Discocelis saleuta gen. nov. et sp. nov. (Protista incertae sedis)

Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa

I discuss how different feeding modes and related cellular structures map onto the eukaryote evolutionary tree. Centrally important for understanding eukaryotic cell diversity are Loukozoa: ancestrally biciliate phagotrophic protozoa possessing a posterior cilium and ventral feeding groove into which ciliary currents direct prey. I revise their classification by including all anaerobic Metamonada as a subphylum and adding Tsukubamonas. Loukozoa, often with ciliary vanes, are probably ancestral to all protozoan phyla except Euglenozoa and Percolozoa and indirectly to kingdoms Animalia, Fungi, Plantae, and Chromista. I make a new protozoan phylum Sulcozoa comprising subphyla Apusozoa (Apusomonadida, Breviatea) and Varisulca (Diphyllatea; Planomonadida, Discocelida, Mantamonadida; Rigifilida). Understanding sulcozoan evolution clarifies the origins from them of opisthokonts (animals, fungi, Choanozoa) and Amoebozoa, and their evolutionary novelties; Sulcozoa and their descendants (collectively called podiates) arguably arose from Loukozoa by evolving posterior ciliary gliding and pseudopodia in their ventral groove. I explain subsequent independent cytoskeletal modifications, accompanying further shifts in feeding mode, that generated Amoebozoa, Choanozoa, and fungi. I revise classifications of Choanozoa, Conosa (Amoebozoa), and basal fungal phylum Archemycota. I use Choanozoa, Sulcozoa, Loukozoa, and Archemycota to emphasize the need for simply classifying ancestral (paraphyletic) groups and illustrate advantages of this for understanding step-wise phylogenetic advances.

Rigifila ramosa n. gen., n. sp., a filose apusozoan with a distinctive pellicle, is related to Micronuclearia

We report the ultrastructure, 18S and 28S rDNA sequences, and phylogenetic position of a distinctive free-living heterotrophic filose protist, Rigifila ramosa n. gen., n. sp., from a freshwater paddyfield. Rigifila lacks cilia and has a semi-rigid, radially symmetric, well-rounded, partially microtubule-supported, dorsal pellicle, and flat mitochodrial cristae. From a central aperture in a ventral depression emerges a protoplasmic stem that branches into several branching filopodia that draw bacteria to it. Electron microscopy reveals a general cell structure similar to Micronuclearia, the only non-flagellate previously known in Apusozoa; the large basal vacuole is probably an unusual giant contractile vacuole. Phylogenetic analysis of concatenated rDNA sequences groups Rigifila and Micronuclearia as sisters with maximal statistical support. However, novel morphological differences unique to Rigifila, notably a double (not single) proteinaceous layer beneath the cell membrane, and cortical microtubules, lead us to place it in a new family Rigifilidae. Our morphological and molecular analyses show that Rigifila is the closest known relative of Micronuclearia. Therefore we group Micronucleariidae and Rigifilidae as a new order Rigifilida within the existing class Hilomonadea, which now excludes planomonads. Rigifilida groups weakly with Collodictyon (Diphyllatea). We discuss the possible relationships of Rigifilida to other Apusozoa and Diphyllatea.

A cytological study of Aulacomonas submarina Skuja 1939, a heterotrophic flagellate with a novel ultrastructural identity

Aulacomonas submarina is a free-living freshwater heterotrophic flagellate, the ultrastructure of which is described here. It has two long, nearly equal flagella that insert apically and beat asynchronously. It engulfs prey as large as itself via a ventral groove. The nucleus is anterior and surrounded by several dictyosomes. The two flagella are similar in structure. The most external part of the transition zone is marked by a constriction of the membrane, and distal to this the periaxonemal space expands for a short distance. The two basal bodies are inclined to each other and connected by roots, in a style reminiscent of some heterokont organisms. The basal body of the dorsal flagellum (Fl) is connected to a dorsal cytoskeletal root that is comprised of 5 superimposed microtubules and which sweeps from right to left in an incomplete loop near the cell apex. This loop is reinforced near its origin by an electron-dense rootlet. Material associated with the dorsal root gives rise to cortical microtubules which form a cape covering the apical region of the cell. The ventral basal body (of flagellum F2) is connected to two microtubular and microfibrillar fibres which support the lips of the ventral groove. More distally the lips of the groove are bordered only by a ridge supported by microfibrillar material. Mitochondria have bleb-shaped tubular cristae similar to those of actinophryid heliozoa. There is no cyst. The shape of the mitochondrial cristae, and the arrangement of basal bodies suggest that this genus is distantly related to the heterokont flagellates. However, most aspects of the ultrastructure are unlike those of other flagellates, such that Aulacomonas cannot be assigned to any familiar subgroup of heterokont organisms or other group of flagellates.

The hemimastigophora (Hemimastix amphikineta nov. gen., nov. spec.), a new protistan phylum from gondwanian soils

The morphology, morphogenesis and ultrastructure of Hemimastix amphikineta nov. gen., nov. spec, are described. This species occurred in some Australian and in 1 Chilean soil, but was absent from more than 1000 soil samples from Laurasian localities. Thus, it has probably a restricted Gondwanian distribution. Hemimastix amphikineta is a small (14-20 × 7-10 μn), colourless organism that looks distinctly Ciliophora-like because of its posteriorly located contractile vacuole and its 2 longitudinal somatic kineties each composed of about 12 cilia-like flagella. These 2 kineties are interposed between 2 large plicated and microtubule-bearing pellicular plates which are arranged inversely mirror-image like ("diagonal symmetry"). Hemimastix amphikineta has saccular to tubular mitochondrial cristae and complex extrusomes. It has 2 microtubular systems and a membranous sac associated with each kinetid. The nucleolus persists throughout nuclear division. A permanent cytostome-cytopharyngeal complex, pharyngeal rods, striated fibres, mastigonemes, and a paraflagellar rod are absent. This unique combination of characters dictates a very separate position for H. amphikineta within the known protists. Thus, the phylum Hemimastigophora nov. phylum (Hemimastigea nov. cl. and Hemimastigida nov. ord.), is established to include H. amphikineta and possibly Spironema multiciliatum Klebs, 1892. The structure of the pellicle and the nuclear apparatus of H. amphikineta indicate some relationship with the Euglenophyta. However, clear evidence for a certain affinity is lacking. Thus, the Hemimastigophora are placed in an incertae sedis position within the kingdom Protista Haeckel, 1866.

The Diversity of Eukaryotes

The discipline of evolutionary protistology has emerged in the past 30 yr. There is as yet no agreed view of how protists are interrelated or how they should be classified. The foundations of a stable taxonomic superstructure for the protists and other eukaryotes lie in cataloging the diversity of the major monophyletic lineages of these organisms. The use of common patterns of cell organization (ultrastructural identity) seems to provide us with the most robust hypotheses of such lineages. These lineages are placed in 71 groups without identifiable sister taxa. These groups are here referred to as "major building blocks." For the first time, the compositions, ultrastructural identities, synapomorphies (where available), and subgroups of the major building blocks are summarized. More than 200 further lineages without clear identities are listed. This catalog includes all known major elements of the comprehensive evolutionary tree of protists and eukaryotes. Different approaches among protistologists to issues of nomenclature, ranking, and definitions of these groups are discussed, with particular reference to two groups-the stramenopiles and the Archezoa. The concept of "extended in-group" is introduced to refer to in-groups and the most proximate sister group and to assist in identifying the hierarchical location of taxa.

Should there be a separate code of nomenclature for the protists?

The present Botanical and Zoological Codes of Nomenclature are often inadequate for resolution of all the peculiar problems caused by the very nature of the numerous and diverse groups of the so-called 'lower' eukaryotic organisms known as protists. Whether or not a separate code should therefore be created for these species--many but not all of which are unicellular in structure and microscopic in size--is complicated by several factors. The principal one is related to the wide dispersal of protists throughout many taxonomic classes and phyla/divisions; sometimes even multiple kingdoms are involved. If recognition of a single kingdom Protista is no longer tenable, then even the concept of one code per kingdom is not applicable. Other difficulties arise primarily from long-standing differences in major provisions of present Botanical and Zoological Codes. Numerous 'ambiregnal' forms exist, species currently under dual code jurisdiction. The matter of names for suprafamilial taxa of protists, irrespective of their ultimate kingdom assignment, poses another set of concerns not yet resolved. A plea is made to recognize the legitimacy of having distinct high-level ranks for protist species that seem to be widely separated phylogenetically from fellow protists or from other eukaryotic assemblages.