A re-investigation of the oxygen permeability of the chorion in relation to the onset of embryonic diapause in the silkworm, Bombyx mori

Abstract

In silkworm eggs, it has been proposed that the decrease in oxygen uptake which accompanies the initiation of diapause may result from a decrease in the permeability of the chorion to air during the early stages of development (“oxygen barrier hypothesis”). Thus, we measured the amount of oxygen that permeated through the chorion using an apparatus designed for this purpose. The pigmented and non-diapausing egg mutant ( pnd mutant) of the silkworm were used. The homozygous embryo of the pnd mutant never enters diapause owing to a genetic defect, but the heterozygous embryo produced by sperm bearing a pnd + gene can enter diapause. The oxygen permeability of the chorion did not decline appreciably even at the period when oxygen uptake decreased in the heterozygote. Furthermore, no significant difference in oxygen permeability of the chorion was detected between the heterozygote and homozygote of the pnd mutant. In the wild-type silkworm, it was demonstrated that although the chorion of the diapause egg was less permeable than that of the non-diapause egg, the oxygen permeability of the chorion does not change appreciably during the early stages in the diapause eggs. Therefore, our results cast doubt on the “oxygen barrier hypothesis”.