Stimulation of cell division and DNA replication in Prototheca richardsi by passage through larval amphibian guts

Population-level analyses indirectly reveal cryptic sex and life history traits of Pseudoperkinsus tapetis (Ichthyosporea, Opisthokonta): a unicellular relative of the animals

We use population genetics to detect the molecular footprint of a sexual cycle, of a haploid vegetative state, and of lack of host specificity in Pseudoperkinsus tapetis, a marine unicellular relative of the animals. Prior to this study, complete life cycles were not known for any of the unicellular lineages sharing common ancestry with multicellular animals and fungi. We established the first collection of conspecific cultures of any member from the unicellular opisthokont lineage ichthyosporea, isolating 126 cultures of P. tapetis from guts of marine invertebrates ranging from clams to sea cucumbers. We sequenced fragments of the elongation factor alpha-like (EFL) and heat-shock protein 70 (HSP70) genes for a subset of our isolates. Absence of heterozygotes from the EFL locus in 52 isolates provided evidence for haploidy. Phylogenetic incongruence and a lack of support for linkage between two loci from 34 sequenced isolates signified a history of recombination consistent with a sexual cycle. Shared haplotypes in different invertebrate species showed that P. tapetis was not host specific. Based on estimates of the frequency of sex and on observations of cultures, we propose that P. tapetis is transmitted between hosts via asexual endospores. New protists are continually being discovered, and, as this study illustrates, analysis of culturable collections from natural habitats can transform a species from a near unknown to a model system for better understanding the evolution of life histories.

Multiple isolations of a culturable, motile Ichthyosporean (Mesomycetozoa, Opisthokonta), Creolimax fragrantissima n. gen., n. sp., from marine invertebrate digestive tracts

A fragrant, spherical, osmotrophic eukaryote was isolated 27 times from the digestive tracts of marine invertebrates collected from the Northeast Pacific. The isolates were cultured from 7 animal collections over a 2-year period, most from the peanut worm, Phascolosoma agassizii. A small subunit ribosomal DNA phylogeny placed the spherical organism within the ichthyosporea, closest to Sphaeroforma arctica and Pseudoperkinsus tapetis. Supporting the close relationship of isolates, the sequences of ribosomal gene internal transcribed spacers determined for 26 isolates were identical, as were the elongation factor 1-alpha-like gene fragments from 7 isolates. Dispersal via amoeboid cells distinguished this species from its closest relatives and led to the erection of a new genus and species, "Creolimax fragrantissima." Vegetative cells reproduced asexually in vitro after they reached 30-60 microm in diameter by producing amoebae or endospores, which escaped through openings in the parent cell wall. Ultrathin sections of vegetative cells prepared by high-pressure-freeze substitution provided some of the first images of ichthyosporean spindle pole bodies and document, for the first time, tubular extensions of the plasma membrane into an electron-translucent inner layer of the cell wall. Ichthyosporeans are parasites and commensals of animals and culturable species are few. Because "C. fragrantissima" can be isolated regularly and repeatedly from nature and then grown easily through cycles of asexual reproduction, it has the potential to serve as a model organism for further research into marine ichthyosporeans.

The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary

When the enigmatic fish pathogen, the rosette agent, was first found to be closely related to the choanoflagellates, no one anticipated finding a new group of organisms. Subsequently, a new group of microorganisms at the boundary between animals and fungi was reported. Several microbes with similar phylogenetic backgrounds were soon added to the group. Interestingly, these microbes had been considered to be fungi or protists. This novel phylogenetic group has been referred to as the DRIP clade (an acronym of the original members: Dermocystidium, rosette agent, Ichthyophonus, and Psorospermium), as the class Ichthyosporea, and more recently as the class Mesomycetozoea. Two orders have been described in the mesomycetozoeans: the Dermocystida and the Ichthyophonida. So far, all members in the order Dermocystida have been pathogens either of fish (Dermocystidium spp. and the rosette agent) or of mammals and birds (Rhinosporidium seeberi), and most produce uniflagellated zoospores. Fish pathogens also are found in the order Ichthyophonida, but so are saprotrophic microbes. The Ichthyophonida species do not produce flagellated cells, but many produce amoeba-like cells. This review provides descriptions of the genera that comprise the class Mesomycetozoea and highlights their morphological features, pathogenic roles, and phylogenetic relationships.