The dynamic behavior of chromatophores marks the transition from bands to spots in leopard geckos

Reptilian skin coloration is spectacular and diverse, yet little is known about the ontogenetic processes that govern its establishment and the molecular signaling pathways that determine it. Here, we focus on the development of the banded pattern of leopard gecko hatchlings and the transition to black spots in the adult. With our histological analyses, we show that iridophores are present in the white and yellow bands of the hatchling and they gradually perish in the adult skin. Furthermore, we demonstrate that melanophores can autonomously form spots in the absence of the other chromatophores both on the regenerated skin of the tail and on the dorsal skin of the Mack Super Snow (MSS) leopard geckos. This color morph is characterized by uniform black coloration in hatchlings and black spots in adulthood; we establish that their skin is devoid of xanthophores and iridophores at both stages. Our genetic analyses identified a 13-nucleotide deletion in the PAX7 transcription factor of MSS geckos, affecting its protein coding sequence. With our single-cell transcriptomics analysis of embryonic skin, we confirm that PAX7 is expressed in iridophores and xanthophores, suggesting that it plays a key role in the differentiation of both chromatophores. Our in situ hybridizations on whole-mount embryos document the dynamics of the skin pattern formation and how it is impacted in the PAX7 mutants. We hypothesize that the melanophores–iridophores interactions give rise to the banded pattern of the hatchlings and black spot formation is an intrinsic capacity of melanophores in the postembryonic skin. Overall design: single cell RNA-seq of a E35 wild-type leopard gecko embryo dorsal skin sample.

爬行动物的皮肤着色绚烂多样，但人们对调控其形成的个体发育过程（ontogenetic processes）以及决定其形成的分子信号通路（molecular signaling pathways）仍知之甚少。本研究以豹纹守宫（leopard gecko）幼体的条纹图案发育，以及其成体阶段向黑斑的转变过程为研究对象。通过组织学分析（histological analyses），我们发现幼体的白色与黄色条纹中存在虹彩细胞（iridophores），且这类细胞会在成体皮肤中逐渐消亡。此外，我们证实，在马克超级雪（Mack Super Snow, MSS）豹纹守宫的尾部再生皮肤与背部皮肤中，即便缺乏其他色素细胞（chromatophores），黑素细胞（melanophores）仍可自主形成斑点。该体色形态型（color morph）的特征为：幼体阶段通体乌黑，成体阶段则长出黑斑；我们确认，在这两个阶段，其皮肤均不含黄色素细胞（xanthophores）与虹彩细胞（iridophores）。我们的遗传分析（genetic analyses）发现，MSS豹纹守宫的PAX7转录因子（PAX7 transcription factor）中存在一处13核苷酸缺失，该变异会影响其蛋白质编码序列（protein coding sequence）。通过对胚胎皮肤（embryonic skin）开展单细胞转录组学（single-cell transcriptomics）分析，我们证实PAX7在虹彩细胞与黄色素细胞中均有表达，这表明PAX7在这两类色素细胞的分化过程中发挥关键作用。我们对整体胚胎（whole-mount embryos）开展原位杂交（in situ hybridizations）实验，记录了皮肤图案形成的动态变化，以及PAX7突变体中该过程受到的影响。我们提出假说：幼体的条纹图案由黑素细胞与虹彩细胞的互作所调控，而成体黑斑的形成则是黑素细胞在胚后皮肤中具备的内在能力。实验设计概况：对E35期野生型豹纹守宫胚胎的背部皮肤样本开展单细胞RNA测序（single cell RNA-seq）。