Nakao Hajime, Takasu Yoko
Insect Genome Research and Engineering Unit, Division of Applied Genetics, Institute of Agrobiological Sciences, National Agriculture and Food Research Organization (NARO), 1-2 Oowashi, Tsukuba, Ibaraki 305-8634, Japan.
Silk Materials Research Unit, Division of Biotechnology, Institute of Agrobiological Sciences, National Agriculture and Food Research Organization (NARO), 1-2 Oowashi, Tsukuba, Ibaraki 305-8634, Japan.
Dev Biol. 2019 Jan 1;445(1):29-36. doi: 10.1016/j.ydbio.2018.10.012. Epub 2018 Oct 24.
Inheritance (sequestration of a localized determinant: germplasm) and zygotic induction are two modes of metazoan primordial germ cell (PGC) specification. vasa and nanos homologs are evolutionarily conserved germline marker genes that have been used to examine the ontogeny of germ cells in various animals. In the lepidopteran insect Bombyx mori, although the lack of vasa homolog (BmVLG) protein localization as well as microscopic observation suggested the lack of germplasm, classical embryo manipulation studies and the localization pattern of Bm-nosO (one of the four nanos genes in Bombyx) maternal mRNA in the egg raised the possibility that an inheritance mode is operating in Bombyx. Here, we generated Bm-nosO knockouts to examine whether the localized mRNA acts as a localized germ cell determinant. Contrary to our expectations, Bm-nosO knockout lines could be established. However, these lines frequently produced abnormal eggs, which failed to hatch, to various extent depending on the individuals. We also found that Bm-nosO positively regulated BmVLG expression at least during embryonic stage, directly or indirectly, indicating that these genes were on the same developmental pathway for germ cell formation in Bombyx. These results suggest that these conserved genes are concerned with stable germ cell production. On the other hand, from the aspect of BmVLG as a PGC marker, we showed that maternal Bm-nosO product(s) as well as early zygotic Bm-nosO activity were redundantly involved in PGC specification; elimination of both maternal and zygotic gene activities (as in knockout lines) resulted in the apparent lack of PGCs, indicating that an inheritance mechanism indeed operates in Bombyx. This, however, together with the fact that germ cells are produced at all in Bm-nosO knockout lines, also suggests the possibility that, in Bombyx, not only this inheritance mechanism but also an inductive mechanism acts in concert to form germ cells or that loss of early PGCs are compensated for by germline regeneration: mechanisms that could enable the evolution of preformation. Thus, Bombyx could serve as an important organism in understanding the evolution of germ cell formation mechanisms; transition between preformation and inductive modes.
遗传（局部决定因素的隔离：种质）和合子诱导是后生动物原始生殖细胞（PGC）特化的两种模式。vasa和nanos同源物是进化上保守的生殖系标记基因，已被用于研究各种动物中生殖细胞的个体发育。在鳞翅目昆虫家蚕中，尽管缺乏vasa同源物（BmVLG）蛋白定位以及显微镜观察表明缺乏种质，但经典的胚胎操作研究以及家蚕中四个nanos基因之一的Bm-nosO母体mRNA的定位模式增加了家蚕中存在遗传模式的可能性。在这里，我们生成了Bm-nosO敲除品系，以研究定位的mRNA是否作为定位的生殖细胞决定因素。与我们的预期相反，Bm-nosO敲除品系可以建立。然而，这些品系经常产生异常卵，这些卵未能孵化，不同个体的异常程度有所不同。我们还发现，Bm-nosO至少在胚胎期直接或间接正向调节BmVLG表达，表明这些基因在家蚕生殖细胞形成的同一发育途径上。这些结果表明，这些保守基因与稳定的生殖细胞产生有关。另一方面，从BmVLG作为PGC标记的角度来看，我们表明母体Bm-nosO产物以及早期合子Bm-nosO活性冗余地参与了PGC特化；消除母体和合子基因活性（如在敲除品系中）导致明显缺乏PGC，表明家蚕中确实存在遗传机制。然而，这与Bm-nosO敲除品系中仍能产生生殖细胞这一事实一起，也表明在家蚕中，不仅这种遗传机制，而且诱导机制也协同作用形成生殖细胞，或者早期PGC的损失通过生殖系再生得到补偿：这些机制可能使预成论得以进化。因此，家蚕可以作为理解生殖细胞形成机制进化的重要生物体；预成论和诱导模式之间的转变。
