Our ultrastructural studies of the sperm duct epithelium in the ascidian Ciona intestinalis document the existence of a light-sensitive assembly of microfilaments. This process coincides with the contraction of the sperm duct epithelium following exposure to blue light. Isolated regions are capable of organizing a microfilament system, thus, the receptoral mechanism regulating this process must be distributed along the length of the epithelium. Cells of the isolated dark-adapted duct contain an amorphous material, but few microfilaments. In contrast, when ducts are actively spawning in response to illumination or when isolated segments of the excised duct are exposed to light, the basal plasmalemma becomes highly convoluted, and the adjacent cytoplasm is packed with a microfilamentous complex. Treatment of dark-adapted segments with A23187 will mimic the effects of light, suggesting that ion fluxes may be involved in generating the contractile network. SDS gel electrophoresis indicates that these cells contain a major band with an electrophoretic mobility indistinguishable from that of actin. The light-induced contraction of cells composing the sperm duct epithelium is implicated in effecting sperm release from individuals and, hence, in controlling synchronous spawning within a population.
Bibliographical noteFunding Information:
We are grateful to Ms. Georgia Hsiau and Lisa Levin for their generous technical assistance with portions of this investigation. Figures 2 and 15 were prepared by Mr. Laszlo Meszoly. We thank Dr. Donald P. Abbott and Mr. 0. R. Blanton for making research facilities at the Hopkins Marine Station available during the summers of 1975 and 1976. Dr. Robert L. Hammill, Lilly Research Laboratories, kindly provided the A23187, and Dr. Richard L. Steinhardt gave assistance in designing ionophore experiments. Dr. John S. Condeelis, Dr. Thomas D. Pollard, and Ms. Mary Troeger read drafts of the manuscript and offered valuable suggestions for its improvement. This project was supported by grants from the National Science Foundation whose assistance is gratefully acknowledged.