Department of Biology, Emory University, Atlanta, GA 30322, USA; National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA; Mutualisms Research Group, Max Planck Institute for Developmental Biology, Tübingen 72076, Germany.
Department of Entomology, Max Planck Institute for Chemical Ecology, Jena 07745, Germany.
Curr Biol. 2020 Aug 3;30(15):2875-2886.e4. doi: 10.1016/j.cub.2020.05.043. Epub 2020 Jun 4.
Numerous adaptations are gained in light of a symbiotic lifestyle. Here, we investigated the obligate partnership between tortoise leaf beetles (Chrysomelidae: Cassidinae) and their pectinolytic Stammera symbionts to detail how changes to the bacterium's streamlined metabolic range can shape the digestive physiology and ecological opportunity of its herbivorous host. Comparative genomics of 13 Stammera strains revealed high functional conservation, highlighted by the universal presence of polygalacturonase, a primary pectinase targeting nature's most abundant pectic class, homogalacturonan (HG). Despite this conservation, we unexpectedly discovered a disparate distribution for rhamnogalacturonan lyase, a secondary pectinase hydrolyzing the pectic heteropolymer, rhamnogalacturonan I (RG-I). Consistent with the annotation of rhamnogalacturonan lyase in Stammera, cassidines are able to depolymerize RG-I relative to beetles whose symbionts lack the gene. Given the omnipresence of HG and RG-I in foliage, Stammera that encode pectinases targeting both substrates allow their hosts to overcome a greater diversity of plant cell wall polysaccharides and maximize access to the nutritionally rich cytosol. Possibly facilitated by their symbionts' expanded digestive range, cassidines additionally endowed with rhamnogalacturonan lyase appear to utilize a broader diversity of angiosperms than those beetles whose symbionts solely supplement polygalacturonase. Our findings highlight how symbiont metabolic diversity, in concert with host adaptations, may serve as a potential source of evolutionary innovations for herbivorous lineages.
许多适应性是在共生生活方式的基础上获得的。在这里，我们研究了龟甲叶甲（鞘翅目：叶甲科）与其果胶分解菌 Stammera 共生体之间的强制性伙伴关系，以详细了解细菌流线型代谢范围的变化如何塑造其食草宿主的消化生理学和生态机会。对 13 株 Stammera 菌株的比较基因组学揭示了高度的功能保守性，其中普遍存在多聚半乳糖醛酸酶，这是一种针对自然界最丰富的果胶类物质同半乳糖醛酸聚糖（HG）的主要果胶酶。尽管存在这种保守性，但我们出人意料地发现了 rhamnogalacturonan 裂解酶的截然不同的分布，rhamnogalacturonan 裂解酶是一种水解果胶杂多糖 rhamnogalacturonan I（RG-I）的次要果胶酶。与缺乏该基因的共生体的甲虫相比，具有 rhamnogalacturonan 裂解酶的叶甲能够使 RG-I 解聚，这与 rhamnogalacturonan 裂解酶在 Stammera 中的注释一致。考虑到 HG 和 RG-I 在叶片中的普遍存在，编码针对这两种底物的果胶酶的 Stammera 允许其宿主克服更多种类的植物细胞壁多糖，并最大限度地获得营养丰富的细胞质。可能是由于它们的共生体扩大了消化范围，额外具有 rhamnogalacturonan 裂解酶的叶甲似乎比那些共生体仅补充多聚半乳糖醛酸酶的甲虫利用更多种类的被子植物。我们的研究结果强调了共生体代谢多样性与宿主适应性相结合，如何为食草谱系的进化创新提供潜在来源。
