Ancyromonadida: A New Phylogenetic Lineage Among the Protozoa Closely Related to the Common Ancestor of Metazoans, Fungi, and Choanoflagellates (Opisthokonta)

Abstract. Molecular and morphological evidence points to the ancyromonad Ancyromonas as a plausible candidate for the closest relative to the common ancestor of metazoans, fungi, and choanoflagellates (the Opisthokonta). Using 18S rDNA sequences from most of the major eukaryotic lineages, maximum-likelihood, minimum-evolution, and maximum-parsimony analyses yielded congruent phylogenies supporting this hypothesis. Combined with ultrastructural similarities between Ancyromonas and opisthokonts, the evidence presented here suggests that Ancyromonas may form an independent lineage, the Ancyromonadida Cavalier-Smith 1997, closer in its relationship to the opisthokonts than is its nearest protist relatives, the Apusomonadida. However, the very low bootstrap support for deep nodes and hypothesis testing indicate that the resolving power of 18S rDNA sequences is limited for examining this aspect of eukaryotic phylogeny. Alternate branching positions for the Ancyromonas lineage cannot be robustly rejected, revealing the importance of ultrastructure when examining the origins of multicellularity. The future use of a multigene approach may additionally be needed to resolve this aspect of eukaryotic phylogeny.