Experiments on Fertilizing Capacity of the Bombyx Silkworm Sperm Entered the Mature Ovum with Special Reference to the Phenomenon of Heterosis

Abstract

1. Inhibitory effect of low temperatures in the late autumn upon fertilization of wild insects is compensated by incubation at 25°C.
2. In the silkworm, the effect of low temperature has a definite critical point between 2.5°C and 5°C, which is related to the suppression of fertilizing capacity of the sperm remaining within the mature ovum.
3. No marked critical point is found between 2.5°C and 5°C with respect to the fertilizability of the mature ovum. In other words, the ovum can resist the inhibitory effect of low temperature fairly well, and fertilization takes place normally as long as the sperm activity is well preserved.
4. Fertilizing activity of the sperm at 2.5°C in the crosses between the same strain and those between different strains shows that the sperm activity is higher in the latter crosses than in the former crosses.
5. Fertilizing activity of the sperm held within the ooplasm of hybrids was found high too.
6. These results point out that the fertilization capacity of the sperm is dependent upon surrounding ooplasm, which is likely to substantiate the preceding observations on ovarian transplantation and favors the view that the phenomenon of heterosis is related not only to the behavior of nuclear genes but also to the ooplasm which is the field of functioning of the nuclear agencies.