Sperm precedence and sperm movement under different copulation intervals in the silkworm, Bombyx mori

Abstract

Sperm precedence and sperm movement were investigated under different copulation intervals in the silkworm, Bombyx mori. The mass of sperm began to enter the spermatheca 1 h after copulation ended, and the number of sperm gradually increased in the spermatheca and attained a plateau when the spermatheca was filled at about 2.5 h after the end of copulation. Thereafter, the sperm proceeded to the fertilization canal at 3 h after the end of copulation. Based on movement of the sperm, arrival time of the sperm at the fertilization canal, and number of sperm in the spermatheca, we estimated the proportion of sperm from the 2nd copulation to the total number of sperm in the spermatheca in double copulations at 0 and 2 h interval between copulations. The estimated proportions of sperm from the 2nd copulations were 1.03 and 0 at 0 and 2 h intervals, respectively. To test the validity of these estimates, we mated females to two different genetically marked males at 0 and 2 h intervals between copulations. The actual P 2 values were 0.95 and 0.06 at 0 and 2 h intervals between copulations respectively, values which were consistent with our estimated values. Difference of sperm precedence in relation to copulation intervals in B. mori may be caused by the timing of sperm transfer from the spermatophore to the spermatheca and from the spermatheca to the vestibulum.