18S rDNA Phylogeny of Lamproderma and Allied Genera (Stemonitales, Myxomycetes, Amoebozoa)

The phylogenetic position of the slime-mould genus Lamproderma (Myxomycetes, Amoebozoa) challenges traditional taxonomy: although it displays the typical characters of the order Stemonitales, it appears to be sister to Physarales. This study provides a small subunit (18S or SSU) ribosomal RNA gene-based phylogeny of Lamproderma and its allies, with new sequences from 49 specimens in 12 genera. We found that the order Stemonitales and Lamproderma were both ancestral to Physarales and that Lamproderma constitutes several clades intermingled with species of Diacheopsis, Colloderma and Elaeomyxa. We suggest that these genera may have evolved from Lamproderma by multiple losses of fruiting body stalks and that many taxonomic revisions are needed. We found such high genetic diversity within three Lamproderma species that they probably consist of clusters of sibling species. We discuss the contrasts between genetic and morphological divergence and implications for the morphospecies concept, highlighting the phylogenetically most reliable morphological characters and pointing to others that have been overestimated. In addition, we showed that the first part (∼600 bases) of the SSU rDNA gene is a valuable tool for phylogeny in Myxomycetes, since it displayed sufficient variability to distinguish closely related taxa and never failed to cluster together specimens considered of the same species

The prokaryote messenger c-di-GMP triggers stalk cell differentiation in Dictyostelium

Cyclic di-(3′:5′)-guanosine monophosphate (c-di-GMP) is a major prokaryote signalling intermediate, which is synthesized by diguanylate cyclases and triggers sessility and biofilm formation(1,2). We detected the first eukaryote diguanylate cyclases (DgcAs) in all major groups of Dictyostelia. Upon food depletion, Dictyostelium discoideum amoebas collect into aggregates, which first transform into migrating slugs and next into sessile fruiting structures. These structures consist of a spherical spore mass that is supported by a column of stalk cells and a basal disk. A polyketide, DIF-1, was isolated earlier, which induces stalk-like cells in vitro(3). However, its role in vivo proved recently to be restricted to basal disk formation(4). Here we show that Dictyostelium DgcA produces c-di-GMP as the morphogen responsible for stalk cell differentiation. D.discoideum DgcA synthesized c-di-GMP in a GTP-dependent manner and was expressed at the slug tip, the site of stalk cell differentiation. Disruption of the DgcA gene blocked the transition from slug migration to fructification and the expression of stalk genes. Fructification and stalk formation were restored by exposing dgca- slugs to wild-type secretion products or to c-di-GMP. Moreover, c-di-GMP, but not c-di-AMP, induced stalk gene expression in dilute cell monolayers. Apart from identifying the long elusive stalk-inducing morphogen, our work also identifies the first role for c-di-GMP in eukaryotes

The Amoebozoa

The model organism Dictyostelium discoideum is a member of the Amoebozoa, one of the six major ­divisions of eukaryotes. Amoebozoa comprise a wide variety of amoeboid and flagellate organisms with single cells measuring from 5 μm to several meters across. They have adopted many different life styles and sexual behaviors and can live in all but the most extreme environments. This chapter provides an overview of Amoebozoan diversity and compares roads towards multicellularity within the Amoebozoa with inventions of multicellularity in other protist divisions. The chapter closes with a scenario for the evolution of Dictyostelid multicellularity from an Amoebozoan stress response