Repression of tyrosine hydroxylase is responsible for the sex-linked chocolate mutation of the silkworm, Bombyx mori.
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 degrees 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 degrees 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 approximately 70-kb sequence was replaced with approximately 4.6 kb of a Tc1-mariner type transposon located approximately 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.
Liu
Chun
C
Key Sericultural Laboratory of Agricultural Ministry, Institute of Sericulture and Systems Biology, Southwest University, Chongqing 400716, China.
Yamamoto
Kimiko
K
Cheng
Ting-Cai
TC
Kadono-Okuda
Keiko
K
Narukawa
Junko
J
Liu
Shi-Ping
SP
Han
Yu
Y
Futahashi
Ryo
R
Kidokoro
Kurako
K
Noda
Hiroaki
H
Kobayashi
Isao
I
Tamura
Toshiki
T
Ohnuma
Akio
A
Banno
Yutaka
Y
Dai
Fang-Ying
FY
Xiang
Zhong-Huai
ZH
Goldsmith
Marian R
MR
Mita
Kazuei
K
Xia
Qing-You
QY
D000818:Animals; D012831:Bombyx / Q000201:enzymology* / Q000235:genetics*; D002874:Chromosome Mapping; D017344:Genes, Insect / Q000235:genetics*; D008040:Genetic Linkage; D016678:Genome / Q000235:genetics; D007814:Larva; D009154:Mutation / Q000235:genetics*; D010641:Phenotype; D010858:Pigmentation / Q000235:genetics*; D015203:Reproducibility of Results; D012727:Sex Characteristics; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism*
D014446:Tyrosine 3-Monooxygenase