Summary: Insights into the Evolutionary Origin and Genome Architecture of the Unicellular
Opisthokonts Capsaspora owczarzaki and Sphaeroforma arctica
IN AKI RUIZ-TRILLO, CHRISTOPHER E. LANE, JOHN M. ARCHIBALD and ANDREW J. ROGER
Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, Canada B3H 1X5
ABSTRACT. Molecular phylogenetic analyses have recently shown that the unicellular amoeboid protist Capsaspora owczarzaki is
unlikely to be a nucleariid or an ichthyosporean as previously described, but is more closely related to Metazoa, Choanoflagellata, and
Ichthyosporea. However, the specific phylogenetic relationship of Capsaspora to other protist opisthokont lineages was poorly resolved.
To test these earlier results we have expanded both the taxonomic sampling and the number of genes from opisthokont unicellular lin-
eages. We have sequenced the protein-coding genes elongation factor 1-a (EF1-a) and heat shock protein 70 (Hsp70) from C. owczarzaki
and the ichthyosporean Sphaeroforma arctica. Our maximum likelihood (ML) and Bayesian analyses of a concatenated alignment of
EF1-a, Hsp70, and actin protein sequences with a better sampling of opisthokont-related protist lineages indicate that C. owczarzaki is not
clearly allied with any of the unicellular opisthokonts, but represents an independent unicellular lineage closely related to animals and
choanoflagellates. Moreover, we have found that the ichthyosporean S. arctica possesses an EF-like (EFL) gene copy instead of the
canonical EF1-a, the first so far described in an ichthyosporean. A maximum likelihood phylogenetic analysis shows that the EF-like gene
of S. arctica strongly groups with the EF-like genes from choanoflagellates. Finally, to begin characterizing the Capsaspora genome, we
have performed pulsed-field gel electrophoresis (PFGE) analyses, which indicate that its genome has at least 12 chromosomes with a total
genome size in the range of 2225 Mb.
Key Words. EFL gene, genome size, molecular systematics, pulsed-field gel electrophoresis.
ONGOING molecular systematic studies have revealed that
two major eukaryotic groups, Metazoa (or Animals) and