Endocrinological studies on reproduction in hynobiid salamanders

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In Hynobius nigrescens, the seminiferous lobules are arranged around the efferent ductules like the spokes of a fan. The spermatogonia are located in the tubules proximal to the efferent ductules. Germ cells in advanced stages are seen in the tubular regions distal to the efferent ductules. Steroid producing cells and myoid cells are always present within the interstitial tissue. Spermiation and ovulation occur soon after entering the breeding pond. Spermatogenesis occurs synchronously in all seminiferous lobules, and spermatozoa appear in August. From the results of the present experiments, it is conceivable that the so-called water drive is not triggered by the stimulation of prolactin in hynobiid salamanders. Male salamanders show pronounced changes in body shape (male secondary sexual character) associated with aquatic reproduction soon after entering the breeding pond. This is a large increase in body mass, head width and tail height. These well-swelled males monopolize egg sacs just deposited by females, and fertilize the eggs. Occasionally, males with the less developed secondary sexual characters embrace the egg sacs also. In that case, other surrounding males have a chance to fertilize the eggs. Ovulation is experimentally caused in gravid females just before entering the breeding pond between 14 and 18 h after a single injection of gonadotropin (600 IU hCG) in water at 8ﾟC. The formation of egg sacs is completed in the uterus at least 50 h after the injection. The larvae of H. nigrescens and H. retardatus grow at low temperature, such as 6ﾟC, but do not metamorphose under such a condition. In those larval-form salamanders, the differentiation of GTH cells is hardly seen, and gonadal maturation does not occur. It is therefore conceivable that artificial induction of neoteny in hynobiid salamanders by keeping them at low temperatures is difficult.