NIBB Core Research Facilities, National Institute for Basic Biology, Myodaiji, Okazaki 444-8585, Japan.
Proc Biol Sci. 2013 Jan 7;280(1750):20121952. doi: 10.1098/rspb.2012.1952.
Aphids evolved novel cells, called bacteriocytes, that differentiate specifically to harbour the obligatory mutualistic endosymbiotic bacteria Buchnera aphidicola. The genome of the host aphid Acyrthosiphon pisum contains many orphan genes that display no similarity with genes found in other sequenced organisms, prompting us to hypothesize that some of these orphan genes are related to lineage-specific traits, such as symbiosis. We conducted deep sequencing of bacteriocytes mRNA followed by whole mount in situ hybridizations of over-represented transcripts encoding aphid-specific orphan proteins. We identified a novel class of genes that encode small proteins with signal peptides, which are often cysteine-rich, that are over-represented in bacteriocytes. These genes are first expressed at a developmental time point coincident with the incorporation of symbionts strictly in the cells that contribute to the bacteriocyte and this bacteriocyte-specific expression is maintained throughout the aphid's life. The expression pattern suggests that recently evolved secretion proteins act within bacteriocytes, perhaps to mediate the symbiosis with beneficial bacterial partners, which is reminiscent of the evolution of novel cysteine-rich secreted proteins of leguminous plants that regulate nitrogen-fixing endosymbionts.
蚜虫进化出了一种新型细胞，称为菌胞，专门分化以容纳必需的互利共生内共生细菌 Buchnera aphidicola。宿主蚜虫 Acyrthosiphon pisum 的基因组包含许多孤儿基因，这些基因与在其他测序生物中发现的基因没有相似性，这促使我们假设其中一些孤儿基因与谱系特异性特征有关，例如共生。我们对菌胞 mRNA 进行了深度测序，然后对编码蚜虫特异性孤儿蛋白的过表达转录物进行了整体原位杂交。我们鉴定出了一类新的基因，这些基因编码具有信号肽的小蛋白，这些小蛋白通常富含半胱氨酸，在菌胞中过表达。这些基因最初在与共生体严格共生的细胞中表达，这些细胞有助于菌胞的形成，并且这种菌胞特异性表达在整个蚜虫的生命周期中都得到维持。这种表达模式表明，最近进化出的分泌蛋白在菌胞内发挥作用，可能是为了介导与有益的细菌伙伴的共生，这让人联想到豆科植物中新型富含半胱氨酸的分泌蛋白的进化，这些蛋白调节固氮共生体。
