Roles of actin during sporocarp culmination in the simple mycetozoan Planoprotostelium aurantium

Phylogenetic placement of the protosteloid amoeba Microglomus paxillus identifies another case of sporocarpic fruiting in Discosea (Amoebozoa)

Protosteloid amoebae are a paraphyletic assemblage of amoeboid protists found exclusively in the eukaryotic assemblage Amoebozoa. These amoebae can facultatively form a dispersal structure known as a fruiting body, or more specifically, a sporocarp, from a single amoeboid cell. Sporocarps consist of one to a few spores atop a noncellular stalk. Protosteloid amoebae are known in two out of three well-established major assemblages of Amoebozoa. Amoebae with a protosteloid life cycle are known in the major Amoebozoa lineages Discosea and Evosea but not in Tubulinea. To date, only one genus, which is monotypic, lacks sequence data and, therefore, remains phylogenetically homeless. To further clarify the evolutionary milieu of sporocarpic fruiting we used single-cell transcriptomics to obtain data from individual sporocarps of isolates of the protosteloid amoeba Microglomus paxillus. Our phylogenomic analyses using 229 protein coding markers suggest that M. paxillus is a member of the Discosea lineage of Amoebozoa most closely related to Mycamoeba gemmipara. Due to the hypervariable nature of the SSU rRNA sequence we were unable to further resolve the phylogenetic position of M. paxillus in taxon rich datasets using only this marker. Regardless, our results widen the known distribution of sporocarpy in Discosea and stimulate the debate between a single or multiple origins of sporocarpic fruiting in Amoebozoa.

The nature and features of organization of reserve polysaccharides in three Pelomyxa species (Archamoebea, Pelobiontida)

The nature and features of organization of reserve polysaccharides in three species of the genus Pelomyxa-P. palustris, P. belevskii, and P. stagnalis-were studied using light and transmission electron microscopy. We applied the periodic acid-Schiff reaction that is a highly selective method for detecting glycogen. The fluorescent dye auramine-SO₂ (Au-SO₂) was used as a Schiff-type reagent. The densely packed aggregates of glycogen that form the morphologically differentiated organelle-like bodies are revealed in the cytoplasm in all studied species. The organization of these bodies is characterized by the species-specific features, while in most cases, their size and number in the cells vary depending on the season of the year. Although in all the cases we studied, these bodies do not have their own boundary membrane, in fact, they are surrounded by membranous structures. These structures differ in a variety of Pelomyxa species. We concluded that there are two groups of species in the genus Pelomyxa. The first one includes organisms containing glycogen structures in the cytoplasm (P. palustris, P. belevskii, P. stagnalis, P. binucleata, P. corona, P. secunda). No inclusions resembling glycogen bodies were found in P. flava, P. paradoxa, P. gruberi, and P. prima that form the second group.

Two potential evolutionary origins of the fruiting bodies of the dictyostelid slime moulds

Dictyostelium discoideum and the other dictyostelid slime moulds ('social amoebae') are popular model organisms best known for their demonstration of sorocarpic development. In this process, many cells aggregate to form a multicellular unit that ultimately becomes a fruiting body bearing asexual spores. Several other unrelated microorganisms undergo comparable processes, and in some it is evident that their multicellular development evolved from the differentiation process of encystation. While it has been argued that the dictyostelid fruiting body had similar origins, it has also been proposed that dictyostelid sorocarpy evolved from the unicellular fruiting process found in other amoebozoan slime moulds. This paper reviews the developmental biology of the dictyostelids and other relevant organisms and reassesses the two hypotheses on the evolutionary origins of dictyostelid development. Recent advances in phylogeny, genetics, and genomics and transcriptomics indicate that further research is necessary to determine whether or not the fruiting bodies of the dictyostelids and their closest relatives, the myxomycetes and protosporangids, are homologous.

Between a Pod and a Hard Test: The Deep Evolution of Amoebae

Amoebozoa is the eukaryotic supergroup sister to Obazoa, the lineage that contains the animals and Fungi, as well as their protistan relatives, and the breviate and apusomonad flagellates. Amoebozoa is extraordinarily diverse, encompassing important model organisms and significant pathogens. Although amoebozoans are integral to global nutrient cycles and present in nearly all environments, they remain vastly understudied. We present a robust phylogeny of Amoebozoa based on broad representative set of taxa in a phylogenomic framework (325 genes). By sampling 61 taxa using culture-based and single-cell transcriptomics, our analyses show two major clades of Amoebozoa, Discosea, and Tevosa. This phylogeny refutes previous studies in major respects. Our results support the hypothesis that the last common ancestor of Amoebozoa was sexual and flagellated, it also may have had the ability to disperse propagules from a sporocarp-type fruiting body. Overall, the main macroevolutionary patterns in Amoebozoa appear to result from the parallel losses of homologous characters of a multiphase life cycle that included flagella, sex, and sporocarps rather than independent acquisition of convergent features.

Variation in the SSUrDNA of the Genus Protostelium Leads to a New Phylogenetic Understanding of the Genus and of the Species Concept for Protostelium mycophaga (Protosteliida, Amoebozoa)

Members of the genus Protostelium (including P. mycophaga, P. nocturnum, and P. okumukumu) are protosteloid amoebae commonly found in terrestrial habitats on dead plant matter. They, along with the closely allied nominal genus Planoprotostelium, containing the single species Pl. aurantium, all have an amoeboid trophic stage with acutely pointed subpseudopodia and orange lipid droplets in the granuloplasm. These amoebae form stalked fruiting bodies topped with a single, usually deciduous spore. The species are identified based on their fruiting body morphologies except for Pl. aurantium which looks similar to P. mycophaga in fruiting morphology, but has amoebae that can make flagella in liquid medium. We built phylogenetic trees using nuclear small subunit ribosomal DNA sequences of 35 isolates from the genera Protostelium and Planoprotostelium and found that (1) the nonflagellated P. nocturnum and P. okumukumu branch basally in the genus Protostelium, (2) the flagellate, Pl. aurantium falls within the genus Protostelium in a monophyletic clade with the nominal variety, P. mycophaga var. crassipes, (3) the cultures initially identified as Protostelium mycophaga can be divided into at least three morphologically recognizable taxa, P. aurantium n. comb., P. apiculatum n. sp., and P. m. rodmani n. subsp., as well as a paraphyletic assemblage that includes the remainder of the P. mycophaga morphotype. These findings have implications for understanding the ecology, evolution, and diversity of these amoeboid organisms and for using these amoebae as models for other amoeboid groups.

Eumycetozoans and molecular systematics

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.

The control of cellulose microfibril deposition in the cell wall of higher plants : II. Freeze-fracture microfibril patterns in maize seedling tissues following experimental alteration with colchicine and ethylene

Cells of maize (Zea mays L.) seedling that are not fixed or cryoprotected contain the impressions of cellulose microfibrils on freeze-fractured plasma membranes. Impressions of the most recently deposited microfibrils have terminal complexes associated with them (see preceding paper). The orientations of microtubules in cytoplasmic fractures are parallel to the newest microfibrils observed on adjacent plasma membrane fractures. Small groups of microfibrils, distinguished from the next older layer by their new orientation, are sometimes observed directly adjacent and parallel to individual microtubules. Whereas microtubules are parallel to microfibril orientations which vary from transverse to occasionally longitudinal, microfilaments are parallel to the longitudinal cell axis. After colchicine treatment, cytoplasmic microtubules are absent, as are the bands of microfibrils that are observed on the membrane of control cells. Parallel orientations of microfibrils and normal pitfield outlines are often still observed after colchicine treatment. However, on some membranes, multidirectionally-oriented microfibril tips occur, associated with perturbations of microfibril orientation and rounded pit-field outlines. In ethylene-treated cells, some membranes have microfibril tips oriented in only one direction in new layers of longitudinal microfibrils. On other membranes, longitudinal bands of microfibril tips are oriented in opposing directions. We propose that after colchicine treatment, the patterns of microfibrils reflect an orientation mechanism which has been uncoupled from the influence of microtubules but which is still under some other form of cellular control. We propose that membrane flow could orient the lateral movement of synthesizing complexes in the membrane and that microtubules modulate this movement, apparently organizing the microfibrils into parallel bands in newly-forming wall layers.