Effects of ooplasmic size on the larval development as observed in the small egg mutant emi of Bombyx mori

摘要

A Bombyx mori population hich produces miniature eggs was found as a spontaneous mutation in the c15 strain stocked in Kyushu University and established as a new strain. The relevant, recessive gene was named emi and mapped to the position 9.5 cM on the 12th linkage group (KAWAGUCHI el al., 1991). Marked smallness of the eggs compared with normal eggs is the major trait of emi, inherited in a pseudo-maternal manner. Although similar to the sm mutations (HAYASHI, 1937, 1940; CHIKUSHI and DOIRA, 1970) in size and shape, the emi eggs are remarkable in that they can be fertilized and hatched (KAWAGUCHI et al., 1993a, 1996b). Moreover, the minuteness of emi eggs was partially recovered by the administration of 20-hydroxyecdysone into the maternal body cavity at the pupal stage (KAWAGUCHI et al., 1998). The different-sized eggs from the hormone-treated and untreated emi individuals are derived from different-sized ooplasms. We investigated the influence of the small emi ooplasm produced in the emi homozygous females, and also the enlarged emi ooplasm induced by the. administration of 20-hydroxyecdysone, upon the larval development of the next generation. The larvae hatched from the emi eggs were smaller and lighter than normal throughout the entire larval stage. On the other hand, the larvae hatched from the hormonally enlarged emi eggs were larger than those from their untreated emi counterparts, but the difference gradually disappeared during larval development. In contrast, the widths of the head capsules were equal in all specimens tested, and thus it were not affected by ooplasmic size.