Rejuvenation and Multiple-Trait Selection in the Five Chinese-Shape of Parental Silkworms

Introduction Introducing new commercial lines is one of the goals of silkworm breeding centers in the world. Iran has the potential to introduce new lines for hybrid silkworm production due to its silkworm genetic resources and silkworm egg production technology. The performance of productive, reproductive and viability traits in the parental silkworms, including Japanese-shaped and Chinese-shaped lines, changes after several years for various reasons, including inbreeding. Therefore, after a few years, it is necessary to introduce new silkworm strains. Most silkworm breeding schemes in most of the countries are based on the introduction of the Chinese and Japanese-shaped lines to make commercial silkworm hybrid.Material and Methods Five Japanese - shape lines of silkworms named 31, 103, 151, 153, and 1524 were selected from silkworm germplasm of Iran Silk Research Center in the spring of 2012. All genetic combinations obtained by using the diallel method of mating system (20 crosses) were reared in autumn 2012 and spring 2013. After two generations of mass rearing in each genetic combination, the reciprocal crosses in the diallel design were mixed to establish the basic generation of this breeding scheme. The selection program was based on independent culling levels (I.C.L.) method applied from 2014 to 2019 in the selected group. At the same time, all parental lines were reared as control by using a random mating system. Of the 10 genetic compounds, six combinations were retained and the rest were rejected. In this breeding program, due to the necessity of eliminating weak families, family selection, and individual selection were both used simultaneously. In the present study, the productive traits of cocoon weight, cocoon shell percentage, good cocoon percentage, number of cocoons per liter, and cocoon weight per liter were compared simultaneously in the genetic combinations and control lines. Two-factor analysis of variance (including 6 genotypes and 5 years) was performed in a completely randomized design with six replications; then the results were analyzed using SAS statistical software.Result and Discussion In general, the effects of genotype, year and interaction effect of genotype and year were significant for all studied traits. Based on the average performance of each trait during selection program from 2016 to 2019 years (average performance of G2, G3, G4 and G5 generations), the cocoon weight in the combinations of IRA1, IRA11 and IRA7 was more than the parental lines. This advantage was also seen in the trait of cocoon weight per liter. For the cocoon shell percentage, none of the genetic combinations in all the studied years were completely superior to parents. The response to selection for cocoon weight based on deviation from the maternal control line in IRA5 and IRA1 was positive and significant. The two genetic compounds, IRA1 and IRA3, whose parent was 103, had the highest genetic progression for cocoon shell percentage. Due to the superiority of the good cocoons percentage in IRA1, this genetic combination performed well in terms of cocoon economic traits. IRA9, whose cocoon weight gain was lower than the others, not only did not show any improvement in cocoon shell weight compared to the parent during these years, but also showed the lowest amount compared to other genetic combinations.Conclusion The six new genetic compounds in this breeding program had the minimum requirements for the economically important traits of the silkworm. In other words, cocoon-related characteristics such as cocoon weight, good cocoon percentage of each family, and cocoon size are superior to all parental lines, then they could introduce to the silkworm gene bank. IRA1 was superior to parents in most of the studied traits. This genetic combination showed the best performance compared to the parent for both cocoon weight and cocoon shell percentage. Therefore it can be considered more in subsequent evaluations. IRA3 also responded relatively well. Other new genetic combinations did not have a significant advantage over their parents in terms of cocoon shell percentage. However, lower performance in some traits is not a reason for the new strain to be undesirable. Because the selection of the best paternal and maternal lines (Chinese-shaped and Japanese-shaped lines) will be determined after that the hybridization program was carried out to produce all hybrids and then the best hybrids were selected based on  estimation of specific combining ability for all important traits.