Wilson, W. 0., Woodard, A. E. and Abplanalp, H., 1964 Exogenous regulation of oviposition in chicken. Poultry Sci. 43 : 1187-1192.. (49).

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