Abstract 109 5.1 Introduction 110 5.2 Animals and Their Phylogeny 110

5.2.1 Molecular Phylogeny 111 5.2.2 Basal Animal Lineages 111

5.3 Fungi and Their Phylogeny 113 5.3.1 Molecular Phylogeny 113 5.3.2 Basal Fungal Lineages 114

5.4 The Protistan Animal-Fungal Allies (Choanozoa) 114 5.4.1 Choanoflagellatea 115 5.4.2 Ichthyosporea 115 5.4.3 Corallochytrea 116 5.4.4 Cristidiscoidea 117

5.5 Choanozoa Phylogeny and the Origin of Animals and Fungi 118 5.5.1 Evidence for an Animal-Fungal Clade (Opisthokonta) 118 5.5.2 Phylogenetic Position of the Choanozoa 118

5.6 Possible Sistergroups of the Opisthokonta 120 5.6.1 Amoebozoa Phylogeny 121

5.7 Conclusions 122 Acknowledgments 123 References 123

The well-established exclusive grouping of animals and fungi (Opisthokonta) has recently been expanded to include a diverse collection of protistan taxa. These single-celled opisthokonts or Choanozoa include choanoflagellates (uniflagellated filter feeders), ichthyosporeans (parasites of aquatic animals), corallochytreans (free-living saprotrophs) and cristidiscoideans (nucleariid and ministeriid amoebae). The exact relationships of the choanozoans to each other and to animals and fungi are unknown, because most studies of

these taxa are based on single-gene trees and taxonomically nonoverlapping datasets. From the limited data available, the taxon Choanozoa appears to be paraphyletic. However, to substantiate this and to reconstruct the early evolution of opisthokonts, broad sampling of the basal animal and fungal lineages, various choanozoans and appropriate outgroups (i.e., Amoebozoa and Apusozoa) is required. As with all deep phylogenetic questions, resolving the relationships among multi-and unicellular opisthokonts might be difficult, if not impossible, with single-gene trees. This is possibly because they lack sufficient phylogenetic information or are burdened by too many substitutions at individual sites. Thus, the origin of opisthokonts and their early divergence will most probably only be resolved with the analysis of taxonomically well-represented multisequence datasets.