Environmental influence on testicular activity in the green frog, Rana esculenta

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Abstract
Adult male Rana esculenta were placed under several combinations of light and temperature in different periods of the year to elucidate the regulation of various testicular phases by environmental factors and to discover the mechanism through which these factors act upon the testis. In nature the quiescence of spermatogenesis in winter is due solely to low temperature. Under controlled experimental conditions a temperature of 4°C, whatever the photoperiod, impairs spermatogenesis at any time of the year. The early spring resurgence of a new wave of spermatogenesis depends upon a gradual rise both in the environmental temperature and the photoperiod. In fact in nature the temperature range favorable for active spermatogenesis is between 12°C, and 25°C with a period of between 10 and 16 hours of light daily. In the laboratory a temperature of ca. 20°C and a period of 12 hours of light daily represent the best combination of these factors for regulating and maintaining active spermatogenesis in any period of the year. Very long (21 hours of light daily) and very short (3 hours of light daily) photoperiods, however, have deleterious effects upon the testis. These effects are manifested mainly in the suppression of primary spermatocytes and successively in a gradual decline in number of secondary spermatocytes and spermatids and appearance of numerous degenerating cell nests. The same occurs when the frogs are treated at very low temperature. Besides this, high temperature stimulates plasma levels of testosterone which does not occur if the animals are kept in the dark. Very low temperature, on the other hand, has the opposite effect.These data suggest that a combination of “critical temperature” and “critical photoperiod” is necessary for regulating and maintaining active spermatogenesis since neither of these factors is effective in the absence of the other. The stimulatory effects of high temperature are due to the increased secretory activity of the pituitary gonadotropin.Furthermore the present results also indicate that the testis of Rana esculenta does not manifest any seasonal change in the degree of its response to thermal and/or photoperiodic regulation i.e., it does not show photo‐ or thermal refractoriness during the year.