Although mutualistic associations between animals and microbial symbionts are widespread in nature, the mechanisms that have promoted their evolutionary persistence remain poorly understood. A vertical mode of symbiont transmission (from parents to offspring) is thought to ensure partner fidelity and stabilization, although the efficiency of vertical transmission has rarely been investigated, especially in cases where hosts harbour a diverse microbial community. Here we evaluated vertical transmission rates of cellulolytic gut oxymonad and parabasalid protists in the wood-feeding termite Reticulitermes grassei. We sequenced amplicons of the 18S rRNA gene of protists from 24 colonies of R. grassei collected in two populations. For each colony, the protist community was characterized from the gut of 14 swarming reproductives and from a pool of 10 worker guts. A total of 98 operational taxonomic units belonging to 13 species-level taxa were found. The vertical transmission rate was estimated for each protist present in a colony based on its frequency among the reproductives. The results revealed that transmission rates were high, with an average of 0.897 (±0.164) per protist species. Overall, the protist community did not differ between reproductive sexes, suggesting that both the queen and the king could contribute to the gut microbiota of the offspring. A positive relationship between the transmission rate of protists and their prevalence within populations was also detected. However, transmission rates alone do not explain the prevalence of protists. In conclusion, these findings reveal key forces behind a conserved, multispecies mutualism, raising further questions on the roles of horizontal transfer and negative selection in shaping symbiont prevalence.

中文翻译：

---

在以木材为食的白蚁中，肠道互助生物的高效垂直传播有时不完美。

尽管动物和微生物共生体之间的相互联系在自然界很普遍，但促进它们进化持久性的机制仍然知之甚少。尽管很少研究垂直传播的效率，特别是在宿主拥有多种微生物群落的情况下，但共生传播的垂直模式（从父母到后代）可以确保伴侣的忠诚和稳定。在这里，我们评估了以草木为食的白蚁网状白蚁中纤维素分解性肠氧单胞菌和parabasalid原生生物的垂直传播速率。我们测序了从两个种群中收集的24个R. grassei菌落的原生生物的18S rRNA基因的扩增子。对于每个殖民地，这个自以为是的社区的特点是有14种成群繁殖的生殖器官和10种工人胆量。总共发现了98个可操作的生物分类单位，属于13个物种级别的分类单位。根据菌落在繁殖体中的频率，估算菌落中每个原生生物的垂直传播速率。结果表明，传播速率很高，每个原生生物平均为0.897（±0.164）。总的来说，不同性别的生殖者社区没有差异，这表明女王和国王都可以为后代的肠道菌群做出贡献。还发现了原生生物的传播率与其在人群中的流行之间存在正相关关系。但是，仅传播速率并不能解释原生生物的流行。综上所述，