Kerner Pierre, Ikmi Aissam, Coen Dario, Vervoort Michel
Evolution et Développement des métazoaires, Centre de Génétique Moléculaire - FRE 3144 CNRS, 1, av, de la terrasse, 91198 Gif-sur-Yvette, France.
BMC Evol Biol. 2009 Apr 15;9:74. doi: 10.1186/1471-2148-9-74.
The iroquois (iro/Irx) genes encode transcriptional regulators that belong to the TALE superclass of homeodomain proteins and have key functions during development in both vertebrates and insects. The Irx genes occur in one or two genomic clusters containing three genes each within the Drosophila and several vertebrate genomes, respectively. The similar genomic organization in Drosophila and vertebrates is widely considered as a result of convergent evolution, due to independent tandem gene duplications. In this study, we investigate the evolutionary history of the Irx genes at the scale of the whole metazoan kingdom.
We identified in silico the putative full complement of Irx genes in the sequenced genomes of 36 different species representative of the main metazoan lineages, including non bilaterian species, several arthropods, non vertebrate chordates, and a basal vertebrate, the sea lamprey. We performed extensive phylogenetic analyses of the identified Irx genes and defined their genomic organizations. We found that, in most species, there are several Irx genes, these genes form two to four gene clusters, and the Irx genes are physically linked to a structurally and functionally unrelated gene known as CG10632 in Drosophila.
Three main conclusions can be drawn from our study. First, an Irx cluster composed of two genes, araucan/caupolican and mirror, is ancestral to the crustaceans+insects clade and has been strongly conserved in this clade. Second, three Irx genes were probably present in the last common ancestor of vertebrates and the duplication that has given rise to the six genes organized into two clusters found in most vertebrates, likely occurred in the gnathostome lineage after its separation from sea lampreys. Third, the clustered organization of the Irx genes in various evolutionary lineages may represent an exceptional case of convergent evolution or may point to the existence of an Irx gene cluster ancestral to bilaterians.
易洛魁（iro/Irx）基因编码转录调节因子，属于同源域蛋白的TALE超类，在脊椎动物和昆虫的发育过程中具有关键功能。Irx基因分别存在于果蝇和几个脊椎动物基因组中的一个或两个基因组簇中，每个簇包含三个基因。果蝇和脊椎动物中相似的基因组组织被广泛认为是趋同进化的结果，这是由于独立的串联基因复制。在本研究中，我们在整个后生动物界的尺度上研究了Irx基因的进化历史。
我们通过计算机分析在36个代表后生动物主要谱系的不同物种的测序基因组中鉴定出了推定的完整Irx基因互补序列，包括非两侧对称动物物种、几种节肢动物、非脊椎脊索动物和一种基础脊椎动物——海七鳃鳗。我们对鉴定出的Irx基因进行了广泛的系统发育分析，并确定了它们的基因组组织。我们发现，在大多数物种中，有几个Irx基因，这些基因形成两到四个基因簇，并且Irx基因在物理上与果蝇中一个名为CG10632的结构和功能无关的基因相连。
我们的研究可以得出三个主要结论。第一，由araucan/caupolican和mirror两个基因组成的Irx簇是甲壳类动物+昆虫类分支的祖先，并且在该分支中得到了强烈的保守。第二，三个Irx基因可能存在于脊椎动物的最后一个共同祖先中，导致大多数脊椎动物中六个基因组织成两个簇的复制，可能发生在颌口动物谱系与海七鳃鳗分离之后。第三，Irx基因在各种进化谱系中的成簇组织可能代表了趋同进化的一个特殊情况，或者可能表明存在两侧对称动物祖先的Irx基因簇。
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