Studies on the increase in egg size in tetraploid silkworms induced from a normal and a giant-egg strains

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Deposition of large-size eggs by tetraploid females of the silkworm has been reported by many investigators, while no detailed analyses have been carried out on the cause of the increase in egg size due to chromosome doubling. In this insect, a sex-linked gene called ‘Giant egg (Ge)’ is known, and the diploid females with Ge-gene lay remarkably large-size eggs.In the present study, tetraploid females of F1 hybrids which do or do not carry Ge-gene were induced by applying a low-temperature treatment, and the effect of the chromosome doubling was compared to that of the doubling of Ge-gene on egg size. Eggs of the tetraploid females for both hybrids showed a remarkable increase in size, although the increment was relatively small in the females with the double Ge-gene dose. Furthermore, especially large eggs laid by some of Ge-progeny were all unfertilized. The results suggested the possibility of a limitation of egg size increase in the silkworm.