美丽的蝴蝶，漂亮的七星瓢虫，都是人们赞美的对象。而事实上，昆虫丰富的色彩，并不仅仅是为了“好看”。对于昆虫自身而言，体色和斑纹在躲避天敌、求偶和适应环境等方面都有重要作用。
西南大学家蚕基因组研究团队利用家蚕进行研究发现，昆虫体色的改变，有时会非常致命。该成果近日在国际著名学术期刊PNAS上在线发表家蚕刚孵化时形如蚂蚁，故称蚁蚕。正常蚁蚕体色为黑色，一种伴性赤蚁（sch，sex-linked chocolate）突变，纯合型具有巧克力色的体色。由于该基因位于家蚕Z染色体上，所以杂合型只有雌蚕变成了巧克力色，而雄蚕仍然为黑色。然而，雌蚕“美丽变身”的代价十分惨重：失去了对高温的耐受能力，遇高温则会因不能孵化而死亡。为何颜色的改变会带来如此严重的后果，其原因一直不明。
该团队刘春等通过定位克隆的方法，对该基因分离群体进行连锁分析，最后锁定了sch突变基因所在区域。经过努力，最终确定了该突变为家蚕编码酪氨酸羟化酶基因(Th，tyrosine hydroxylase)调控区域突变所致：在sch突变及等位致死突变中，两个不同类型的转座子分别通过重组替换和插入方式破坏该基因的转录调控序列，导致BmTh基因的表达量变化，降低了由酪氨酸生成多巴的反应，最终给家蚕带来致命影响。
该研究发现的酪氨酸羟化酶是位于黑色素合成途径中的第一个关键酶。黑色素与其它色素一起构成了昆虫，特别是蝴蝶丰富的斑纹和体色。刘春等人的研究还表明，家蚕Th基因突变位点所在的调控区域可能具有高温应激功能，行使高温刺激下增加基因表达量，以弥补因高温带来的酶活性损失的功能。这一结果暗示了自然界中昆虫体色斑纹受环境因素等影响的一个重要机制，并对人们深层次认识昆虫色素功能具有重要的参考价值。
由西南大学家蚕基因组团队代方银等完成的另一个家蚕色素合成相关突变，即家蚕暗化型（mln）突变基因的定位克隆研究，也于今年发表于JBC上。两篇论文都得到了国家“973”计划等项目的资助。这也是该团队在“发现基因，研究基因，利用基因”方面的又一个重要进展。(生物谷Bioon.net)
西南大学近期部分研究成果
PLoS ONE：家蚕对病原菌黑胸败血芽孢杆菌侵染的分子应答
JCB：细胞周期与原肠期胚胎模式建成之间偶联协调分子机制
Nature Biotechnology：家蚕丝腺甲基化谱完成
生物谷推荐原文出处：
PNAS doi: 10.1073/pnas.1001725107
Repression of tyrosine hydroxylase is responsible for the sex-linked chocolate mutation of the silkworm, Bombyx mori
Chun Liua, Kimiko Yamamotob, Ting-Cai Chengc, Keiko Kadono-Okudab, Junko Narukawab, Shi-Ping Liua, Yu Hana, Ryo Futahashid, Kurako Kidokorob, Hiroaki Nodab, Isao Kobayashib, Toshiki Tamurab, Akio Ohnumae, Yutaka Bannof, Fang-Ying Daia, Zhong-Huai Xianga, Marian R. Goldsmithg, Kazuei Mitab,1, and Qing-You Xiaa,c,1
aKey Sericultural Laboratory of Agricultural Ministry, Institute of Sericulture and Systems Biology, Southwest University, Chongqing 400716, China;
bNational Institute of Agrobiological Sciences, Owashi, Tsukuba, Ibaraki 305-8634, Japan;
cInstitute of Agronomy and Life Science, Chongqing University, Chongqing 400044, China;
dResearch Institute of Genome-based Biofactory, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8566, Japan;
eInstitute of Sericulture, Ami, Ibaraki 300-0324, Japan;
fInstitute of Genetic Resources, Graduate School of Bioresource and Bioenvironmental Science, Kyushu University, Fukuoka 812-8581, Japan; and
gBiological Sciences Department, University of Rhode Island, Kingston, RI 02881
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 (schl) 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.
