A novel method for induction of pairing in Xenopus by addition of steroids into the water.

The African clawed frog, Xenopus laevis, is widely used in biological studies. Ovulation of Xenopus is normally induced by the injection of human chorionic gonadotropin (hCG) into the dorsal lymph sac of fully-grown female frogs. Previously, we reported a novel method for inducing Xenopus ovulation by adding a mixture of steroids into the surrounding water. In the present study, we demonstrate how to induce reproductive behavior in male frogs using the same methodology. The types and concentrations of steroids were evaluated, and the efficiency of the selected steroid for the induction of ejaculation was examined. New procedures were also examined for inducing mating by mixing both females and males activated by steroids. In male frogs, testosterone was effective for the induction of physiological changes, accumulation of melanin in the hands and induction of amplexus. Time course experiments revealed that eight hours were sufficient to induce male reproductive behavior and ovulation in females. Finally, we established an efficient means of inducing pairing in frogs that involved pre-treatment of frogs with salt solution followed by testosterone for males and a mixture of estradiol and progesterone for females. Although the numbers of oocytes obtained were relatively fewer than those resulting from hCG injection, the fertilization rate of eggs ovulated using the new treatment method was similar to that with eggs obtained by hCG-injection, and juveniles developed normally. In conclusion, we have developed a novel method to induce pairing in frogs without the need for injections.