The genome of a daddy-long-legs (Opiliones) illuminates the evolution of arachnid appendages

Chelicerate arthropods exhibit dynamic genome evolution, with ancient whole genome duplication (WGD) events affecting several orders. Yet, genomes remain unavailable for a number of poorly studied orders, such as Opiliones (daddy-long-legs), which has hindered comparative study. We assembled the first opilionid draft genome for the species Phalangium opilio, which bears elongate, prehensile appendages, made possible by numerous distal articles called tarsomeres. Here, we show that the genome of P. opilio exhibits a single Hox cluster and no evidence of WGD. To investigate the developmental genetic basis for the quintessential trait of this group—the elongate legs—we interrogated the function of the Hox genes Deformed (Dfd) and Sex combs reduced (Scr), and a homolog of Epidermal growth factor receptor (Egfr). Knockdown of Dfd incurred homeotic transformation of two pairs of legs into pedipalps, with dramatic shortening of leg segments in the longest leg pair, whereas homeosis in L3 is on...螯肢节肢动物（Chelicerate arthropods）呈现出动态的基因组演化模式，古老的全基因组复制（whole genome duplication, WGD）事件曾波及多个演化支系。然而，包括盲蛛目（Opiliones，俗称长脚盲蛛 daddy-long-legs）在内的诸多研究薄弱的类群，目前仍缺乏可用的基因组数据，这一现状已阻碍了比较基因组学研究的推进。我们为普通长脚盲蛛（Phalangium opilio）组装了首个盲蛛目级别的草图基因组，该物种拥有细长的抓握型附肢，其结构由大量被称为跗节（tarsomeres）的远端肢节构成。本研究证实，普通长脚盲蛛的基因组仅包含单个Hox基因簇（Hox cluster），且未检测到全基因组复制的相关证据。为探究该类群标志性特征——细长附肢的发育遗传基础，我们解析了Hox基因变形（Deformed, Dfd）、性梳减少（Sex combs reduced, Scr）以及表皮生长因子受体（Epidermal growth factor receptor, Egfr）的同源基因的功能。对Dfd进行基因敲低（Knockdown）后，会引发两对附肢发生同源异型转化（homeotic transformation），使其转变为须肢（pedipalps）；其中最长的一对附肢的肢节出现显著缩短，而L3的同源异型转化则……