Direct cell-to-cell transmission of retrotransposons [Minion]

Transposable elements are abundant in host genomes but are generally considered to be confined to the cell in which they are expressed, with the notable exception of endogenous retroviruses. Here, we identify a group of LTR retrotransposons that infect the germline from somatic cells within the Drosophila ovary, despite lacking the fusogenic Envelope protein typically required for retroviral entry. Instead, these elements encode a short transmembrane protein, sORF2, with structural features reminiscent of viral cell-cell fusogens. Through genetics, imaging, and electron microscopy, we show that sORF2 localizes to invasive somatic protrusions, enabling the direct transfer of retrotransposon capsids into the oocyte. Remarkably, sORF2-like proteins are widespread among insect retrotransposons and also occur in piscine nackednaviruses and avian picornaviruses. These findings reveal a noncanonical, Envelope-independent transmission mechanism shared by retrotransposons and non-enveloped viruses, offering important insights into host-pathogen evolution and soma-germline interactions. Comparative poly(A)-enriched RNA-seq of ovaries from somatic piRNA pathway knockdown Drosophila flies vs. control (Tj-vretenoGD vs. Tj>arrestinGD) Comparative poly(A)-enriched RNA-seq of OSCs with Piwi knockdown vs. control

转座因子（Transposable elements）在宿主基因组中丰度颇高，但通常被认为仅局限于其表达所在的细胞内，内源性逆转录病毒是这一认知的显著例外。本研究鉴定出一类长末端重复序列反转录转座子（LTR retrotransposons），这类因子可在果蝇（Drosophila）卵巢内从体细胞侵染生殖系，尽管其缺乏逆转录病毒入侵通常所需的融合素包膜（Envelope）蛋白。取而代之的是，这类因子编码一种短小的跨膜蛋白sORF2，其结构特征与病毒细胞间融合素（viral cell-cell fusogens）高度相似。通过遗传学分析、成像技术与电子显微镜观察，我们证实sORF2定位于侵袭性体细胞突起，可介导反转录转座子衣壳直接转移至卵母细胞（oocyte）中。值得注意的是，类似sORF2的蛋白广泛存在于昆虫反转录转座子中，同时也出现在鱼类裸DNA病毒（piscine nackednaviruses）与禽小RNA病毒（avian picornaviruses）内。本研究揭示了一类非经典、不依赖包膜蛋白的传播机制，该机制为反转录转座子与非包膜病毒所共有，为宿主-病原体进化以及体细胞-生殖系互作研究提供了重要见解。本数据集包含两组比较性poly(A)富集RNA测序（poly(A)-enriched RNA-seq）数据：其一为体细胞piRNA通路敲低的果蝇卵巢与对照组（Tj-vretenoGD vs. Tj>arrestinGD）的测序数据；其二为Piwi蛋白敲低的卵巢干细胞（OSCs）与对照组的测序数据。