Institute of Biology, Leiden University, Sylviusweg 72, 2333 BE Leiden, The Netherlands.
Proc Biol Sci. 2013 Jun 19;280(1764):20131082. doi: 10.1098/rspb.2013.1082. Print 2013 Aug 7.
Insects have been extraordinarily successful in occupying terrestrial habitats, in contrast to their mostly aquatic sister group, the crustaceans. This success is typically attributed to adult traits such as flight, whereas little attention has been paid to adaptation of the egg. An evolutionary novelty of insect eggs is the serosa, an extraembryonic membrane that enfolds the embryo and secretes a cuticle. To experimentally test the protective function of the serosa, we exploit an exceptional possibility to eliminate this membrane by zerknüllt1 RNAi in the beetle Tribolium castaneum. We analyse hatching rates of eggs under a range of humidities and find dramatically decreasing hatching rates with decreasing humidities for serosa-less eggs, but not for control eggs. Furthermore, we show serosal expression of Tc-chitin-synthase1 and demonstrate that its knock-down leads to absence of the serosal cuticle and a reduction in hatching rates at low humidities. These developmental genetic techniques in combination with ecological testing provide experimental evidence for a crucial role of the serosa in desiccation resistance. We propose that the origin of this extraembryonic membrane facilitated the spectacular radiation of insects on land, as did the origin of the amniote egg in the terrestrial invasion of vertebrates.
昆虫在占据陆地生境方面取得了非凡的成功，与它们主要在水中的姐妹群体甲壳类动物形成鲜明对比。这种成功通常归因于成虫的特征，如飞行，而对于卵的适应却很少受到关注。昆虫卵的一个进化新颖之处是浆膜，这是一种围绕胚胎并分泌角质层的胚胎外膜。为了通过在甲虫 Tribolium castaneum 中用 zerknüllt1 RNAi 消除这种膜来实验测试浆膜的保护功能，我们利用了一个特殊的可能性。我们分析了在一系列湿度下的卵的孵化率，发现对于没有浆膜的卵，孵化率随着湿度的降低而急剧下降，但对于对照卵则不然。此外，我们还显示了浆膜对 Tc-几丁质合酶 1 的表达，并证明其敲低导致浆膜角质层缺失，在低湿度下孵化率降低。这些发育遗传学技术与生态测试相结合，为浆膜在抗干燥方面的关键作用提供了实验证据。我们提出，这种额外胚胎膜的起源促进了昆虫在陆地上的惊人辐射，就像羊膜卵在脊椎动物的陆地入侵中一样。
