The opalozoanApusomonasis related to the common ancestor of animals, fungi, and choanoflagellates

Abstract

Recent molecular evidence confirms that fungi are distinctly more closely related to animals and choanoflagellate protozoa than to plants. Animals, fungi, and choanoflagellates have been grouped together as the Opisthokonta, which are characterized by a single posterior cilium in their unicellular motile stage and by (non-discoid) flattened, plate-like mitochondrial cristae. It has been proposed that the common ancestor of the Opisthokonta arose from an opalozoan zooflagellate possessing two cilia and tubular mitochondrial cristae, by the loss of one cilium and a drastic change in the morphology of the mitochondrial cristae. We have tested this idea by sequencing the 18S ribosomal ribonucleic acid (rRNA) gene of the biciliate opalozoan Apusomonas proboscidea, which, like almost all Opalozoa, has tubular mitochondrial cristae. Our phylogenetic analysis, by maximum likelihood, maximum parsimony and two distance methods, shows that Apusomonas (the first member of the protozoan phylum Opalozoa to have its 18S rRNA gene sequenced) is indeed specifically related to the Opisthokonta; they are more closely related than any other protozoa for which sequences are available. This confirms that the Opisthokonta arose by the radical transformation of a biciliate with tubular cristae. Our analysis also shows that the plant kingdom evolved its irregularly flattened mitochondrial cristae from tubular ancestors independently of the Opisthokonta.