Close association between vasa-positive germ plasm granules and mitochondria correlates with cytoplasmic localization of 12S and 16S mtrRNAs during zebrafish spermatogenesis

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The phenomenon of the cytoplasmic localisation of mitochondrial ribosomal subunits (12 S mitochondrial rRNA and 16 S mitochondrial rRNA) has been discovered by scientific teams working with spermatogenic cells of mice. Previous reports showed that the release of mitochondrial substance occurs during interaction of mitochondria with the germ plasm granules (GG). To determine if the interplay between the vasa-positive GG and the mitochondria is associated with cytoplasmic localisation of mtrRNAs, we studied the spermatogenic cells of zebrafish, Danio rerio. It was revealed that in type A undifferentiated spermatogonia the GG did not contact mitochondria, and the extra-mitochondrial localisation of the mtrRNAs was not found. In type A differentiated spermatogonia, the amount of GG in contact with mitochondria increased, but the extra-mitochondrial localisation of the mtrRNAs was not found either. In type B late spermatogonia, which are pre-meiotic cells, the GG/mitochondrion complexes were typically found in contact with the nucleus. This stage was associated with the intra-mitochondrial localisation of GG-originated vasa and extra-mitochondrial localisation of 12 S mtrRNA and 16 S mtrRNA. Until the onset of meiosis, which was determined by the observation of synaptonemal complexes in zygotene-pachytene spermatocytes I, the GG/mitochondrion complexes disappeared, but both types of mtrRNAs persisted in the cytoplasm of spermatids and spermatozoa.