Bmlark is essential for embryonic development

Transcription factor lark has been demonstrated to play multiple functions in Drosophila, but the function of this gene in embryonic development remains to be elucidated.

In this study, the CRISPR/Cas9 gene-editing method was used to construct a Bmlark mutant strain of Bombyx mori to investigate the roles of this gene. The results showed that the homozygous mutant Bmlark^-/- was lethal. The Bmlark^-/- embryos showed obvious developmental defects, such as defective sclerotization and melanization of the exoskeleton. A transcriptomic comparison of Bmlark^-/- and wild-type embryos showed that the differentially expressed genes were mainly enriched in the structure and metabolic processes of chitin and cuticles. While the expression levels of chitin metabolism-related enzyme genes did not significantly change, in the mutant embryos compared to the wild-type embryos, the expression levels of 63 putative cuticle protein genes showed significant differences. Among which, 35 genes were downregulated and 28 genes were upregulated. The expression levels of the transcription factor BmPOUM2 and eight wing disc cuticle protein genes (WCP) also changed. BmPOUM2, WCP5, WCP9, WCP10, WCP11 were downregulated and WCP1, WCP2, WCP3, WCP6 were upregulated in Bmlark^-/- embryos. While the expression level of TH in the tyrosine-mediated pigmentation pathway was upregulated in the mutant embryos, the expression levels of the four key pigment synthesis genes DDC, aaNAT, Laccase2A, and yellow-f2 were significantly downregulated.

The expression levels of 63 putative cuticle protein genes, eight WCP genes, and five pigment synthesis genes significantly changed in Bmlark mutant B. mori compared to those of the wildtype. These results suggest that Bmlark is essential for normal development of cuticle and tyrosine-mediated melanization in silkworm embryos.