Effect of chemical dechorionation on silkworm embryo viability.

The chorion covering/protecting insect egg, which has some effective functions such as providing mechanical strength, protecting eggs from external environments, and keeping moisture adjustment, is one of the principal barriers to manipulation, cryopreservation, and study of insect embryos. Here we evaluated the silkworm embryo viability after dechorionation using chemical reagents. We have developed an easy and effective method for chemical dechorionation that enables embryos to develop in culture, so that the larvae could normally grow. Eggs attached to a nylon net were treated with potassium hydroxide (KOH) and sodium hypochlorite (NaClO) to remove the chorion, washed with the Grace's insect medium, and then cultured using a dry-moist method which we created. The most effective treatment with regard to embryonic development, hatching, and production of second instar larvae was 30% KOH for 7 min and 2% NaClO for 5 min at 27 °C. Embryos at later embryonic stages were more tolerant to chemical dechorionation and over 75% of embryos treated at 168 h-old (Stage 25, appearance of taenidium) survived to the second larval instar, moreover, the larvae derived from the dechorionated embryos have developed into the moths which can lay the fertilized eggs. Our method would contribute to the establishment of cryopreservation using embryos and analysis of silkworm embryogenesis and might also be applicable to other insect species.