Watanabe Kohei, Koga Hajime, Nakamura Kodai, Fujita Akiko, Hattori Akimasa, Matsuda Masaru, Koga Akihiko
a Matsuyama High School, Higashimatsuyama 355-0018, Japan.
Genome. 2014 Apr;57(4):193-9. doi: 10.1139/gen-2014-0011. Epub 2014 May 12.
DNA-based transposable elements are ubiquitous constituents of eukaryotic genomes. Vertebrates are, however, exceptional in that most of their DNA-based elements appear to be inactivated. The Tol1 element of the medaka fish, Oryzias latipes, is one of the few elements for which copies containing an undamaged gene have been found. Spontaneous transposition of this element in somatic cells has previously been demonstrated, but there is only indirect evidence for its germline transposition. Here, we show direct evidence of spontaneous excision in the germline. Tyrosinase is the key enzyme in melanin biosynthesis. In an albino laboratory strain of medaka fish, which is homozygous for a mutant tyrosinase gene in which a Tol1 copy is inserted, we identified de novo reversion mutations related to melanin pigmentation. The gamete-based reversion rate was as high as 0.4%. The revertant fish carried the tyrosinase gene from which the Tol1 copy had been excised. We previously reported the germline transposition of Tol2, another DNA-based element that is thought to be a recent invader of the medaka fish genome. Tol1 is an ancient resident of the genome. Our results indicate that even an old element can contribute to genetic variation in the host genome as a natural mutator.
基于DNA的转座元件是真核生物基因组中普遍存在的组成部分。然而，脊椎动物是个例外，因为它们大多数基于DNA的元件似乎都已失活。青鳉（Oryzias latipes）的Tol1元件是少数几个发现含有未受损基因拷贝的元件之一。此前已经证明了该元件在体细胞中的自发转座，但只有其在生殖系中转座的间接证据。在这里，我们展示了生殖系中自发切除的直接证据。酪氨酸酶是黑色素生物合成中的关键酶。在一个青鳉白化病实验室品系中，该品系对于插入了一个Tol1拷贝的突变酪氨酸酶基因是纯合的，我们鉴定出了与黑色素沉着相关的从头回复突变。基于配子的回复率高达0.4%。回复鱼携带了已切除Tol1拷贝的酪氨酸酶基因。我们之前报道了Tol2在生殖系中的转座，Tol2是另一种基于DNA的元件，被认为是青鳉基因组的近期入侵者。Tol1是基因组的古老驻留元件。我们的结果表明，即使是一个古老的元件也可以作为一种自然诱变剂，对宿主基因组的遗传变异做出贡献。
