Welwitschia mirabilis, which is endemic to the Namib Desert, is the only living species within the family Welwitschiaceae. This species has an extremely long lifespan of up to 2,000 years and bears a single pair of opposite leaves that persist whilst alive. However, the underlying genetic mechanisms and evolution of the species remain poorly elucidated. Here, we report on a chromosome-level genome assembly for W. mirabilis, with a 6.30-Gb genome sequence and contig N50 of 27.50 Mb. In total, 39,019 protein-coding genes were predicted from the genome. Two brassinosteroid-related genes (BRI1 and CYCD3), key regulators of cell division and elongation, were strongly selected in W. mirabilis and may contribute to their long ever-growing leaves. Furthermore, 29 gene families in the mitogen-activated protein kinase signalling pathway showed significant expansion, which may contribute to the desert adaptations of the plant. Three positively selected genes (EHMT1, EIF4E, SOD2) may be involved in the mechanisms leading to long lifespan. Based on molecular clock dating and fossil calibrations, the divergence time of W. mirabilis and Gnetum montanum was estimated at ~123.5 million years ago. Reconstruction of population dynamics from genome data coincided well with the aridification of the Namib Desert. The genome sequence detailed in the current study provides insight into the evolution of W. mirabilis and should be an important resource for further study on gnetophyte and gymnosperm evolution.

中文翻译：

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千岁兰的染色体水平基因组组装，一种独特的纳米布沙漠物种

千岁兰是纳米布沙漠的特有种，是千岁兰科中唯一的现存物种。该物种的寿命极长，可达 2,000 年，并且有一对相对的叶子，在活着的时候仍然存在。然而，该物种的潜在遗传机制和进化仍然不清楚。在这里，我们报告了奇异变形杆菌的染色体水平基因组组装，具有 6.30 Gb 的基因组序列和 27.50 Mb 的重叠群 N50。从基因组中总共预测了 39,019 个蛋白质编码基因。在奇异果中强烈选择了两个油菜素类固醇相关基因（BRI1和CYCD3），它们是细胞分裂和伸长的关键调节因子并且可能有助于它们长长的叶子。此外，丝裂原活化蛋白激酶信号通路中的 29 个基因家族显示出显着扩展，这可能有助于植物的沙漠适应。三个正选择基因（EHMT1，EIF4E，SOD2）可能参与导致长寿命的机制。根据分子钟年代测定和化石校准，W. mirabilis和Gnetum montanum的分歧时间估计在大约 1.235 亿年前。从基因组数据重建种群动态与纳米布沙漠的干旱化相吻合。当前研究中详述的基因组序列提供了对进化的深入了解W. mirabilis应该是进一步研究裸子植物和裸子植物进化的重要资源。