Effect of cross breeding on the tolerance of Chinese based silkworms (Bombyx mori L.) to nuclear polyhedrosis virus

The nuclear polyhedrosis virus (NPV), producing grasserie disease in silkworm, is accounted as one of the most important reason for production loss in sericulture industry. Cross breeding is one of the most important and effective breeding methods which could improve silkworm performance. In the current study, the resistance of larvae to NPV in 104 and 154 Chinese based lines and their hybrids in F1 and F2 generation were investigated. Fourth instar larvae were inoculated by NPV and their mortality were checked. A great value of heterosis and heterobeltiosis was observed in F1 hybrids, due to the susceptibility of line 104 and the tolerance of line 154 to NPV. The value of heterosis and heterobeltosis in reciprocal crosses varied from 82.91 to 86.19 and 27.26 to 29.54, respectively. Reciprocal crosses, however, did not declared significant difference in the susceptibility of larvae to NPV. Heterosis was declined in F2 and maternal heterosis was completely expressed on this generation (13.80 to 17.89 percent). Reciprocal cross did not have considerable effect on the maternal heterosis which indicates that the use of tolerant line as maternal line or paternal line do not make considerable difference.