Background Pleuropodia are limb-derived glandular organs that transiently appear on the first abdominal segment in embryos of insects from majority of "orders". They are missing in the genetic model Drosophila and little is known about them. Experiments carried out on orthopteran insects 80 years ago indicated that the pleuropodia secrete a "hatching enzyme" that digests the serosal cuticle to enable the larva to hatch, but evidence by state-of-the-art molecular methods is missing. Results We used high-throughput RNA-sequencing to identify the genes expressed in the pleuropodia of the locust Schistocerca gregaria (Orthoptera). First, using transmission electron microscopy we studied the development of the pleuropodia during 11 stages of the locust embryogenesis. We show that the glandular cells differentiate and start secreting just before the definitive dorsal closure of the embryo and the secretion granules outside the cells become more abundant prior to hatching. Next, we generated a comprehensive embryonic reference transcriptome for the locust and used it to study genome wide gene expression across ten morphologicaly defined stages of the pleuropodia. We show that when the pleuropodia have morphological markers of functional organs and produce secretion, they are primarily enriched in transcripts associated with transport functions. They express genes encoding enzymes capable of digesting cuticular protein and chitin. These include the potent cuticulo-lytic Chitinase 5, whose transcript rises just before hatching. Unexpected finding was the enrichment in transcripts for immunity-related enzymes. This indicates that the pleuropodia are equipped with epithelial immunity similarly as barrier epithelia in postembryonic stages. Conclusions These data provide transcriptomic support for the historic hypothesis that pleuropodia produce cuticle-degrading enzymes and function in hatching. They may also have other functions, such as facilitation of embryonic immune defense. By the genes that they express the pleuropodia are specialized embryonic organs and apparently an important though neglected part of insect physiology.

中文翻译：

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来自蝗虫胚胎的胸膜转录组支持这些器官产生使幼虫孵化的酶。

背景 胸足是肢体衍生的腺体器官，瞬时出现在大多数“目”昆虫胚胎的第一个腹部节段上。它们在果蝇的遗传模型中缺失，对它们知之甚少。80 年前对直翅目昆虫进行的实验表明，胸足类分泌一种“孵化酶”，可以消化浆膜角质层，使幼虫能够孵化，但缺少最先进的分子方法的证据。结果 我们使用高通量 RNA 测序来鉴定在蝗虫 Schistocerca gregaria (Orthoptera) 的胸膜中表达的基因。首先，我们使用透射电子显微镜研究了蝗虫胚胎发生的 11 个阶段中胸膜的发育。我们表明，腺细胞在胚胎最终背侧闭合之前分化并开始分泌，并且细胞外的分泌颗粒在孵化前变得更加丰富。接下来，我们为蝗虫生成了一个全面的胚胎参考转录组，并用它来研究胸膜足十个形态学定义阶段的全基因组基因表达。我们表明，当胸膜具有功能器官的形态标记并产生分泌物时，它们主要富含与运输功能相关的转录物。它们表达编码能够消化表皮蛋白和几丁质的酶的基因。其中包括有效的角质溶解几丁质酶 5，其转录本在孵化前上升。出乎意料的发现是免疫相关酶的转录本富集。这表明胸膜与胚胎后阶段的屏障上皮细胞一样具有上皮免疫功能。结论 这些数据为历史假设提供了转录组学支持，即胸足产生角质层降解酶并在孵化中发挥作用。它们还可能具有其他功能，例如促进胚胎免疫防御。通过它们表达胸膜的基因，它们是专门的胚胎器官，显然是昆虫生理学中一个重要但被忽视的部分。例如促进胚胎免疫防御。通过它们表达胸膜的基因，它们是专门的胚胎器官，显然是昆虫生理学中一个重要但被忽视的部分。例如促进胚胎免疫防御。通过它们表达胸膜的基因，它们是专门的胚胎器官，显然是昆虫生理学中一个重要但被忽视的部分。