Department of Biology, Indiana University, Bloomington, IN 47405;
INIBIOMA, Universidad Nacional del Comahue-CONICET, Bariloche, 8400, Argentina.
Proc Natl Acad Sci U S A. 2017 Nov 7;114(45):12021-12026. doi: 10.1073/pnas.1714895114. Epub 2017 Oct 24.
Complex traits like limbs, brains, or eyes form through coordinated integration of diverse cell fates across developmental space and time, yet understanding how complexity and integration emerge from uniform, undifferentiated precursor tissues remains limited. Here, we use ectopic eye formation as a paradigm to investigate the emergence and integration of novel complex structures following massive ontogenetic perturbation. We show that down-regulation via RNAi of a single head patterning gene--induces ectopic structures externally resembling compound eyes at the middorsal adult head of both basal and derived scarabaeid beetle species (Onthophagini and Oniticellini). Scanning electron microscopy documents ommatidial organization of these induced structures, while immunohistochemistry reveals the presence of rudimentary ommatidial lenses, crystalline cones, and associated neural-like tissue within them. Further, RNA-sequencing experiments show that after down-regulation, the transcriptional signature of the middorsal head-the location of ectopic eye induction-converges onto that of regular compound eyes, including up-regulation of several retina-specific genes. Finally, a light-aversion behavioral assay to assess functionality reveals that ectopic compound eyes can rescue the ability to respond to visual stimuli when wild-type eyes are surgically removed. Combined, our results show that knockdown of a single gene is sufficient for the middorsal head to acquire the competence to ectopically generate a functional compound eye-like structure. These findings highlight the buffering capacity of developmental systems, allowing massive genetic perturbations to be channeled toward orderly and functional developmental outcomes, and render ectopic eye formation a widely accessible paradigm to study the evolution of complex systems.
肢体、大脑或眼睛等复杂特征是通过在发育时空内协调整合多样化的细胞命运而形成的，但对于复杂性和整合性如何从均匀、未分化的前体细胞组织中出现，我们的理解仍然有限。在这里，我们使用异位眼形成作为范例，研究在大规模个体发生扰动后新的复杂结构的出现和整合。我们表明，通过 RNAi 下调单个头部模式形成基因会在基眼和衍生的金龟子科甲虫物种（Onthophagini 和 Oniticellini）的成年头部中诱导出外部类似于复眼的异位结构。扫描电子显微镜记录了这些诱导结构的小眼组织，而免疫组织化学显示它们内部存在原始小眼晶状体、晶锥和相关的神经样组织。此外，RNA 测序实验表明，下调后，异位眼诱导部位的中背头部的转录特征会趋同于正常复眼的特征，包括几个视网膜特异性基因的上调。最后，一项用于评估功能的避光行为测定表明，当野生型眼睛被手术切除时，异位复眼可以恢复对视觉刺激的反应能力。综上所述，我们的研究结果表明，单个基因的敲低足以使中背头部获得异位生成功能性复眼样结构的能力。这些发现突出了发育系统的缓冲能力，允许大规模的遗传扰动被引导到有序和功能性的发育结果中，并使异位眼形成成为研究复杂系统进化的广泛适用范例。
