FINE STRUCTURE AND TAXONOMY OF MONOMORPHINA AENIGMATICA COMB. NOV. (EUGLENOPHYTA)(1)

The pyrenoid: the eukaryotic CO2-concentrating organelle

The pyrenoid is a phase-separated organelle that enhances photosynthetic carbon assimilation in most eukaryotic algae and the land plant hornwort lineage. Pyrenoids mediate approximately one-third of global CO2 fixation, and engineering a pyrenoid into C3 crops is predicted to boost CO2 uptake and increase yields. Pyrenoids enhance the activity of the CO2-fixing enzyme Rubisco by supplying it with concentrated CO2. All pyrenoids have a dense matrix of Rubisco associated with photosynthetic thylakoid membranes that are thought to supply concentrated CO2. Many pyrenoids are also surrounded by polysaccharide structures that may slow CO2 leakage. Phylogenetic analysis and pyrenoid morphological diversity support a convergent evolutionary origin for pyrenoids. Most of the molecular understanding of pyrenoids comes from the model green alga Chlamydomonas (Chlamydomonas reinhardtii). The Chlamydomonas pyrenoid exhibits multiple liquid-like behaviors, including internal mixing, division by fission, and dissolution and condensation in response to environmental cues and during the cell cycle. Pyrenoid assembly and function are induced by CO2 availability and light, and although transcriptional regulators have been identified, posttranslational regulation remains to be characterized. Here, we summarize the current knowledge of pyrenoid function, structure, components, and dynamic regulation in Chlamydomonas and extrapolate to pyrenoids in other species.

Phylogenetic Reappraisal of the Genus Monomorphina (Euglenophyceae) Based on Molecular and Morphological Data

A morphological and molecular examination of the genus Monomorphina was conducted on 46 strains isolated mainly from Korea. The strains were divided into two types based on morphological data: Monomorphina aenigmatica and M. pyrum - like species. Phylogenetic analysis based on a combined data set of nuclear SSU and LSU and plastid SSU and LSU rDNA showed that the strains could be divided into eight clades: Clade A of M. aenigmatica, Clade B of the isolates (M. pyropsis) from Michigan, USA, Clade C of M. pseudopyrum, Clade D of the isolates (M. pyroria) from Bremen, Germany, Clade E of M. soropyrum, Clade F of M. pyriformis, Clade G of M. parapyrum, and Clade H of M. pyrum. Six of these clades came from strains that would be considered M. pyrum sensu Kosmala et Zakryś, one of which could be recognized as a traditional species (M. pyrum) and five were designated as new species; each species had unique molecular signatures at nr SSU rDNA helix 17 and 17' and spacer E23_14'-E23_15. The species of Monomorphina had a wide range of genetic diversity with interspecies sequence similarity of 85.6%-97.1% and intraspecies similarity of 96.4%-99.9%. Our results suggested that genetic diversity found in the M. pyrum complex justifies the recognition of a minimum of eight species within this genus, based on specific molecular signatures and gene divergence of the nr SSU rDNA sequences.

Evidence for transitional stages in the evolution of euglenid group II introns and twintrons in the Monomorphina aenigmatica plastid genome

Background

Photosynthetic euglenids acquired their plastid by secondary endosymbiosis of a prasinophyte-like green alga. But unlike its prasinophyte counterparts, the plastid genome of the euglenid Euglena gracilis is riddled with introns that interrupt almost every protein-encoding gene. The atypical group II introns and twintrons (introns-within-introns) found in the E. gracilis plastid have been hypothesized to have been acquired late in the evolution of euglenids, implying that massive numbers of introns may be lacking in other taxa. This late emergence was recently corroborated by the plastid genome sequences of the two basal euglenids, Eutreptiella gymnastica and Eutreptia viridis, which were found to contain fewer introns.

Methodology/Principal Findings

To gain further insights into the proliferation of introns in euglenid plastids, we have characterized the complete plastid genome sequence of Monomorphina aenigmatica, a freshwater species occupying an intermediate phylogenetic position between early and late branching euglenids. The M. aenigmatica UTEX 1284 plastid genome (74,746 bp, 70.6% A+T, 87 genes) contains 53 intron insertion sites, of which 41 were found to be shared with other euglenids including 12 of the 15 twintron insertion sites reported in E. gracilis.

Conclusions

The pattern of insertion sites suggests an ongoing but uneven process of intron gain in the lineage, with perhaps a minimum of two bursts of rapid intron proliferation. We also identified several sites that represent intermediates in the process of twintron evolution, where the external intron is in place, but not the internal one, offering a glimpse into how these convoluted molecular contraptions originate.

PHYLOGENY OF THE EUGLENALES INFERRED FROM PLASTID LSU rDNA SEQUENCES(1)

To gain insights into the phylogeny of the Euglenales, we analyzed the plastid LSU rDNA sequences from 101 strains of the photosynthetic euglenoids belonging to nine ingroup genera (Euglena, Trachelomonas, Strombomonas, Monomorphina, Cryptoglena, Colacium, Discoplastis, Phacus, and Lepocinclis) and two outgroup genera (Eutreptia and Eutreptiella). Bayesian and maximum-likelihood (ML) analyses resulted in trees of similar topologies and four major clades: a Phacus and Lepocinclis clade; a Colacium clade; a Trachelomonas, Strombomonas, Monomorphina, and Cryptoglena clade; and a Euglena clade. The Phacus and Lepocinclis clade was the sister group of all other euglenalian genera, followed by Discoplastis spathirhyncha (Skuja) Triemer and the Colacium clade, respectively, which was inconsistent with their placement based on nuclear rDNA genes. The Trachelomonas, Strombomonas, Monomorphina, and Cryptoglena clade was sister to the Euglena clade. The loricate genera, Trachelomonas and Strombomonas, were closely related to each other, while Monomorphina and Cryptoglena also grouped together. The Euglena clade formed a monophyletic lineage comprising most species from taxa formerly allocated to the subgenera Calliglena and Euglena. However, within this genus, none of the subgenera was monophyletic.

NOVEL PELLICLE SURFACE PATTERNS ON EUGLENA OBTUSA (EUGLENOPHYTA) FROM THE MARINE BENTHIC ENVIRONMENT: IMPLICATIONS FOR PELLICLE DEVELOPMENT AND EVOLUTION(1)

Euglena obtusa F. Schmitz possesses novel pellicle surface patterns, including the greatest number of strips (120) and the most posterior subwhorls of strip reduction in any euglenid described so far. Although the subwhorls form a mathematically linear pattern of strip reduction, the pattern observed here differs from the linear pattern described for Euglena mutabilis F. Schmitz in that it contains seven linear subwhorls, rather than three, and is developmentally equivalent to three whorls of exponential reduction, rather than two. These properties imply that the seven-subwhorled linear pattern observed in E. obtusa is evolutionarily derived from an ancestral bilinear pattern, rather than from a linear pattern, of strip reduction. Furthermore, analysis of the relative lateral positions of the strips forming the subwhorls in E. obtusa indicates that (1) the identity (relative length, lateral position, and maturity) of each strip in any mother cell specifies that strip's identity in one of the daughter cells following pellicle duplication and cell division, (2) the relative length of any given pellicle strip regulates the length of the nascent strip it will produce during pellicle duplication, and (3) pellicle pores develop within the heels of the most mature pellicle strips. These observations suggest that continued research on pellicle development could eventually establish an ideal system for understanding mechanisms associated with the morphogenesis and evolution of related eukaryotic cells.

ISOLATION OF A NON-PHAGE-LIKE LYTIC VIRUS INFECTING AUREOCOCCUS ANOPHAGEFFERENS(1)

We have been working to characterize viruses that infect the HAB-forming pelagophyte Aureococcus anophagefferens Hargraves et Sieburth. Field samples were collected during brown-tide events in 2002 and tested for the presence of lytic agents. Here, we describe a recently isolated, lytic virus-like particle (VLP) that is morphologically similar to particles observed in thin sections of infected A. anophagefferens cells from natural samples. TEM and SEM have revealed VLPs consistent with the morphological characteristics of previously described Phycodnaviridae. Large icosahedral particles (∼140 nm) of similar shape and morphology dominate cell lysates and are accompanied by smaller phage-like particles and heterotrophic prokaryotes that appear to be incurable from our cultures. To determine which of these particles interacts with the Aureococcus cells, we preserved cultures during the early stage of infection so that SEM could be used to visualize those particles that attach to the surface of naïve cultures. SEM revealed that 63% of the large icosahedral-shaped particles attached to A. anophagefferens cells after only 30 min of exposure, while no significant frequency of attachment to the alga was observed for the phage-like particles. The results of these observations are in contrast to previous studies, where phage-like particles were reported to infect cells. When considered in conjunction with field observations, the results suggest that this newly isolated virus represents the dominant virus-morphotype associated with bloom collapse and termination.