Repression of tyrosine hydroxylase is responsible for the sex-linked chocolate mutation of the silkworm, Bombyx mori

Abstract

Pigmentation patterning has long interested biologists, integrating topics in ecology, development, genetics, and physiology. Wild-type neonatal larvae of the silkworm, Bombyx mori , are completely black. By contrast, the epidermis and head of larvae of the homozygous recessive sex-linked chocolate ( sch ) mutant are reddish brown. When incubated at 30 °C, mutants with the sch allele fail to hatch; moreover, homozygous mutants carrying the allele sch lethal ( sch l ) do not hatch even at room temperature (25 °C). By positional cloning, we narrowed a region containing sch to 239,622 bp on chromosome 1 using 4,501 backcross (BC1) individuals. Based on expression analyses, the best sch candidate gene was shown to be tyrosine hydroxylase ( BmTh ). BmTh coding sequences were identical among sch , sch l , and wild-type. However, in sch the ∼70-kb sequence was replaced with ∼4.6 kb of a Tc1-mariner type transposon located ∼6 kb upstream of BmTh , and in sch l , a large fragment of an L1Bm retrotransposon was inserted just in front of the transcription start site of BmTh . In both cases, we observed a drastic reduction of BmTh expression. Use of RNAi with BmTh prevented pigmentation and hatching, and feeding of a tyrosine hydroxylase inhibitor also suppressed larval pigmentation in the wild-type strain, pnd + and in a pS (black-striped) heterozygote. Feeding L-dopa to sch neonate larvae rescued the mutant phenotype from chocolate to black. Our results indicate the BmTh gene is responsible for the sch mutation, which plays an important role in melanin synthesis producing neonatal larval color.

Genetic Basis of Melanin Pigmentation in Butterfly Wings

“…Of these, pale, yellow, and Ddc are previously known as melanin-promoting factors. CRISPR-induced knockouts in these three genes produced melanin repression phenotypes in accordance with expectations from Drosophila (Morgan 1916;Wright et al 1976a;True et al 1999;Wittkopp et al 2002b) and other insects including P. xuthus (Futahashi and Fujiwara 2005), Manduca sexta (Gorman et al 2007), Bombyx mori (Futahashi et al 2008;Liu et al 2010), Tribolium castaneum (Gorman and Arakane 2010), and Oncopeltus fasciatus (Liu et al 2016). Our findings support the idea that pale, yellow, and Ddc play deeply conserved roles in insect melanin pigmentation.…”

Hemolymph Melanization in the Silkmoth Bombyx mori Involves Formation of a High Molecular Mass Complex That Metabolizes Tyrosine

“…The only known tyrosinase-like enzymes in B. mori plasma are PPO1/PO1 and PPO2/PO2. However, the B. mori genome does encode a Tyr hydroxylase previously implicated in cuticle pigmentation (42) and whose ortholog in M. sexta is expressed in hemocytes (43). Three lines of evidence, however, argue against Tyr hydroxylase playing a role in the rapid melanization of B. mori plasma.…”

The transcription factor Apontic-like controls diverse colouration pattern in caterpillars

“…Molecular genetic studies have identified the genes responsible for several colour pattern mutants in the silkworm [27][28][29] . Turner 30 introduced four different mutations of this silkworm, Moricaud (p M ), Zebra (Ze), Multilunar (L) and quail (q), which are good models for investigating caterpillar mimicry with various patterns.…”