Ward Chris M, Onetto Cristobal A, Borneman Anthony R
Australian Wine Research Institute, Glen Osmond, Australia.
School of Agriculture, Food and Wine, University of Adelaide, Adelaide, Australia.
Genome Biol Evol. 2024 Dec 4;16(12). doi: 10.1093/gbe/evae251.
Fungi have been found to be associated with many insect species, with some species transitioning to reside within insects as symbionts. However, the evolutionary pressures and genomic consequences associated with this transition are not well understood. Pathogenic fungi of the genus Ophiocordyceps have undergone multiple, independent transitions from pathogen to endosymbiont lifestyles, where they reside within the fatty tissues of infected soft-scale insects transgenerationally without killing their hosts. To gain an understanding of the genomic adaptations underlying this life history shift, long-read sequencing was utilized to assemble the genomes of both the soft-scale insect Parthenolecanium corni and its Ophiocordyceps endosymbiont from a single insect. Assembly and metagenomic-based binning produced a highly contiguous genome for Pa. corni and a chromosome-level assembly for the Ophiocordyceps endosymbiont. The endosymbiont genome was characterized by 524 gene loss events compared to free-living pathogenic Ophiocordyceps relatives, with predicted roles in hyphal growth, cell wall integrity, metabolism, gene regulation, and toxin production. Contrasting patterns of selection were observed between the nuclear and mitochondrial genomes specific to the endosymbiont lineage. Intensified selection was most frequently observed across orthologs in the nuclear genome, whereas selection on most mitochondrial genes was found to be relaxed. Scans for positive selection were enriched within the fatty acid metabolism pathway with endosymbiont specific selection within three adjacent enzymes catalyzing the conversion of acetoacetate to acetyl-coenzyme A, suggesting that the endosymbiont lineage is under selective pressure to effectively exploit the lipid rich environment of the insect fat bodies in which it is found.
人们发现真菌与许多昆虫物种有关联，其中一些物种转变为以共生体的形式寄生于昆虫体内。然而，与这种转变相关的进化压力和基因组后果尚未得到充分理解。虫草属的致病真菌已经历了多次从病原体到内共生体生活方式的独立转变，它们在受感染的软蚧昆虫的脂肪组织中世代生存，而不杀死宿主。为了了解这种生活史转变背后的基因组适应性，利用长读长测序技术从一只昆虫中组装了软蚧昆虫角蜡蚧扁角跳小蜂及其虫草内共生体的基因组。组装和基于宏基因组的分箱为角蜡蚧扁角跳小蜂生成了高度连续的基因组，并为虫草内共生体生成了染色体水平的组装。与自由生活的致病虫草属亲缘种相比，内共生体基因组具有524个基因丢失事件，这些基因在菌丝生长、细胞壁完整性、代谢、基因调控和毒素产生方面具有预测作用。在内共生体谱系特有的核基因组和线粒体基因组之间观察到了不同的选择模式。在核基因组的直系同源基因中最常观察到强化选择，而在大多数线粒体基因上的选择则较为宽松。对正向选择的扫描在脂肪酸代谢途径中富集，在内共生体中，在催化乙酰乙酸转化为乙酰辅酶A的三种相邻酶中存在特异性选择，这表明内共生体谱系面临着有效利用其所处昆虫脂肪体富含脂质环境的选择压力。
