Mutation Breeding in Bombyx mori: Current Trends and Future Avenues

Genetic improvement of silkworm has been a prime focus since decades through conventional and molecular approaches, and heritable variation is an important prerequisite. Silkworm breeding for crop enhancement necessitates genetic variation in beneficial properties. In order to create new impulses in silkworm breeding, mutation breeding is rather effective in generating genetic variation targeting the desired trait. Owing to the availability of more than 400 visible mutations, the silkworm, Bombyx mori, is the model insect for genetic studies next only to Drosophila. Of which, 200 visible mutations have been well established to conventional linkage groups of B. mori spanning 900.2 CM. These mutations have been reported to affect various basic characteristics of silkworm that comprises different qualitative and quantitative traits, viz., larval weight, fecundity, disease tolerance/resistance, and thermotolerance, which are responsible for development and distinction of silkworm races and inbred lines. Mutation breeding estimates higher genetic variability which could be sparse through the conventional methods in achieving the targeted traits within short breeding time. In silkworm, it has been observed that the naturally occurred mutagenesis is not successful in generating commercially viable breeding lines. However, it could be successful through artificially induced mutagenesis by employing chemical mutagens and radiation which could be quick enough to develop new alleles and phenotypes. Through induction of X-ray irradiation, sex-limited breeds had been developed by researchers which gained commercial importance before its downfall which will be discussed in detail in this chapter. In recent years, silkworm mutants had been developed by utilizing various genome editing tools like zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly interspaced palindromic repeats/CRISPR-associated (CRISPR/Cas) system by enabling efficient gene knockouts. In this chapter, the silkworm mutants developed through different mutagens and gene editing tools will be reviewed, and the future prospects of mutation breeding in sericulture improvement will be discussed.