A review on correlation, heritability and selection in silkworm breeding

Mating compatibility and offspring traits evaluation among different strains of Tenebrio molitor

The fast-growing sector of insects for food and feed stimulates researchers and the industry to explore uncharted territories, such as insect breeding, to improve economically important insect fitness traits. The yellow mealworm, Tenebrio molitor L., is one of the most thoroughly studied insect species as food and feed. However, data on mating compatibility and the effect of cross-breeding between different strains on the performance and fitness of the hybrids are scarce. In the present study, we comparatively evaluated the mating compatibility between two T. molitor inbred strains (Greek and Italian) and their outbred strains, i.e., Italian (♀)-Greek (♂) and Greek (♀)-Italian (♂), as well as the performance of their hybrid offspring. Based on the results, there is good mating compatibility among adults of the strains tested. Offspring performance, quantified as larval survival and final larval weight, followed a similar pattern among the crossings examined. Even though differences were insignificant, the outbreeding of females of the Italian strain resulted in a higher cumulative number of eggs and hatching rate and higher offspring weight. The present study sheds light on the mating compatibility of different T. molitor strains and their hybrids' economically important life table characteristics to take the mass production of insects one step further.

Effect of environmental conditions on the changes of voltinism in three lines of Bombyx mori

This work aims to estimate the voltinism differences in the studied lines and the influence of biological and environmental variables on this trait. Three Bombyx mori. lines were used. One polyvoltine (Indian, C. Nichi) and two bivoltine (Chinese6-C6 and Japanese7-J7). Silkworms were reared under controlled conditions of temperature (24±1, 26±1, 28±1°C) and photoperiod (8-16, 12-12, 16-8 light/darkness) during the life cycle. The measured biological characters were duration of larval period (DLP), larval weight on fifth day of V instar (LW(V-5)), rate of weight (RWV), duration of life cycle (DLC), fecunditiy (NE), fertility (F) and voltinism (V). Voltinism was estimated using a linear regression model; the variables that influence voltinism were found using a multiple regression analysis; and the effect of environmental conditions on change of voltinism employing an ANOVA. The results show that the variables that affect voltinism are different for each line, which demonstrates the effect of the line factor. Nichi voltinism was affected by the six measured variables, while in C6 and J7 where voltinisms depends on the duration of diapause, some variables were excluded from the model. In Indian tropical polyvoltine, the temperature was the influencing factor, while in Chinese and Japanese bivoltines lines was the photoperiod.

Genetic parameters in traits of productive importance in lines of Bombyx mori L

In the textile industry, complex cocoon traits are closely related to silk production. The aim of the present study was to estimate genetic parameters of economic importance traits-cocoon length (CL), cocoon weight (CW) and shell weight (SW)-in three B. mori lines-Chinese (C6), Japanese (J7) and Indian (C. Nichi)-reared under different temperature and photoperiod conditions. For each of these lines, data were collected from several generations with a full-sibling family structure and variance-covariance component were obtained via restricted maximum likelihood (REML) estimates based on a bi-trait animal model analysed through the multiple-trait derivative-free restricted maximum likelihood (MTDFREML) software. Genetic parameters of the traits varied between the silkworm lines were evaluated. Heritabilities were highest in J7 line (0.71, 0.89 and 0.93 for CL, CW and SW, respectively) followed by C6 (0.48, 0.54 and 0.50 for CL, CW and SW, respectively) and C. Nichi (0.36, 0.43 and 0.40 for CL, CW and SW, respectively). Phenotypic correlations among these lines were positive, with values ranging between 0.36 and 0.767. Similarly, genetic correlations between the analysed silkworm lines were observed to be positive, with high values ranging from 0.86 to 0.94. The evidence for environmental correlation in these lines was found only between CW-SW traits with moderate to high values ranging from 0.600 to 0.940. The magnitude of heritability and genetic correlations implies that phenotypic variations of the CL, CW and SW traits depend mainly on genotypic variation within the J7, C6 and C. Nichi lines, and that simultaneous genetic gains are possible by implementing selection processes for any of the evaluated traits.

Genetic and genomic analysis for cocoon yield traits in silkworm

Domestic species provides a powerful model for examining genetic mechanisms in the evolution of yield traits. The domestic silkworm (Bombyx mori) is an important livestock species in sericulture. While the mechanisms controlling cocoon yield are largely unknown. Here, using B. mori and its wild relative B. mandarina as intercross parents, 100 BC₁ individuals were sequenced by restriction site-associated DNA sequencing (RAD-Seq). The linkage map contained 9,632 markers was constructed. We performed high-resolution quantitative trait locus (QTL) mapping for four cocoon yield traits. A total of 11 QTLs were identified, including one yield-enhancing QTL from wild silkworm. By integrating population genomics and transcriptomic analysis with QTLs, some favourable genes were revealed, including 14 domestication-related genes and 71 differentially expressed genes (DEGs) in the fifth-instar larval silk gland transcriptome between B. mori and B. mandarina. The relationships between the expression of two important candidate genes (KWMTBOMO04917 and KWMTBOMO12906) and cocoon yield were supported by quantitative real-time PCR (qPCR). Our results provide some new insights into the molecular mechanisms of complex yield traits in silkworm. The combined method might be an efficient approach for identifying putative causal genes in domestic livestock and wild relatives.