| Left-right symmetry plays an important role in the evolution of organisms, the formation of left-right axis is crucial important to the development of left-right symmetry animal. But there is also some evidence that vertebrate symmetrical characters were broken in the evolution. For example, although the out shape of human body is symmetric, the internal organs are located asymmetrically, such as the heart is located on the left side while the liver on the right side. Another example is that the zebrafish parapineal organ is located on the midline of brain, but at some certain development stage it would migrate to the left side and start the asymmetry morphogenesis. The most famous symmetry broken example in vertebrate is flatfish asymmetry. The most remarkable character of flatfish is that one eye will migrate through the head to the other side, and the skull twist towards the ocular side, then start the asymmetry morphogenesis process. The controversy about the genesis of flatfish ended in 2008 when the intermediate species fossil was found by Friedman, and it made us clear that the evolution process of eye migration of flatfish was gradual, but the mechanism of eye migration is still unclear. Most of the research thought that the eye migration was caused by the twist of skull. However, the skull not only twisted later than the eye migration, but was also far from the migrating eye, accordingly, the most effective force should not come from the skull twist. Furthermore, it was clear that the metamorphosis of flatfish was regulated by thyroid hormone, but the research on the receptor gene expression was limited in the whole body level. So the research on the spatial-temporal expression pattern is important to find the co-relationship between the thyroid hormone receptor expression and the tissue or organ development.In this research, we thought the force of eye migration was from the cell proliferation of suborbital tissue, but not from the twist of skull. Whole-mount BrdU and whole-mount in situ hybridization were employed to research the pattern of cell proliferation and thyroid hormone receptor expression during metamorphosis. Four species in pleuronectiforms were investigated, including Solea senegalensis (left eye migration), Cynoglossus semilaevis (right eye migration), Pseudopleuronectes yokohamae (left eye migration), and Paralichthys olivaceus (right eye migration). We found that the cell proliferation was left-right asymmetrical during eye migrating. Before eye migration, the cell proliferation in the suborbital tissue of migrating eye was stronger than that in the other side, the left-right side asymmetric cell proliferation was still obvious at climax of metamorphosis, and the cell proliferation in the suborbital tissue formed a proliferation band. The eye migration could be blocked by the inhibitor of cell proliferation microinjected into the suborbital tissue of migrating eye via microinjection technology, and the no eye migration symmetrical flatfish were obtained for the first time. Further detection results of cell proliferation confirmed that the cell proliferation was inhibited for certain, it proved that the force of eye migration was from the cell proliferation of suborbital tissue of migrating eye, and it corrected the hypothesis that eye migration was caused by skull twist. Additionally, the expression pattern of thyroid hormone receptor (TR) during eye migration was researched for the first time in Paralichthys olivaceus by whole-mount in situ hybridization. It was found that four subtypes of TR were also expressed in the tissue around the eyes, including suborbital tissue, suprorbital tissue, lateral ethmoid, and frontal bone, and TRβ1 was the most possible candidate gene of regulating eye migration. It was also found that TR expression was also related with other metamorphosis events, such as the growth of somite, skeleton development, fin development, digestion system reconstruction, et al. |
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