Origin and evolution of the slime molds (Mycetozoa)

Abstract

The Mycetozoa include the cellular (dictyostelid), acellular (myxogastrid), and protostelid slime molds. However, available molecular data are in disagreement on both the monophyly and phylogenetic position of the group. Ribosomal RNA trees show the myxogastrid and dictyostelid slime molds as unrelated early branching lineages, but actin and beta-tubulin trees place them together as a single coherent (monophyletic) group, closely related to the animal-fungal clade. We have sequenced the elongation factor-1alpha genes from one member of each division of the Mycetozoa, including Dictyostelium discoideum, for which cDNA sequences were previously available. Phylogenetic analyses of these sequences strongly support a monophyletic Mycetozoa, with the myxogastrid and dictyostelid slime molds most closely related to each other. All phylogenetic methods used also place this coherent Mycetozoan assemblage as emerging among the multicellular eukaryotes, tentatively supported as more closely related to animals + fungi than are green plants. With our data there are now three proteins that consistently support a monophyletic Mycetozoa and at least four that place these taxa within the "crown" of the eukaryote tree. We suggest that ribosomal RNA data should be more closely examined with regard to these questions, and we emphasize the importance of developing multiple sequence data sets.

Figure 1

EF-1α sequence alignment and intron positions. The deduced amino acid sequences of the Dictyostelium discoideum (Ddi), Physarum polycephalum (Ppo), Planoprotostelium aurantium (Pau), and Rhodotorula mucilaginosa (Rho) EF-1αs are shown aligned with those of the fungus Neurospora crassa (Ncr) and the fish Danio rerio (Dre). Gaps in the alignment are indicated by hyphens and missing data by periods. An insertion and deletion, which are together diagnostic of fungi, are indicated above the alignment by asterisks (see text). Intron positions are shown below the alignment with open, shaded, or solid triangles to indicate phase 0, 1, or 2 introns, respectively; phase 0 introns are indicated below their 3′ flanking amino acid. Organisms in which the indicated introns are found are indicated below the alignment and abbreviated as follows: Ag, Absidia glauca; Am, Apis mellifera; Ap, Aureobasidium pullulans; As, Artemia salina; C, Caenorhabditis elegans (two loci); D2, Drosophila melanogaster F2; Fp, flowering plants; Hs, Homo sapiens; M, Mucor racemosus (three loci); Nc, Neurospora crassa; Pa, Podospora anserina; Pg, Puccinia graminis; Tr, Trichoderma reseei; and Xl, Xenopus laevis. All sequences are available from the GenBank database.

Figure 2

Phylogenetic analyses of EF-1α amino acid sequences show a monophyletic, late-branching Mycetozoa. The tree shown is one of two shortest trees found by parsimony analysis; the other tree at this length places Schizosaccharomyces at the base of the Zygomycete + Basidiomycete clade. The tree is 2,319 steps long, and branches are drawn to scale as indicated. Dotted lines indicate the differences in the tree topology reconstructed by distance analysis. Bootstrap values over 50% are indicated above the nodes for parsimony analysis and below the nodes and to the left of the “/” for distance analysis. Dots (·) below the nodes (to the right of a “/”) indicate which nodes were constrained for maximum likelihood analysis. The results of bootstrap analyses with the Porphyra and Stylonychia sequences excluded are indicated in parentheses to the left and right of the “/,” respectively, for distance and maximum likelihood analyses.