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Jeseter (Acipenser) představuje evoluční přechod od holoblastického k meroblastickému rýhování a unikátní způsob vývoje střeva
SHAH, Mujahid Ali
A vertebrate embryo's cleavage pattern is either holoblastic (complete) or meroblastic (partial). Holoblastic cleavage is thought to be ancestral to vertebrates and is most likely to occur in amphibians, mammals, and chondrosteans. Meroblastic cleavage has evolved five times in vertebrate lineages, including hagfish, elasmobranchs, coelacanths, teleosts, and amniotes. In holoblastic cleavage (as in Xenopus laevis embryos), all blastomeres contribute to one of the germ layers. On the contrary, in meroblastic cleavage pattern (as in teleosts and amniotes-including birds and reptiles), only the animal pole contributes the formation of the germ layers. The transition from holoblastic to meroblastic is usually occurred by an increase in egg size in comparison to the lineage's ancestral state. Sturgeons evolved about 200 million years ago (mya). Their eggs are significantly larger than that of X. laevis. Despite the variation in sizes, their embryos retain nearly characteristics the same as that of X. laevis. Nevertheless, vegetal blastomeres of sturgeons are bigger and divide slower than that of X. laevis. It was speculated that vegetal blastomeres of sturgeon are extraembryonic as in yolk of teleost (zebrafish) and Yolk cells of (YCs) of bichir-earliest diverged living group of actinopterygian fishes, agnathan lampreys (Petromyzontidae)-an extant lineage of jawless fishes and an Eleutherodactylus coqui (direct developing frog). Furthermore, the gut development pattern of sturgeon (Acipenser) and its evolutionary conservation was poorly understood so far. First, we developed the robust technique for specific blastomeres inhibition of sturgeon embryos using diatoms-derived polyunsaturated aldehydes, 2, 4-Decadienal (DD; a model aldehyde for experimental studies). The sturgeon's embryos were injected with optimal DD percentage (0.01 v/v) and subsequently irradiating them by visible light (91.15 - 44.86 W m2). Notably, DD plus light, and not DD injection or light irradiation alone can inhibit cleavage. Furthermore, qPCR-tomography revealed that localized pattern of maternal mRNA remained constant through animal-vegetal axis in partially cleaved embryos when compared to normal. Second, fate-mapping of sturgeon vegetal blastomeres revealed that these blastomeres gave rise to primordial germ cells (PGCs), and the rest of the descendants were vegetal yolk cells. Plastic section histology showed that the nuclei of vegetal yolky cells sharply declined as embryos developed. In addition, inhibition of vegetal blastomeres, RT-qPCR and BrdU pulse revealed that yolk cells become transcriptionally inactive after mid-blastula transition. Here, our results suggested that the meroblastic cleavage in actinopterygian lineage had evolved by the fusion of vegetal blastomeres, which is parallel to the closely related group, e.g., gar (Lepisosteidae), that evolved at approximately 57 mya. Lastly, we continued the observation of sturgeon gut development and its comparison with other taxa including holoblastic (X. laevis, bichir, and mice) and meroblastic (chicks, gars, and zebrafish) representatives. For this purpose, we used histology, in-situ hybridization (HCR), and Immunohistochemistry. We found that sturgeon's endodermal cells formed the Archenteron (primitive gut) as frog and bichir. However, these cells continued to proliferate lateroventrally to encompass a massive amount of yolk mass to give rise "yolk inside the gut." Cross-species comparison revealed that sturgeon retained a unique mode of gut developmental pattern during vertebrate evolution. In conclusion, our current findings suggest that sturgeon embryo development represents a distinct transition from holoblastic to meroblastic cleavage, as well as a distinct archaic mode of gut-endoderm development.
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