The Holozoa clade comprises animals and several unicellular lineages (choanoflagellates, filastereans and teretosporeans). Understanding their full diversity is essential to address the origins of animals and other evolutionary questions. However, they are poorly known. To provide more insights into the real diversity of holozoans and check for undiscovered diversity, we here analysed 18S rDNA metabarcoding data from the global Tara Oceans expedition. To overcome the low phylogenetic information contained in the metabarcoding dataset (composed of sequences from the short V9 region of the gene), we used similarity networks by combining two datasets: unknown environmental sequences from Tara Oceans and known reference sequences from GenBank. We then calculated network metrics to compare environmental to reference sequences. These metrics reflected the divergence between both types of sequences and provided an effective way to search for evolutionary relevant diversity, further validated by phylogenetic placements. Our results showed that the percentage of unicellular holozoan diversity remains hidden. We found novelties in several lineages, especially in Acanthoecida choanoflagellates. We also identified a potential new holozoan group that could not be assigned to any of the described extant clades. Data on geographical distribution showed that, although ubiquitous, each unicellular holozoan lineage exhibits a different distribution pattern. We also identified a positive association between new animal hosts and the ichthyosporean symbiont Creolimax fragrantissima, as well as for other holozoans previously reported as free-living. Overall, our analyses provide a fresh perspective into the diversity and ecology of unicellular holozoans, highlighting the amount of undescribed diversity.

中文翻译：

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基因相似性网络揭示了一个潜在的新型单细胞群体，与塔拉海洋探险队的动物密切相关。

Holozoa 进化枝由动物和几个单细胞谱系（领鞭毛动物、丝虫动物和圆孢动物）组成。了解它们的全部多样性对于解决动物起源和其他进化问题至关重要。然而，他们却鲜为人知。为了更深入地了解全动物的真实多样性并检查未发现的多样性，我们在此分析了来自全球塔拉海洋探险的 18S rDNA 元条形码数据。为了克服元条形码数据集（由基因短 V9 区域的序列组成）中包含的低系统发育信息，我们通过组合两个数据集来使用相似性网络：来自Tara Oceans 的未知环境序列和来自 GenBank 的已知参考序列。然后，我们计算网络指标以将环境序列与参考序列进行比较。这些指标反映了两种类型序列之间的差异，并提供了搜索进化相关多样性的有效方法，并通过系统发育放置进一步验证。我们的结果表明，单细胞全动物多样性的百分比仍然是隐藏的。我们在几个谱系中发现了新奇之处，特别是在棘鞭毛虫中。我们还发现了一个潜在的新全动物群，它不能归属于任何所描述的现存进化枝。地理分布数据表明，尽管普遍存在，但每个单细胞全动物谱系都表现出不同的分布模式。我们还发现新的动物宿主与鱼孢子共生体Creolimax fragrantissima以及之前报道的自由生活的其他全生动物之间存在正相关关系。总的来说，我们的分析为单细胞全动物的多样性和生态学提供了一个新的视角，强调了未描述的多样性的数量。