Effect of Micropylar Morphology and Size on Rapid Sperm Entry into the Eggs of the Medaka

An experimental study on sperm entry into the micropyle of the medaka egg was carried out in order to analyze the effect of micropylar size on fertilization. The eggs and micropyles of Oryzias latipes were larger than those of Oryzias melastigma. When eggs of O. latipes and O. melastigma were simultaneously inseminated by spermatozoa of O. latipes, O. melastigma or Aplocheilus panchax, O. melastigma spermatozoa rapidly entered the conspecies eggs as well as the large micropyles of O. latipes eggs, while most of the O. latipes and A. panchax spermatozoa failed to rapidly enter the small O. melastigma micropyles. Moreover, when O. latipes eggs with a small or a large vestibule were simultaneously inseminated by O. melastigma spermatozoa, the size of the micropylar vestibules (range of 17-23 μm in diameter) also affected rapid sperm entry into the egg. However, this effect was not recognized when the eggs were inseminated by conspecies spermatozoa. On the other hand, when O. latipes eggs were inseminated by a mixture of O. latipes and O. melastigma spermtozoa or O. latipes and A. panchax spermatozoa, the faster swimming spermatozoa fertilized significantly more frequently than the slower O. latipes spermatozoa. These results suggest that rapid sperm entry into the micropyle is not only affected by differences in the morphology and size of the micropyles, but also might be accelerated by the linear swimming velocity of the spermatozoa.