Eumycetozoans and molecular systematics

Abstract

Eumycetozoans, the myxomycetes, protostelids, and dictyostelids, were first hypothesized to be a monophyletic group by L.S. Olive, who suggested that the primitive members of the group were similar to some of the extant protostelids. A review of morphological evidence supporting some aspects of this hypothesis is presented along with explicit explanations of the shortcomings of morphological data as tests of other aspects. For the hypothesis to be supported, modified, or rejected, data from other areas such as the sequences of the nuclear ribosomal small subunit genes (SSrDNA) will have to be used. Presently, sequences for this gene are known only from Physarum polycephalum and Dictyostelium discoideum. These two slime molds are treated as separate, deep clades in the grand eukaryote phylogenies derived from the sequences of SSrDNA. That is, each species represents an independent lineage that diverged early in the history of the eukaryotes. Insufficient taxon sampling may account for the molecular trees which suggest that the dictyostelids and myxomycetes are not members of a monophyletic group. We have begun to examine the SSrDNA sequence in the protostelid Protostelium mycophaga. Preliminary phylogenetic reconstructions using 11 eukaryotic outgroups suggest that the protostelids, myxomycetes, and dictyostelids are members of a single monophyletic group which may be most closely related to the Chromista. It is interesting that these results coincide with earlier phylogenetic hypotheses based on the morphological characters of these slime molds. Key words: dictyostelids, myxomycetes, protostelids, ribosomal DNA, slime molds.