Early evolution and the origin of eukaryotes

Pelagomonas calceolata gen. et sp. nov., an ultra-planktonic marine alga, is described using electron microscopy and the cytoplasmic small subunit (18S) ribosomal RNA (rRNA) gene sequence. Cells are uniflagellate, about 1.5 × 3 μm in size. The flagellium has two rows of bipartite hairs, the paraxonemal rod has a dentate appearance, and a two-gyred transitional helix is present between two transitional plates. Microtubular roots, striated roots, and a second basal body are absent. A thin organic theca surrounds most of the cell. There is a single chloroplast with a girdle lamella and a single, dense mitochondrion with tubular cristae. A single Golgi body with swelled cisternae lies beneath the flagellum, and each cell has an ejectile organelle that putatwely releases a cylindrical structure. A vacuole, or cluster of vacuoles, contains the putative carbohydrate storage product. The 18S rRNA gene was sequenced completely in both directions, excluding three primer regions. When compared to the same gene sequence from other organisms, Pelagomonas calceolata gen. et sp. nov. occupies an unresolved position among other chromophyte algae and is distinct from members of any of these classes. Based on morphological, ultrastructural, and molecular data, we describe this alga as a new species, and we place this highly unusual new species in a new genus, family, order, and class.

Ultrastructure and 18s rrna gene sequence for pelagomonas calceolata gen. et sp. nov. and the description of a new algal class, the pelagophyceae classis nov.¹

Evolution of the protists and protistan parasites from the perspective of molecular systematics.

In this paper, I review what is currently known of phylogenetic relationships of heterokont and haptophyte algae. Heterokont algae are a monophyletic group that is classified into 17 classes and represents a diverse group of marine, freshwater, and terrestrial algae. Classes are distinguished by morphology, chloroplast pigments, ultrastructural features, and gene sequence data. Electron microscopy and molecular biology have contributed significantly to our understanding of their evolutionary relationships, but even today class relationships are poorly understood. Haptophyte algae are a second monophyletic group that consists of two classes of predominately marine phytoplankton. The closest relatives of the haptophytes are currently unknown, but recent evidence indicates they may be part of a large assemblage (chromalveolates) that includes heterokont algae and other stramenopiles, alveolates, and cryptophytes. Heterokont and haptophyte algae are important primary producers in aquatic habitats, and they are probably the primary carbon source for petroleum products (crude oil, natural gas).

https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.3732/ajb.91.10.1508

Biology and systematics of heterokont and haptophyte algae

Algae are ubiquitous. They are the primary producers for all the oceans and seas, an area that covers 71% of the Earth's surface. Algae also occur in freshwater lakes, ponds and streams as well as on and in soil, rocks, ice, snow, plants and animals. In total, 40% of global photosynthesis is contributed by algae. The algae are tremendously diverse. These are at least seven distinct phylogenetic lineages that arose independently during geological time and that evolved at different rates (based upon molecular clocks). Consequently, the algaein toto do not form a single, cohesive group, and they must be considered a polyphyletic assemblage. The red algal lineage has approximately 5000 recognized species (excluding nomenclatural synonyms, superfluous names, etc.) that are extant, and an estimated 500 to 15 000 new species remain to be described. The green algal lineage has approximately 16 000 recognized extant species and up to 100 000 species that remain to be described. The chromophyte lineage has approximately 15 000 recognized extant species and from one to ten million species that remain to be described. The dinophycean lineage (primarily dinoflagellates) has approximately 3000 recognized, extant species and workers estimate that from 500 to 8000 species remain to be described. The euglenophyte lineage has approximately 900 recognized extant species, and experts estimate that up to 1000 species remain to be described. The cryptophyte lineage is undergoing total revision and the numbers of species is very uncertain; perhaps 200 extant species are recognized and 1000 species remain to be described. The glaucophyte lineage has 13 recognized species, and perhaps several dozen species remain to be described. Since the 1960s, clonal culture followed by electron microscopic examination is often required before an alga can be identified or described as new. Recently, electron microscopy is being augmented by, or replaced by, gene sequence comparisons. Both electron microscopy and molecular sequencing are time consuming and expensive methods that slow the descriptions of new species. Time consuming methods for describing new algal species, a reduction in the number of algal systematists, a reduction in funds to support systematic research, and phycologically unexplored geographical areas act synergistically to hamper the rate of accessing algal biodiversity.

Diversity of eukaryotic algae

The 16S-like ribosomal RNA genes from Mallomonas papillosa Harris et Bradley (Synurophyceae) and Tribonema aequale Pascher (Xanthophyceae) were sequenced and compared to those of other eukaryotes. Mallomonas is closely related to Ochromonas (Chrysophyceae) and supports the general hypothesis of a close phylogenetic relationship between the Synurophyceae and Chrysophyceae. Tribonema is specifically related to Costaria costata (C. A. Agardh) Saunders (Phaeophyceae) demonstrating an unexpected phylogenetic relationship between the Xanthophyceae and Phaeophyceae. Distance and parsimony analysis place these four chromophyte genera in a complex evolutionary assemblage that includes the Bacillariophyceae and Oomycetes but excludes the Dinophyceae. The close relationship between the chromophyte algae and the Oomycete fungi supports the hypothesis that protists with tripartite hairs form a natural assemblage.

Edgardo V. Ariztia

Papers

A new phylogeny for chromophyte algae using 16s-like rrna sequences from mallomonas papillosa (synurophyceae) and tribonema aequale (xanthophyceae)1