202309Kina

研究発表を行った学会；第56回日本発生生物学会
2023年 7月22日〜25日　（仙台）
タイトル； Distinct thresholds of piRNA abundance required for germ cell formation and transposon silencing in Drosophila oogenesis.
発表者；喜納　寛野氏
（熊本大学　発生医学研究所　生殖発生分野）
要旨；
In many animals, germ cell formation relies on the germ plasm, which contains maternal RNAs and proteins required for germline development. During Drosophila oogenesis, the germ plasm is assembled at the posterior pole of the oocyte. Many germ plasm proteins including Aubergine (Aub) are also assembled around nurse cell nuclei in the ovary, called nuage. Nuage is essential for producing PIWI-interacting RNAs (piRNAs) and silencing their target transposons. Of note, Aub is thought to carry piRNAs to the germ plasm and inherit piRNA-encoded information of invaders to the germ cells in offspring. The mechanism of how functions and assemblies of the nuage and germ plasm are coordinated remains elusive.
We have identified a new maternal factor involved in the germ plasm assembly, named tiny pole plasm (tpp), which encodes an evolutionarily conserved protein in bilaterians. Embryos from tpp- females formed fewer germ cells. In tpp- oocyte, the posterior localization of Aub was severely reduced, which appeared to be the primary cause of pole cell-less phenotype. Tpp colocalized with Aub in the nuage, but not in the germ plasm, suggesting that their interaction in the nuage is crucial for the localization of Aub to the germ plasm. During oogenesis, the loss of Tpp decreased Aub-bound piRNAs, although transposon was almost properly silenced. Therefore, the reduction of Aub localization to the germ plasm is likely to be caused by the decreased piRNA loading to Aub. Consistent with this idea, mutant Aub in the piRNA binding site failed to localize to the germ plasm. Interestingly, we provide evidence that the threshold of piRNA abundance required for germ cell formation was much higher than that for transposon silencing. We propose that a physiological reason why a huge amount of piRNA has to be produced in oogenesis is to carry enough amount of piRNAs to the next generation.