Hemiselmis Parke

Introduction

Described by Parke; type species Hemiselmis rufescens (Parke 1949).

Characteristics

• Types of biliprotein: PC630, PC612, PC612 or PE555 (with subtype PE545/555)
• No dimorphism known, but differences in cell sizes have been observed in some cultures
  
• Light microscopy: blue to blue-green or red cryptophytes, small, often reniform cells (< 10 µm in cell length) with laterally inserting flagella, one plastid, predominantly marine (one freshwater species described: Hemiselmis amylosa; Clay and Kugrens 1999)
• Ultrastructure: hexagonal plates on both sides of the periplast, short gullet, thylakoids traverse the pyrenoid matrix, nucleomorph free in periplastidial space (Santore 1984)
  

Discussion of Phylogenetic Relationships

The type strain of Hemiselmis rufescens, established and examined by Parke, is still available from the Plymouth Algal Culture Collection (UK), and thus was genetically characterized in the revision of the genus Hemiselmis (Lane and Archibald 2008). Hemiselmis appears to represent a consistent genus in molecular phylogenies without any evidence for dimorphism thus far (Figure 1; Lane and Archibald 2008). At least four different biliprotein types, three blue phycocyanins and one red phycoerythrin, can be found (Lane and Archibald 2008; Hoef-Emden 2008). The reddish Hemiselmis rufescens and Hemiselmis andersenii evolved from blue-green Hemiselmis species (Lane and Archibald 2008). The problems inherent in the morphospecies concept also apply to Hemiselmis: molecular phylogenetic trees show a higher genetic variation than the morphological characters do.

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Figure 1. Molecular phylogeny of the genera Chroomonas, Hemiselmis and Komma. Maximum likelihood tree inferred from nucleomorph SSU rDNA sequences. PC, phycocyanin; PE, phycoerythrin; scale bar = substitutions per site. © 2008 Kerstin Hoef-Emden

Nucleomorph Genome

Preliminary analyses of the nucleomorph genomes of several members of the genus Hemiselmis have been performed (Lane and Archibald 2006, 2008). As in the model cryptophyte Guillardia theta (Douglas et al. 2001), all examined Hemiselmis species possess three nucleomorph chromosomes. Estimated genome sizes for the group range from ~560 kb in H. tepida to ~600 kb in H. rufescens. Unlike G. theta, which possesses complete and near identical ribosomal DNA operons on both ends of each nucleomorph chromosome (Douglas et al. 2001; Archibald 2007), Hemiselmis species are characterized by the absence of 18S and 28S loci on chromosome II (only the 5S locus is present on all 6 chromosome ends in examined species). Nucleomorph karyotype similarities within and between Hemiselmis species correlate with the molecular phylogenetic analyses performed thus far (Lane and Archibald 2008).

The nucleomorph genome of Hemiselmis andersenii CCMP644 has been completely sequenced (Lane et al. 2007). It is 571,872 bp in size and possesses 525 genes (472 protein genes and 53 non-mRNA genes). It encodes the identical set of 30 plastid-targeted proteins as Guillardia theta and also possesses a high gene density (1.09 kb/gene). The most striking feature of the H. andersenii genome is the complete absence of spliceosomal introns and genes for splicing RNAs and proteins (Lane et al. 2007). The G. theta nucleomorph genome possesses 17 small introns (42-52 bp) and genes for spliceosomal components (Douglas et al. 2001).

References

Archibald JM (2007) Nucleomorph genomes: structure, function, origin and evolution. BioEssays 29: 392-402

Clay B, Kugrens P (1999). Characterization of Hemiselmis amylosa sp. nov. and phylogenetic placement of the blue-green cryptomonads H. amylosa and Falcomonas daucoides. Protist 150: 297-310

Douglas SE, Zauner S, Fraunholz M, Beaton M, Penny S, Deng LT, Wu X, Reith M, Cavalier-Smith T, Maier UG (2001) The highly reduced genome of an enslaved algal nucleus. Nature 410: 1090-1096

Hoef-Emden K (2008) Molecular phylogeny of phycocyanin-containing cryptophytes: Evolution of biliproteins and geographical distribution. J. Phycol. 44: 985-993

Lane CE, Archibald JM (2006) Novel nucleomorph genome architecture in the cryptomonad genus Hemiselmis. J. Eukaryot. Microbiol. 44: 339-350

Lane CE, Archibald JM (2008) New marine members of the genus Hemiselmis (Cryptomonadales, Cryptophyceae). J. Phycol. 44: 339-350

Lane CE, van den Heuvel K, Kozera C, Curtis BA, Parsons B, Bowman S, Archibald JM (2007) Nucleomorph genome of Hemiselmis andersenii reveals complete intron loss and compaction as a driver of protein structure and function. Proc. Natl. Acad. Sci. USA 104: 19908-19913

Parke M (1949) Studies on marine flagellates. J. Mar. Biol. Ass. UK 28: 255-285

Santore UJ (1984) Some aspects of taxonomy in the Cryptophyceae. New Phytol. 98: 627-646