1. Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
2. Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
3. †Present address: School of Biological and Chemical Sciences, University of Exeter, Washington Singer Laboratories, Perry Road, Exeter EX4 4QG, UK

Correspondence to: Thomas A. Richards^1,2,3 Correspondence and requests for materials should be addressed to T.A.R. (Email: thomr@nhm.ac.uk).

Abstract

Eukaryotic cells have two contrasting cytoskeletal and ciliary organizations. The simplest involves a single cilium-bearing centriole, nucleating a cone of individual microtubules (probably ancestral for unikonts: animals, fungi, Choanozoa and Amoebozoa). In contrast, bikonts (plants, chromists and all other protozoa) were ancestrally biciliate with a younger anterior cilium, converted every cell cycle into a dissimilar posterior cilium and multiple ciliary roots of microtubule bands. Here we show by comparative genomic analysis that this fundamental cellular dichotomy also involves different myosin molecular motors. We found 37 different protein domain combinations, often lineage-specific, and many previously unidentified. The sequence phylogeny and taxonomic distribution of myosin domain combinations identified five innovations that strongly support unikont monophyly and the primary bikont/unikont bifurcation. We conclude that the eukaryotic cenancestor (last common ancestor) had a cilium, mitochondria, pseudopodia, and myosins with three contrasting domain combinations and putative functions.

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