Allele-specific knockouts reveal a role for apontic-like in the evolutionary loss of larval melanin pigmentation in the domesticated silkworm, Bombyx mori

Disruption of a BTB-ZF transcription factor causes female sterility and melanization in the larval body of the silkworm, Bombyx mori

The dilute black (bd) of the silkworm Bombyx mori is a recessive mutant that produces a grayish-black color in the larval integument, instead of the characteristic white color found in wild-type larvae. In addition, eggs produced by bd females are sterile due to a deficiency in the micropylar apparatus. We identified candidate genes responsible for the bd phenotype using publicly available RNA-seq data. One of these candidate genes was homologous to the maternal gene required for meiosis (mamo) of Drosophila melanogaster, which encodes a broad-complex, tramtrack, and bric-à-brac-zinc finger (BTB-ZF) transcription factor essential for female fertility. In three independent bd strains, the expression of the B. mori mamo (Bmmamo) was downregulated in the larval integument. Using a CRISPR/Cas9-mediated knockout strategy, we found that Bmmamo knockout mutants exhibit a grayish-black color in the larval integument and female infertility. Moreover, larvae obtained from the complementation cross between bd/+ mutants and heterozygous knockouts for the Bmmamo also exhibited a grayish-black color, indicating that Bmmamo is responsible for the bd phenotype. Gene expression analysis using Bmmamo knockout mutants suggested that the BmMamo protein suppresses the expression of melanin synthesis genes. Previous comparative genome analysis revealed that the Bmmamo was selected during silkworm domestication, and we found that Bmmamo expression in the larval integument is higher in B. mori than in the wild silkworm B. mandarina, suggesting that the Bmmamo is involved in domestication-associated pigmentation changes of the silkworm.

Recessive embryonic lethal mutations uncovered in heterozygous condition in silkworm semiconsomic strains

In this study, we found two embryonic lethal mutations, t04 lethal (l-t04) and m04 lethal (l-m04), in semiconsomic strains T04 and M04, respectively. In these semiconsomic strains, the entire diploid genome, except for one chromosome 4 of the wild silkworm Bombyx mandarina, is substituted with chromosomes of the domesticated silkworm B. mori, and l-t04 and l-m04 mutations are located on B. mandarina-derived chromosome 4. To clarify the cause of the lethalities and the genes responsible for these mutations, positional cloning and CRISPR/Cas9 mediated knockout screening were performed. Finally, genetic complementation tests identified the mutations responsible for the l-t04 and l-m04 as the Bombyx homolog of imaginal discs arrested (Bmida) and TATA box binding protein-associated factor 5 (BmTaf5), respectively. Lethal stages of each knockout mutant indicated the importance of these genes in B. mori late embryogenesis. The lethal mutations responsible for l-t04 and l-m04 were not found in parental strains or wild B. mandarina collected from 39 distinct locations in Japan, indicating that both mutations were independently introduced during or after the development of the semiconsomic strains. We conclude that the recessive embryonic lethality in the T04 and M04 strains is due to deleterious mutations produced in B. mandarina-derived chromosome 4.

Sericin Ser3 Ectopic Expressed in Posterior Silk Gland Affects Hemolymph Immune Melanization Response via Reducing Melanin Synthesis in Silkworm

The transgenesis of silkworms is an important way to innovate genetic resources and silk function. However, the silk-gland (SG) of transgenic silkworms, which is the most concerned target tissue of sericulture, often suffers from low vitality, stunting and other problems, and the reasons are still unknown. This study trans engineered recombinant Ser3, a middle silk gland (MSG) specific expression gene, in the posterior silk gland (PSG) of the silkworm, and studied hemolymph immune melanization response changes in mutant pure line SER (Ser3^+/+). The results showed that although the mutant had normal vitality, the melanin content and phenoloxidase (PO) activity in hemolymph related to humoral immunity were significantly reduced, and caused significantly slower blood melanization and weaker sterilization ability. The mechanism investigation showed that the mRNA levels and enzymatic activities of phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH) and dopamine decarboxylase (DDC) in the melanin synthesis pathway in mutant hemolymph, as well as the transcription levels of the PPAE, SP21 and serpins genes in the serine protease cascade were significantly affected. Moreover, the total antioxidant capacity, superoxide anion inhibition capacity and catalase (CAT) level related to the redox metabolic capacity of hemolymph were significantly increased, while the activities of superoxide dismutase (SOD) and glutathione reductase (GR), as well as the levels of hydrogen peroxide (H₂O₂) and glutathione (GSH), were significantly decreased. In conclusion, the anabolism of melanin in the hemolymph of PSG transgenic silkworm SER was inhibited, while the basic response level of oxidative stress was increased, and the hemolymph immune melanization response was decreased. The results will significantly improve the safe assessment and development of genetically modified organisms.