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

Chun Liu, Kimiko Yamamoto, Ting Cai Cheng, Keiko Kadono-Okuda, Junko Narukawa, Shi Ping Liu, Yu Han, Ryo Futahashi, Kurako Kidokoro, Hiroaki Noda, Isao Kobayashi, Toshiki Tamura, Akio Ohnuma, Yutaka Banno, Fang Ying Dai, Zhong Huai Xiang, Marian R. Goldsmith, Kazuei Mita, Qing You Xia

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87 Citations (Scopus)


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, schl, 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 schl, 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.

Original languageEnglish
Pages (from-to)12980-12985
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number29
Publication statusPublished - Jul 20 2010

All Science Journal Classification (ASJC) codes

  • General


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