Left Lateral Plate Research Articles

Redefinition of the Dinoflagellate GenusAlexandriumBased onCentrodinium: Reinstatement ofGessneriumandProtogonyaulax, andEpisemicolongen. nov. (Gonyaulacales, Dinophyceae)

The genusCentrodiniumcontains oceanic and predominantly tropical species that have received little attention. Three species ofCentrodiniumwere examined using thecal plate dissociation, scanning electron microscopy, and molecular sequences. The apical horn ofCentrodinium intermediumandC. eminensis formed by the elongation of the fourth apical plate, and a second apical split into two plates. InC. punctatumtwo apical plates (2′ and 4′) almost completely encircle the apical pore plate (Po), while the contact with the plate 1′ in the ventral side is much reduced, and the plate 3′ does not reach the Po. Moreover, its left posterior lateral sulcal plate is longer than its right pair, while reversed in the typicalCentrodiniumspp. The sulcal posterior plate ofC. punctatumis located in the left-ventral side below the plates 1′′′ and 2′′′, while the sulcal posterior plate located in the right face below the plates 4′′′ and 5′′′ in the typicalCentrodiniumspp. Phylogenetic analyses based on the small and large subunit of the rRNA gene showed thatCentrodiniumspp. andAlexandrium affine/A. gaarderaeclustered as a sister clade of theAlexandrium tamarense/catenella/fraterculusgroups. The clade of the subgenusGessnerium, and the clade of the type species ofAlexandrium,A. minutum, with four divergent species, clustered in more basal positions. The polyphyly ofAlexandriumis solved with the split into four genera: (1)Alexandriumsensu stricto for the species of the clade ofA. minutumand four divergent species; (2) the reinstatement of the genusGessneriumfor the species of the clade ofA. monilatum; (3) the reinstatement of genusProtogonyaulaxfor the species of thetamarense/catenella/fraterculusgroups, and (4) the new genusEpisemicolongen. nov. forA. affineandA. gaarderae.New combinations in the generaGessnerium,Protogonyaulax,andEpisemicolonare proposed.

Progressive segmentation and systematic block rotation within a plutonic body: palaeomagnetism of the Cretaceous Kurihashi granodiorite in northeast Japan

SUMMARY Intensive palaeomagnetic and rock magnetic studies clarify the history of multiple block rotational events within an active plate margin. The Cretaceous Kurihashi Granodiorite in northeast Japan preserves multi-component remanent magnetization: high-unblocking-temperature (TUB) thermoremanent magnetization (TRM) acquired during the initial cooling of the granodiorite, and middle-/low-TUB thermoviscous remanent magnetization (TVRM) acquired during succeeding hydrothermal activities. The observed directional difference between the three magnetic components requires the first counter-clockwise rotation event within a part (>1 km) of northeast Japan as a result of left-lateral plate motion in the Late Cretaceous. Later clockwise rotation, probably related with right-lateral faulting during the course of the Miocene backarc spreading of the Japan Sea, is indicated by significant easterly deflections in the low-TUB component. On the other hand, scatter in the high-TUB TRM declinations implies that the second deformation phase was achieved as non-rigid rotation of fault-bounded blocks as wide as 100 m. The latest and most confined rotation events are confirmed by complicated deflections of the TRM and TVRM in a fault zone. A shear zone as wide as 30 m with dense fractures near the main fault is sharply divided into three compartments, which have been rotated around three orthogonal poles. Systematic block rotations may accommodate differential slips on fault planes within a plutonic body, genetically containing orthogonal fractures as reactivated intrinsic cooling joints. We interpret our results to show that the Kurihashi granodiorite has been progressively segmented since the Cretaceous under varied stresses.

Regulation of midline development by antagonism of lefty and nodal signaling.

The embryonic midline is crucial for the development of embryonic pattern including bilateral symmetry and left-right asymmetry. In zebrafish, lefty1 (lft1) and lefty2 (lft2) have distinct midline expression domains along the anteroposterior axis that overlap with the expression patterns of the nodal-related genes cyclops and squint. Altered expression patterns of lft1 and lft2 in zebrafish mutants that affect midline development suggests different upstream pathways regulate each expression domain. Ectopic expression analysis demonstrates that a balance of lefty and cyclops signaling is required for normal mesendoderm patterning and goosecoid, no tail and pitx2 expression. In late somite-stage embryos, lft1 and lft2 are expressed asymmetrically in the left diencephalon and left lateral plate respectively, suggesting an additional role in laterality development. A model is proposed by which the vertebrate midline, and thus bilateral symmetry, is established and maintained by antagonistic interactions among co-expressed members of the lefty and nodal subfamilies of TGF-beta signaling molecules.