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BackgroundThe 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.ResultsEmbryos 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.ConclusionsThese 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.

It has been generally thought that a tiny freshwater fish, medaka (Oryzias latipes), has expanded its habitat into the Japanese archipelago as wet-rice cultivation spread across the region, and hence the distribution of medaka should be a matter of anthropological interest. However, there has been no study to verify this popular belief. To address the issue, we sampled wild medaka, and undertook genetic analyses of its populations. We collected 976 individual medaka from 13 local wild sites, which included 11 paddy-field irrigation channels and two ponds. The gene tree constructed based on nucleotide sequences of mitochondrial genome D-loop region showed no discrepancy in the topology, which is presumed to reflect the northern Kyushu origin of the southern Japanese medaka. Population genetic statistics indicated that the medaka populations in west Japan have greater genetic diversity (heterozygosity) than those in east Japan, supporting the hypothesis that the medaka originate from northern Kyushu. Hence, we argue that the current medaka distribution can be attributed to their past migration event(s) following the expansion of paddy fields from northern Kyushu to the eastern part of the Japanese archipelago.

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