Igai Katsura, Kitade Osamu, Fu Jieyang, Omata Kazumi, Yonezawa Takahiro, Ohkuma Moriya, Hongoh Yuichi
School of Life Science and Technology, Tokyo Institute of Technology, Tokyo, Japan.
College of Science, Ibaraki University, Ibaraki, Japan.
Mol Ecol. 2022 Feb;31(4):1317-1331. doi: 10.1111/mec.16309. Epub 2021 Dec 17.
The hindgut of lower termites is generally coinhabited by multiple morphologically identifiable protist species. However, it is unclear how many protist species truly coexist in this miniaturized environment, and moreover, it is difficult to define the fundamental unit of protist diversity. Species delineation of termite gut protists has therefore been guided without a theory-based concept of species. Here, we focused on the hindgut of the termite Reticulitermes speratus, where 10 or 11 morphologically distinct oxymonad cell types, that is, morphospecies, coexist. We elucidated the phylogenetic structure of all co-occurring oxymonads and addressed whether their diversity can be explained by the "ecotype" hypothesis. Oxymonad-specific 18S rRNA gene amplicon sequencing analyses of whole-gut samples, combined with single-cell 18S rRNA sequencing of the oxymonad morphospecies, identified 210 one-nucleotide-level variants. The phylogenetic analysis of these variants revealed the presence of microdiverse clusters typically within 1% sequence divergence. Each known oxymonad morphospecies comprised one to several monophyletic or paraphyletic microdiverse clusters. Using these sequence data sets, we conducted computational simulation to predict the rates of ecotype formation and periodic selection, and to demarcate putative ecotypes. Our simulations suggested that the oxymonad genetic divergence is constrained primarily by strong selection, in spite of limited population size and possible bottlenecks during intergenerational transmission. A total of 33 oxymonad ecotypes were predicted, and most of the putative ecotypes were consistently detected among different colonies and host individuals. These findings provide a possible theoretical basis for species diversity and underlying mechanisms of coexistence of termite gut protists.
低等白蚁的后肠通常聚居着多种形态可识别的原生生物物种。然而，目前尚不清楚在这个微型环境中究竟有多少原生生物物种真正共存，而且，很难界定原生生物多样性的基本单位。因此，白蚁肠道原生生物的物种划分一直缺乏基于物种理论概念的指导。在此，我们聚焦于黄胸散白蚁（Reticulitermes speratus）的后肠，其中有10种或11种形态各异的毛滴虫细胞类型（即形态种）共存。我们阐明了所有共存毛滴虫的系统发育结构，并探讨了它们的多样性是否可以用“生态型”假说来解释。对全肠样本进行毛滴虫特异性18S rRNA基因扩增子测序分析，并结合毛滴虫形态种的单细胞18S rRNA测序，鉴定出210个单核苷酸水平的变异体。对这些变异体的系统发育分析表明，通常在1%的序列差异范围内存在微观多样化的聚类。每个已知的毛滴虫形态种都包含一个到几个单系或并系的微观多样化聚类。利用这些序列数据集，我们进行了计算模拟，以预测生态型形成和周期性选择的速率，并划定假定的生态型。我们的模拟结果表明，尽管种群规模有限且在代际传递过程中可能存在瓶颈，但毛滴虫的遗传分化主要受强烈选择的限制。总共预测出33种毛滴虫生态型，并且在不同蚁群和宿主个体中大多能持续检测到假定的生态型。这些发现为白蚁肠道原生生物的物种多样性及其共存的潜在机制提供了一个可能的理论基础。
