Opisthokonts, one of the largest eukaryotic supergroups, is divided into two major clades: the Holozoa, encompassing animals and their unicellular relatives, and the Holomycota, including fungi and their unicellular relatives. The earliest-branching lineage in Holomycota, which has received many names (e.g. Nucleariidae, Cristidiscoidia, Discicristata), remains poorly studied. This group of phagotrophic filose amoebae bears contrasting features in comparison with those of their close relatives, the parasitic Opisthosporidia and the osmotrophic fungi. It originally contained a single genus, Nuclearia, which includes naked amoebae of ca. 40 μm cell diameter that feed on filamentous cyanobacteria in freshwater environments. More recently, Fonticula alba was included as sister to Nuclearia spp. With much smaller cell size and bacterivore, this amoeba presents an aggregative multicellular fruiting body. Here we describe the new genus Parvularia nov. gen., a small filose amoeba formerly called Nuclearia sp. ATCC50694, and we compare its morphological features with those of the genera Nuclearia and Fonticula. We also review the whole nucleariid lineage at the onset of Holomycota, focusing on their diversity, ecology and evolutionary importance. SSU rRNA-based phylogenetic analyses including environmental sequences suggest that nucleariids are relatively scarce and thrive exclusively in freshwater systems. Based on existing transcriptomic data for Parvularia and novel data for a canonical Nuclearia strain, we carried out phylogenomic analyses to study the internal phylogeny of the whole group. Nucleariids occupy a key evolutionary position, such that mapping phenotypic traits on the phylogeny of Opisthokonts will help understanding important evolutionary transitions such multicellularity, parasitism or osmotrophy.

Trabajo presentado en el Moscow Forum PROTIST 2016, celebrado en Moscú del 6 al 10 de junio de 2016.

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