Sericulture has been an important agricultural industry since 2650 BC (Seidavi et al. 2006). There are about 10 million farmers and 0.5 million related industrial workers in the world. The present capacity of silkworm eggs production in Iran is about 70000 boxes per year which are distributed among 40000 silkworm house holders. Annual cocoon production in Iran is about 6000 metric tons which is small in comparison with production in previous centuries. Silkworm breeding using selection systems can improve line and hybrid performance. Individual selection based on cocoon weight in GGP generation (great grandparent) improved GP (grandparent) and P (parent) parental line generation (Seidavi et al. 2007), but it did not consider the effects on resistance performance of hybrid generation. In this experiment, the effects of two phenotypic selections of GGP parents on vitality (percentage of survival pupae) and resistance properties in six of their hybrids (including 31×32, 32×31, 103×104, 104×103, 107×110 and 110×107) were studied.
Silkworms are agricultural insects producing silk fibres. Because of economic importance of sericulture, breeding programs are performed in order to improve bio-economic traits of lines (production and resistance traits) and also developing new lines containing interesting characteristics. Silkworms in Iran are reared twice a year, in spring and autumn. Spring is the natural rearing period of silkworms because of good environmental conditions. In autumn the quantity and quality of the cocoon produced by silkworm lines decreases due to increasing mortality and access to mulberry leaves with low nutritive quality. This decrease in performance will be lower in the lines which are genetically resistant to severe environmental conditions. Therefore, it is necessary to breed and develop new silkworm lines suitable for such a situation. It is obvious that silkworm lines having lower performance but higher resistance can be introduced for autumn season rearing. Appropriate breeding programs have to be defined and conducted for the autumn rearing lines. Thus, we have to estimate genetic parameters of autumn and spring lines separately when reared in each season. The present study was undertaken to identify heritability of important traits in the spring and autumn seasons for 6 silkworm lines.
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