INRA, UMR 1333 Laboratoire Diversité, Génomes & Interactions Microorganismes - Insectes (DGIMI), CC54, 2 place E. Bataillon, 34095 Montpellier, France.
Insect Biochem Mol Biol. 2013 Jan;43(1):54-64. doi: 10.1016/j.ibmb.2012.10.012. Epub 2012 Nov 9.
The proteins of the X-tox family have imperfectly conserved tandem repeats of several defensin-like motifs known as cysteine-stabilized αβ (CS-αβ) motifs. These immune-related proteins are inducible and expressed principally in hemocytes, but they have lost the antimicrobial properties of the ancestral defensins from which they evolved. We compared x-tox gene structure and expression in three lepidopteran species (Spodoptera frugiperda, Helicoverpa armigera and Bombyx mori). Synteny and phylogenetic analyses showed that the x-tox exons encoding CS-αβ motifs were phylogenetically closely related to defensin genes mapping to chromosomal positions close to the x-tox genes. We were able to define two groups of paralogous x-tox exons (three in Noctuids) that each followed the expected species tree. These results suggest that the ancestor of the three species already possessed an x-tox gene with at least two proto-domains, and an additional duplication/fusion should have occurred in the ancestor of the two noctuid species. An expansion of the number of exons subsequently occurred in each lineage. Alternatively, the proto x-tox gene possessed more copy and each group of x-tox domains might undergo concerted evolution through gene conversion. Accelerated protein evolution was detected in x-tox domains when compared to related defensins, concomitantly to multiplication of exons and/or the possible activation of concerted evolution. The x-tox genes of the three species have similar structural organizations, with repeat motifs composed of CS-αβ-encoding exons flanked by introns in phase 1. Diverse mechanisms underlie this organization: (i) the acquisition of new repeat motifs, (ii) the duplication of preexisting repeat motifs and (iii) the duplication of modules. A comparison of gDNA and cDNA structures showed that alternative splicing results in the production of multiple X-tox protein isoforms from the x-tox genes. Differences in the number and sequence of CS-αβ motifs in these isoforms were found between species, but also between individuals of the same species. Thus, our analysis of the genetic organization and expression of x-tox genes in three lepidopteran species suggests a rapid evolution of the organization of these genes.
X 毒素家族的蛋白具有不完美保守的串联重复序列，这些重复序列由几个防御素样基序组成，称为半胱氨酸稳定的 αβ（CS-αβ）基序。这些免疫相关蛋白是可诱导的，主要在血细胞中表达，但它们已经失去了祖先防御素的抗菌特性，而这些防御素是它们进化而来的。我们比较了三种鳞翅目昆虫（草地贪夜蛾、棉铃虫和家蚕）的 x-tox 基因结构和表达。基因同线性和系统发育分析表明，编码 CS-αβ基序的 x-tox 外显子在系统发育上与定位在接近 x-tox 基因的染色体位置的防御素基因密切相关。我们能够定义两组同源 x-tox 外显子（夜蛾中的三个），它们各自遵循预期的种系发生树。这些结果表明，这三个物种的祖先已经拥有了至少两个原型结构域的 x-tox 基因，并且在两个夜蛾物种的祖先中还发生了额外的复制/融合。随后，每个谱系中的外显子数量都增加了。或者，原始的 x-tox 基因具有更多的拷贝，并且每组 x-tox 结构域可能通过基因转换协同进化。与相关防御素相比，在 x-tox 结构域中检测到蛋白质进化加速，同时伴随着外显子的倍增和/或协同进化的可能激活。这三个物种的 x-tox 基因具有相似的结构组织，重复基序由编码 CS-αβ的外显子组成，侧翼是 1 相的内含子。这种组织背后有多种机制：（i）获得新的重复基序，（ii）复制现有重复基序，（iii）复制模块。对 gDNA 和 cDNA 结构的比较表明，可变剪接导致 x-tox 基因产生多种 X-tox 蛋白同工型。在这些同工型中，不同物种之间以及同一物种的个体之间发现了 CS-αβ 基序的数量和序列的差异。因此，我们对三种鳞翅目昆虫的 x-tox 基因的遗传组织和表达进行了分析，表明这些基因的组织迅速进化。
