Origins of plastids and glyceraldehyde-3-phosphate dehydrogenase genes in the green-colored dinoflagellate Lepidodinium chlorophorum

The dinoflagellate Lepidodinium chlorophorum possesses "green" plastids containing chlorophylls a and b (Chl a+b), unlike most dinoflagellate plastids with Chl a+c plus a carotenoid peridinin (peridinin-containing plastids). In the present study we determined 8 plastid-encoded genes from Lepidodinium to investigate the origin of the Chl a+b-containing dinoflagellate plastids. The plastid-encoded gene phylogeny clearly showed that Lepidodinium plastids were derived from a member of Chlorophyta, consistent with pigment composition. We also isolated three different glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes from Lepidodinium-one encoding the putative cytosolic "GapC" enzyme and the remaining two showing affinities to the "plastid-targeted GapC" genes. In a GAPDH phylogeny, one of the plastid-targeted GapC-like sequences robustly grouped with those of dinoflagellates bearing peridinin-containing plastids, while the other was nested in a clade of the homologues of haptophytes and dinoflagellate genera Karenia and Karlodinium bearing "haptophyte-derived" plastids. Since neither host nor plastid phylogeny suggested an evolutionary connection between Lepidodinium and Karenia/Karlodinium, a lateral transfer of a plastid-targeted GapC gene most likely took place from a haptophyte or a dinoflagellate with haptophyte-derived plastids to Lepidodinium. The plastid-targeted GapC data can be considered as an evidence for the single origin of plastids in haptophytes, cryptophytes, stramenopiles, and alveolates. However, in the light of Lepidodinium GAPDH data, we need to closely examine whether the monophyly of the plastids in the above lineages inferred from plastid-targeted GapC genes truly reflects that of the host lineages.     

A haptophyte bearing siliceous scales: ultrastructure and phylogenetic position of Hyalolithus neolepis gen. et sp. nov. (Prymnesiophyceae, Haptophyta)

A haptophyte alga bearing hyaline but conspicuous scales was discovered in surface water samples of Shiribeshi Seamount, Japan. X-ray elemental analysis confirmed that silica was the major element in these scales. These scales were hat-shaped, ellipsoidal in top view, 4-6mum wide and 5-7mum long, perforated by several small pores, and were deposited on cells in several layers. Beneath the siliceous scale layers, organic scales were present, which are typical of haptophytes. The cells were non-motile despite having two short flagella hidden in the scale case. The haptonema, extended over the scale case, and one and a half times the cell length. The intracellular features were typical of haptophytes, including the peripheral endoplasmic reticulum (PER), Golgi cisternae with peculiar dilations, and the flagellar apparatus. The siliceous scales were produced in vesicles in the posterior region of the cell. Motile cells lacking silica scales were observed occasionally when cultures were maintained at lower temperatures and under oligotrophic conditions. This alga was described as Hyalolithus neolepis gen. et sp. nov. Phylogenetic analysis using the SSU rDNA and rbcL gene sequences indicated that Hyalolithus is a member of the Prymnesiales and falls in a clade including Prymnesium, Platychrysis, and Chrysochromulina polylepis. Based on these results, the evolutionary implications of the presence of silicified scales in haptophytes is discussed.