Thyroxine-induced haemolymph protein and ecdysteroid increases in the silkworm, Bombyx mori: Effect on larval growth and silk production

Abstract

The effect of feeding mulberry leaves treated with thyroid powder on larval growth and cocoon formation was analysed following treatment of different larval instars. Larvae treated during the second instar only, showed maximal growth along with other enhanced cocoon characters. This suggests that thyroxine could be used effectively for commercial silkworm rearing by treatment at an early stage of silkworm development. The larval period was shortened by 30–40 h with increased cocoon and shell (silk) weight in all treated groups, reaching 150% in some cases with no loss in silk quality. An increase in haemolymph ecdysteroid level was observed during the fourth instar, suggesting that thyroxine in some way affects ecdysteroid metabolism. As compared to controls, where haemolymph ecdysteroid levels reached 1.4 ng/μl during the fourth instar, ecdysteroid levels showed an almost 2-fold increase during the fourth instar when animals were treated with thyroxine from the first through the fifth instar (2.3 ng/μl) or when treated in the fourth instar only (2.8 ng/μl), and a 3.5-fold increase (4.8 ng/μl) for treatment in the second instar only. A peak of haemolymph ecdysteroid was observed on day 10 of the fifth instar for all treated groups, but not until day 11 for the control group. Thyroxine treatment from the first through the fifth instar caused a 2-fold increase in haemolymph protein level during the first (15.07 μg/μl) and second (10.99 μg/μl) instars, as compared to controls (7.73 and 5.48 μg/μl, respectively). Thyroxine treatment of second or fourth instars alone resulted in a 2-fold increase in haemolymph protein levels to 12.83 and 13.49 μg/μl, respectively, during the fourth instar, when compared to that of the control group (6.20 μg/μl). This suggests that thyroxine, in addition to affecting ecdysteroid metabolism, also affects protein metabolism.