Hemiselmis

Abstract: Summary The unique and diagnostic characteristics of the Cryptophyceae are discussed and present knowledge of their taxonomy is summarized. It is proposed that the pigmented organisms can be assigned to four genera, Hemiselmis, Chroomonas, Cryptomonas and Pyrenomonas nom. nov.; the characteristics of these genera are reviewed. The validity of the pigmented genera Isoselmis, Plagioselmis and Rhodomonas and the apochlorotic genera Chilomonas, Cyathomonas and Phyllomitus is considered and present knowledge of their taxonomy is summarized.

Abstract: Cryptophytes are an enigmatic group of unicellular eukaryotes with plastids derived by secondary (i.e., eukaryote-eukaryote) endosymbiosis. Cryptophytes are unusual in that they possess four genomes–a host cell-derived nuclear and mitochondrial genome and an endosymbiont-derived plastid and 'nucleomorph' genome. The evolutionary origins of the host and endosymbiont components of cryptophyte algae are at present poorly understood. Thus far, a single complete mitochondrial genome sequence has been determined for the cryptophyte Rhodomonas salina. Here, the second complete mitochondrial genome of the cryptophyte alga Hemiselmis andersenii CCMP644 is presented. The H. andersenii mtDNA is 60,553 bp in size and encodes 30 structural RNAs and 36 protein-coding genes, all located on the same strand. A prominent feature of the genome is the presence of a ~20 Kbp long intergenic region comprised of numerous tandem and dispersed repeat units of between 22–336 bp. Adjacent to these repeats are 27 copies of palindromic sequences predicted to form stable DNA stem-loop structures. One such stem-loop is located near a GC-rich and GC-poor region and may have a regulatory function in replication or transcription. The H. andersenii mtDNA shares a number of features in common with the genome of the cryptophyte Rhodomonas salina, including general architecture, gene content, and the presence of a large repeat region. However, the H. andersenii mtDNA is devoid of inverted repeats and introns, which are present in R. salina. Comparative analyses of the suite of tRNAs encoded in the two genomes reveal that the H. andersenii mtDNA has lost or converted its original trnK(uuu) gene and possesses a trnS-derived 'trnK(uuu)', which appears unable to produce a functional tRNA. Mitochondrial protein coding gene phylogenies strongly support a variety of previously established eukaryotic groups, but fail to resolve the relationships among higher-order eukaryotic lineages. Comparison of the H. andersenii and R. salina mitochondrial genomes reveals a number of cryptophyte-specific genomic features, most notably the presence of a large repeat-rich intergenic region. However, unlike R. salina, the H. andersenii mtDNA does not possess introns and lacks a Lys-tRNA, which is presumably imported from the cytosol.

Abstract: Hemiselmis rufescens Parke and three species of the genus Chroomonas have been examined by electron microscopy. They demonstrate certain characteristic features of the Cryptophyceae, such as the presence of trichocysts and the arrangement of thylakoids in pairs in the plastid. The prominent pyrenoid of the genus Chroomonas is penetrated longitudinally by a tongue of cytoplasmic matrix which originates from between the two pairs of plastid membranes. In the genus Hemiselmis, however, the pyrenoid is traversed by a pair of thylakoids. The nucleus, Golgi apparatus and Corps de Maupas also occupy characteristic positions. These features indicate a close relationship between these two genera and the other major genus Cryptomonas, and support the suggestion that the Cryptophyceae is a discrete taxonomic group.