Characterization of marine diatom-infecting virus promoters in the model diatom Phaeodactylum tricornutum.

Viruses are considered key players in phytoplankton population control in oceans. However, mechanisms that control viral gene expression in prominent microalgae such as diatoms remain largely unknown. In this study, potential promoter regions isolated from several marine diatom-infecting viruses (DIVs) were linked to the egfp reporter gene and transformed into the Pennales diatom Phaeodactylum tricornutum. We analysed their activity in cells grown under different conditions. Compared to diatom endogenous promoters, novel DIV promoter (ClP1) mediated a significantly higher degree of reporter transcription and translation. Stable expression levels were observed in transformants grown under both light and dark conditions, and high levels of expression were reported in cells in the stationary phase compared to the exponential phase of growth. Conserved motifs in the sequence of DIV promoters were also found. These results allow the identification of novel regulatory regions that drive DIV gene expression and further examinations of the mechanisms that control virus-mediated bloom control in diatoms. Moreover, the identified ClP1 promoter can serve as a novel tool for metabolic engineering of diatoms. This is the first report describing a promoter of DIVs that may be of use in basic and applied diatom research.

Induction of ambicoloration by exogenous cortisol during metamorphosis of spotted halibut Verasper variegatus.

Cortisol, the main glucocorticoid in fish, increases during flatfish metamorphosis and peaks before the surge of thyroxine. A large body of evidence indicates the essential role of thyroxine in flatfish metamorphosis, whereas information on cortisol is limited. We administered cortisol to spotted halibut Verasper variegatus larvae in order to examine the effect on pigmentation during metamorphosis. Administration of 10 µg cortisol per mL of water from before the onset of metamorphosis (stage E) to metamorphic climax (stage G) induced the development of adult type pigment cells on the blind side of the metamorphosed juveniles and increased the occurrence of ambicolored juveniles. When 10 µg/mL cortisol was administered during stage D, stages E-F, stage G or stage H, only the administration during stages E-F induced the development of adult type pigment cells on the blind side. In addition, the expression of the gene dopachrome tautomerase (dct), a marker of melanoblasts, was enhanced at Stage E by cortisol administration. These results clearly indicated, for the first time, the enhancement of pigmentation by exogenous high-dose cortisol. Since endogenous cortisol is secreted in response to various kinds of stress in rearing conditions, these results indicate a possible influence of stress conditions in the occurrence of ambicoloration in flatfish.

Origin of adult-type pigment cells forming the asymmetric pigment pattern, in Japanese flounder (Paralichthys olivaceus).

The flatfish-specific asymmetric pigment pattern depends on the asymmetric appearance of adult-type pigment cells after the late metamorphic stages. To understand the mechanism enabling the formation of this asymmetric pattern, we investigated the behavior of pigment cell latent precursors in postembryonic Japanese flounder, Paralichthys olivaceus, by analysis of the expression patterns of pigment lineage markers (colony stimulating factor 1 receptor, dopachrome tautomerase, kit) and the DiI (DiO) labeling test for latent precursors. We found that, throughout the larval stages, pigment cell latent precursors were predominantly localized along the dorsal and ventral margins of the flank symmetrically and migrated continuously from these regions to the lateral sides symmetrically, and after late metamorphic stages these precursors differentiated into adult-type pigment cells on the lateral side asymmetrically. We conclude that adult-type pigment cells that form the asymmetric pigment pattern are continuously derived from the dorsal and ventral margins of the flank during larval development.