Environmental regulation of annual reproductive cycle in the mosquitofish,Gambusia affinis

To clarify the environmental factors regulating the annual reproductive cyc le of the female mosquitofish, Gambusia affinis, a viviparous teleost, hist ological. changes of the ovary in natural population, and laboratory experi ments were examined. The results, extending over two years, suggested that ovarian recrudescence is initiated by the rise in temperature during spring and that ovarian regression is caused by the shorter daylength during late summer. The first rearing experiments using four photoperiod-temperature g roups to investigate the factors triggering the onset of reproduction revea led that females with regressing ovaries began reproduction with the rise o f temperature regardless of the photoperiod during spring. The results of t he second experiment using three different temperature groups indicated tha t vitellogenesis occurred at over 14 degrees C and pregnancy at over 18 deg rees C. The third experiment with four photoperiod-temperature groups was a rranged to investigate the factors in the cessation of reproduction. Sexual ly active females ceased vitellogenesis of the next clutch of oocytes due t o the shorter daylength regardless of temperatures during late summer; howe ver, temperature seemed to influence the rate of embryo development. The cr itical photoperiod is estimated at about 12.5 hr. In nature, it is supposed that vitellogenesis starts when the temperature rises to about 14 degrees C, and final maturation of oocytes occurs when the temperature reaches abou t 18 degrees C during spring. Then, vitellogenesis of the next clutch of oo cytes ceases when the daylength becomes shorter than 12.5 hr during late su mmer; the last gestation proceeds at a rate dependent on the temperature, a nd finally reproduction ends by the last parturition. (C) 2000 Wiley-Liss, Inc.