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Summary How the mitosis of neuroepithelial stem cells is restricted to the apical ventricular area remains unclear. In zebrafish, the mosaic eyes rw306 ( moe / epb41l5 rw306 ) mutation disrupts the interaction between the putative adaptor protein Moe and the apicobasal polarity regulator Crumbs (Crb), and impairs the maintenance of neuroepithelial apicobasal polarity. While Crb interacts directly with Notch and inhibits its activity, Moe reverses this inhibition. In the moe rw306 hindbrain, Notch activity is significantly reduced, and the number of cells that proliferate basally away from the apical area is increased. Surprisingly, activation of Notch in the moe rw306 mutant rescues not only the basally localized proliferation but also the aberrant neuroepithelial apicobasal polarity. We present evidence that the Crb⋅Moe complex and Notch play key roles in a positive feedback loop to maintain the apicobasal polarity and the apical-high basal-low gradient of Notch activity in neuroepithelial cells, both of which are essential for their apically restricted mitosis.

We mapped 633 markers (488 AFLPs, 28 RAPDs, 34 IRSs, 75 ESTs, 4 STSs, and 4 phenotypic markers) for the Medaka Oryzias latipes, a teleost fish of the order Beloniformes. Linkage was determined using a reference typing DNA panel from 39 cell lines derived from backcross progeny. This panel provided unlimited DNA for the accumulation of mapping data. The total map length of Medaka was 1354.5 cM and 24 linkage groups were detected, corresponding to the haploid chromosome number of the organism. Thirteen to 49 markers for each linkage group were obtained. Conserved synteny between Medaka and zebrafish was observed for 2 independent linkage groups. Unlike zebrafish, however, the Medaka linkage map showed obvious restriction of recombination on the linkage group containing the male-determining region (Y) locus compared to the autosomal chromosomes.

Background The development of blood flow in the heart is crucial for heart function and embryonic survival. Recent studies have revealed the importance of the extracellular matrix and the mechanical stress applied to the valve cushion that controls blood flow to the formation of the cardiac valve during embryogenesis. However, the events that trigger such valve formation and mechanical stress, and their temperature dependence have not been explained completely. Medaka (Oryzias latipes) inhabits a wide range of East Asia and adapts to a wide range of climates. We used medaka embryos from different genomic backgrounds and analyzed heartbeat characteristics including back-and-forth blood flow and bradyarrhythmia in embryos incubated at low temperature. We also used high-speed imaging analysis to examine the heartbeat of these animals after transient exposure to low temperature. Results Embryos of the Hd-rR medaka strain exhibited back-and-forth blood flow in the heart (blood regurgitation) after incubation at 15°C. This regurgitation was induced by exposure to low temperature around the heartbeat initiation period and was related to abnormalities in the maintenance or pattern of contraction of the atrium or the atrioventricular canal. The Odate strain from the northern Japanese group exhibited normal blood flow after incubation at 15°C. High-speed time-lapse analysis of the heartbeat revealed that bradyarrhythmia occurred only in Hd-rR embryos incubated at 15°C. The coefficient of contraction, defined as the quotient of the length of the atrium at systole divided by its length at diastole, was not affected in either strain. The average heart rate after removing the effect of arrhythmia did not differ significantly between the two strains, suggesting that the mechanical stress of individual myocardial contractions and the total mechanical stress could be equivalent, regardless of the presence of arrhythmia or the heart rate. Test-cross experiments suggested that this circulation phenotype was caused by a single major genomic locus. Conclusions These results suggest that cardiogenesis at low temperature requires a constant heartbeat. Abnormal contraction rhythms at the stage of heartbeat initiation may cause regurgitation at later stages. From the evolutionary viewpoint, strains that exhibit normal cardiogenesis during development at low temperature inhabit northern environments.