Abstract：
Ciliated protists are among the oldest unicellular organisms with a heterotrophic lifestyle and share a common ancestor with Plantae. Unlike any other eukaryotes, there are two distinct nuclei in ciliates with separate germline and somatic cell functions. Here, we assembled a near-complete macronuclear genome of Fabrea salina, which belongs to one of the oldest clades of ciliates. Its extremely minimized genome (18.35 Mb) is the smallest among all free-living heterotrophic eukaryotes and exhibits typical streamlined genomic features, including high gene density, tiny introns, and shrinkage of gene paralogs. Gene families involved in hypersaline stress resistance, DNA replication proteins, and mitochondrial biogenesis are expanded, and the accumulation of phosphatidic acid may play an important role in resistance to high osmotic pressure. We further investigated the morphological and transcriptomic changes in the macronucleus during sexual reproduction and highlighted the potential contribution of macronuclear residuals to this process. We believe that the minimized genome generated in this study provides novel insights into the genome streamlining theory and will be an ideal model to study the evolution of eukaryotic heterotrophs.
摘要：
纤毛原生生物是具有异养生活方式的最古老的单细胞生物之一，与植物有共同的祖先。不像其他真核生物,纤毛虫有两个不同的细胞核，具有不同的种系和体细胞功能。这里,我们组装了Fabreasalina的几乎完整的大核基因组，属于最古老的纤毛虫之一。其极其最小化的基因组（18.35Mb）是所有自由生活的异养真核生物中最小的，并表现出典型的流线型基因组特征。包括高基因密度，微小的内含子，和基因旁系同源物的萎缩。参与高盐胁迫抗性的基因家族，DNA复制蛋白,线粒体生物发生扩大，磷脂酸的积累可能在抵抗高渗透压中起重要作用。我们进一步研究了有性生殖过程中大核的形态和转录组变化，并强调了大核残留物对这一过程的潜在贡献。我们相信，这项研究中产生的最小化基因组为基因组精简理论提供了新的见解，并将成为研究真核异养生物进化的理想模型。
