The key to understanding the evolutionary origin and modification of phenotypic traits is revealing the responsible underlying developmental genetic mechanisms. An important organismal trait of ray‐finned fishes is the gas bladder, an air‐filled organ that, in most fishes, functions for buoyancy control, and is homologous to the lungs of lobe‐finned fishes. The critical morphological difference between lungs and gas bladders, which otherwise share many characteristics, is the general direction of budding during development. Lungs bud ventrally and the gas bladder buds dorsally from the anterior foregut. We investigated the genetic underpinnings of this ventral‐to‐dorsal shift in budding direction by studying the expression patterns of known lung genes (Nkx2.1, Sox2, and Bmp4) during the development of lungs or gas bladder in three fishes: bichir, bowfin, and zebrafish. Nkx2.1 and Sox2 show reciprocal dorsoventral expression patterns during tetrapod lung development and are important regulators of lung budding; their expression during bichir lung development is conserved. Surprisingly, we find during gas bladder development, Nkx2.1 and Sox2 expression are inconsistent with the hypothesis that they regulate the direction of gas bladder budding. Bmp4 is expressed ventrally during lung development in bichir, akin to the pattern during mouse lung development. During gas bladder development, Bmp4 is not expressed. However, Bmp16, a paralogue of Bmp4, is expressed dorsally in the developing gas bladder of bowfin. Bmp16 is present in the known genomes of Actinopteri (ray‐finned fishes excluding bichir) but absent from mammalian genomes. We hypothesize that Bmp16 was recruited to regulate gas bladder development in the Actinopteri in place of Bmp4.

中文翻译：

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鳍鳍鱼肺-气膀胱进化过渡的Nkx2.1，Sox2，Bmp4和Bmp16表达变化

理解表型性状的进化起源和修饰的关键是揭示负责任的潜在发育遗传机制。鳍鳍鱼的重要生物特征是气囊，它是一种充满空气的器官，在大多数鱼类中，其功能是控制浮力，并且与鳍鳍鱼的肺部同源。肺和气囊之间的关键形态差异（否则具有许多特征）是发育过程中出芽的总体方向。肺从前前腹向腹侧发芽，气囊从背面向后发芽。我们通过研究已知的肺部基因（Nkx2.1，Sox2和Bmp4）的表达模式，研究了这种在发芽方向上从腹背到背侧移位的遗传基础。）在三种鱼的肺部或膀胱发育过程中：比希尔、,和斑马鱼。Nkx2.1和Sox2在四足动物肺部发育过程中显示出相互的背腹表达模式，并且是肺出芽的重要调控因子。它们在比希尔肺发育期间的表达是保守的。出乎意料的是，我们发现在膀胱发育过程中，Nkx2.1和Sox2的表达与它们调节膀胱出芽方向的假设不一致。Bmp4在比希尔的肺发育过程中在腹侧表达，类似于小鼠肺发育过程中的模式。在气囊发育过程中，不表达Bmp4。但是，Bmp16，Bmp4的旁系同源物，在弓形翅的发育中背侧表达。Bmp16存在于放线菌的已知基因组中（比比希尔除外的射线鳍鱼），而哺乳动物基因组中却没有。我们假设Bmp16被招募来调节气体膀胱发展Actinopteri到位的Bmp4。