Apusomonads are sediment-dwelling bacterivorous protists that are sister to all Opisthokonta. They have been found to show a negative phototactic r...

Apusomonads are sediment-dwelling bacterivorous protists that are sister to all Opisthokonta. They have been found to show a negative phototactic r...

Apusomonads are a clade of understudied sediment-dwelling bacterivorous protists sister to Opisthokonta. Recently, apusomonads have been found to show a negative phototactic response to blue light. He...

A light-avoidance response is described in apusomonads, key organisms for understanding the evolutionary origins of opisthokonts. This response involves Ca²⁺-dependent dynamic changes in both cell contraction and flagellar beating.

Eukaryotes radiated from their last common ancestor, diversifying into several supergroups with unresolved deep evolutionary connections. Heterotrophic flagellates, often branching deeply in phylogenetic trees, are arguably the most diverse eukaryotes. However, many of them remain undersampled and/or incertae sedis . Here, we conducted comprehensive phylogenomics analyses with an expanded taxon sampling of early-branching protists including 22 newly sequenced transcriptomes (apusomonads, ancyromonads, Meteora ). They support the monophyly of Opimoda, one of the largest eukaryotic supergroups, with CRuMs being sister to the Amorphea (amoebozoans, breviates, apusomonads, and opisthokonts – including animals and fungi–), and the ancyromonads+malawimonads clade. By mapping traits onto this phylogenetic framework, we infer a biflagellate opimodan ancestor with an excavate-like feeding groove. Breviates and apusomonads retained the ancestral biflagellate state. Other Amorphea lost one or both flagella, enabling the evolution of amoeboid shapes, novel feeding modes, and palintomic cell division resulting in multinucleated cells, which likely facilitated the subsequent evolution of fungal and metazoan multicellularity. ### Competing Interest Statement The authors have declared no competing interest.

The sodium leak channel NALCN, a key regulator of neuronal excitability, associates with three ancillary subunits that are critical for its function: an extracellular subunit called FAM155, and two cytoplasmic subunits called UNC79 and UNC80. Interestingly, NALCN and FAM155 have orthologous phylogenetic relationships with the fungal calcium channel Cch1 and its extracellular subunit Mid1, however, UNC79 and UNC80 have not been reported outside of animals. In this study, we leveraged expanded gene sequence data available for eukaryotes to re-examine the evolutionary origins of NALCN and Cch1 channel subunits. Our analysis corroborates the direct phylogenetic relationship between NALCN and Cch1 and identifies a larger clade of related channels in additional eukaryotic taxa. We also identify homologues of FAM155/Mid1 in Cryptista algae, and UNC79 and UNC80 homologues in numerous non-metazoan eukaryotes including basidiomycete and mucoromycete fungi, and the microbial eukaryotic taxa Apusomonadida, Malawimonadida, and Discoba. Furthermore, we find that most major animal lineages, except ctenophores, possess a full complement of NALCN subunits. Comparing structural predictions with the solved structure of the human NALCN complex supports orthologous relationships between metazoan and non-metazoan FAM155/Mid1, UNC79, and UNC80 homologues. Together, our analyses reveal unexpected diversity and ancient eukaryotic origins of NALCN/Cch1 channelosome subunits and raise interesting questions about the functional nature of this conserved channel complex within a broad, eukaryotic context. ### Competing Interest Statement The authors have declared no competing interest.

Our study on the first rhodopsin channels known to respond to UV light is now published in PNAS! And they come from our favourite protists, apusomonads! #protistonsky https://www.pnas.org/doi/full/10.1073/pnas.2510619122 [contains quote post or other embedded content]