Data from: Vestigial organs alter fossil placements in an ancient group of terrestrial chelicerates

Vestigial organs provide a link between ancient and modern traits, and
therefore have great potential to resolve the phylogeny of contentious
fossils that bear features not seen in extant species. Here we show that
extant daddy-longlegs (Arachnida, Opiliones), a group once thought to
possess only one pair of eyes, in fact additionally retain a pair of
vestigial median eyes and a pair of vestigial lateral eyes.
Neuroanatomical gene expression surveys of eye-patterning transcription
factors, opsins, and other structural proteins in the daddy-longlegs
Phalangium opilio show that the vestigial median and lateral eyes
innervate regions of the brain positionally homologous to the median and
lateral eye neuropils, respectively, of chelicerate groups like spiders
and horseshoe crabs. Gene silencing of eyes absent shows that the
vestigial eyes are under the control of the retinal determination gene
network. Gene silencing of dachshund disrupted the lateral eyes but not
the median eyes, paralleling loss-of-function phenotypes in insect models.
The existence of lateral eyes in extant daddy-longlegs bears upon the
placement of the oldest harvestmen fossils, a putative stem-group that
possessed both a pair of median eyes and a pair of lateral eyes.
Phylogenetic analysis of harvestman relationships with an updated
understanding of lateral eye incidence resolved the four-eyed fossil group
as a member of an extant daddy-longlegs suborder, which in turn resulted
in older estimated ages of harvestman diversification. This work
underscores that developmental vestiges in extant taxa can influence our
understanding of character evolution, placement of fossils, and inference
of divergence times.

痕迹器官（vestigial organ）可连接远古与现生性状，因此在解析那些带有现生物种所不具备特征的有争议化石的系统发育关系方面，具有巨大潜力。本研究表明，曾被认为仅具一对单眼的现生长脚盲蛛（daddy-longlegs，属于蛛形纲（Arachnida）、盲蛛目（Opiliones）），实际上还保留着一对痕迹中眼与一对痕迹侧眼。本研究对欧洲长脚盲蛛（Phalangium opilio）的眼模式形成转录因子（eye-patterning transcription factor）、视蛋白（opsin）及其他结构蛋白开展神经解剖学基因表达分析，结果显示：痕迹中眼与痕迹侧眼分别向脑部的特定区域发出神经投射，该区域在位置上与蜘蛛、鲎等螯肢动物（chelicerate）类群的中眼神经叶（neuropil）与侧眼神经叶同源。对无眼基因（eyes absent）进行基因沉默实验表明，痕迹眼受视网膜决定基因网络（retinal determination gene network）调控。对达克斯尚德基因（dachshund）进行基因沉默实验会破坏侧眼，但不会影响中眼，这与昆虫模型中的功能丧失表型一致。现生长脚盲蛛侧眼的存在，对解析最古老的盲蛛（harvestmen）化石的分类位置具有重要意义——这类被认为是基干类群的化石同时具有一对中眼与一对侧眼。结合对侧眼出现情况的最新认知，对盲蛛类群亲缘关系开展的系统发育分析，将具四眼的化石类群归为现生长脚盲蛛的一个亚目成员，这一结果也使得盲蛛类群分化时间的估算值更为古老。本研究强调，现生类群中的发育痕迹器官，能够影响我们对性状演化、化石分类位置以及分化时间推断的认知。