BackgroundThe Amoebozoa constitute one of the primary divisions of eukaryotes, encompassing taxa of both biomedical and evolutionary importance, yet its genomic diversity remains largely unsampled. Here we present an analysis of a whole genome assembly of Acanthamoeba castellanii (Ac) the first representative from a solitary free-living amoebozoan.ResultsAc encodes 15,455 compact intron-rich genes, a significant number of which are predicted to have arisen through inter-kingdom lateral gene transfer (LGT). A majority of the LGT candidates have undergone a substantial degree of intronization and Ac appears to have incorporated them into established transcriptional programs. Ac manifests a complex signaling and cell communication repertoire, including a complete tyrosine kinase signaling toolkit and a comparable diversity of predicted extracellular receptors to that found in the facultatively multicellular dictyostelids. An important environmental host of a diverse range of bacteria and viruses, Ac utilizes a diverse repertoire of predicted pattern recognition receptors, many with predicted orthologous functions in the innate immune systems of higher organisms.ConclusionsOur analysis highlights the important role of LGT in the biology of Ac and in the diversification of microbial eukaryotes. The early evolution of a key signaling facility implicated in the evolution of metazoan multicellularity strongly argues for its emergence early in the Unikont lineage. Overall, the availability of an Ac genome should aid in deciphering the biology of the Amoebozoa and facilitate functional genomic studies in this important model organism and environmental host.

The social amoebas (Dictyostelia) display conditional multicellularity in a wide variety of forms. Despite widespread interest in Dictyostelium discoideum as a model system, almost no molecular data exist from the rest of the group. We have constructed the first molecular phylogeny of the Dictyostelia with parallel small subunit ribosomal RNA and α-tubulin datasets, and we found that dictyostelid taxonomy requires complete revision. A mapping of characters onto the phylogeny shows that the dominant trend in dictyostelid evolution is increased size and cell-type specialization of fruiting structures, with some complex morphologies evolving several times independently. Thus, the latter may be controlled by only a few genes, making their underlying mechanisms relatively easy to unravel. TEXTMulticellular animals and plants display an enormous variety of forms, but their underlying genetic diversity is small compared with the genetic diversity of microbes. Eukaryotic microbes include a broad range of unicellular life forms, with multiple independent inventions of multicellularity. One of the most intriguing challenges in biology is to understand the reason behind the repeated occurrence of this particular evolutionary stratagem.The social amoebas, or Dictyostelia, are a group of organisms that hover on the borderline between uni-and multicellularity. Each organism starts its life as a unicellular amoeba, but they aggregate to form a multicellular fruiting body when starved. This process has been