Abstract
Opisthokonta represents a major lineage of eukaryotes and includes fungi and metazoans, as well as other less known unicellular groups. The latter are paraphyletic assemblages that branch in between the former two groups, and thus are important for understanding the origin and early diversification of opisthokonts. The full range of their diversity, however, has not yet been explored from diverse ecological habitats. Freshwater environments are crucial sources for new diversity; they are considered even more heterogeneous than marine ecosystems. This heterogeneity implies more ecological niches where local eukaryotic communities are located. However, knowledge of the unicellular opisthokont diversity is scarce from freshwater environments. Here, we performed an 18S rDNA metabarcoding study in the Middle Parana River, Argentina, to characterize the molecular diversity of microbial eukaryotes, in particular unicellular members of Opisthokonta. We identified a potential novel clade branching as a sister-group to Fungi. We also detected in our data that more than 60% operational taxonomic units classified as unicellular holozoans (animals and relatives) represent new taxa at the species level. Of the remaining, the majority was assigned to the newly described holozoan species, Syssomonas multiformis. Together, our results show that a large hidden diversity of unicellular members of opisthokonts still remain to be uncovered. We also found that the geographical and ecological distribution of several taxa considered exclusive to marine environments is wider than previously thought.
Highlights
Understanding eukaryotic diversity is an essential need for humankind
Our analyses showed that Amoebozoa stands out in terms of potential novelty; with around half of its diversity having less than 90% of blast identity against our reference database (Figure 2 and Supplementary Table)
We decided to sequence both V4 and V8-9 regions of the 18S rRNA gene. Both V4 and V9 are considered suitable regions for biodiversity studies given their great variability and the combination of high and low entropy in these sections, which allows forgood primer binding (Behnke et al, 2011; Hadziavdic et al, 2014; Tanabe et al, 2016). They have been widely used in molecular diversity studies, so the research community knows the advantages and limitations of both regions (AmaralZettler et al, 2009; Stoeck et al, 2010; Massana et al, 2015)
Summary
Understanding eukaryotic diversity is an essential need for humankind. It does provide more knowledge about evolutionary processes, but it helps to fully describe complex ecological networks, which can lead to better conservation policies and new sources of food and medicines (Pawlowski et al, 2016). Animals, fungi, and plants have stood up, they account for the vast majority of described species (Pawlowski et al, 2012; del Campo et al, 2014). Molecular surveys of biodiversity have redrawn a completely different paradigm, in which unicellular eukaryotes represent the majority of the total eukaryotic richness, while remaining mostly undescribed (Pawlowski et al, 2012; del Campo et al, 2014). The complete view of the molecular eukaryotic diversity remains unsolved
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