Diversity of Balbiani body formation in internally and externally fertilizing representatives of Osteoglossiformes (Teleostei: Osteoglossomorpha)

During the early stages of oogenesis, the Balbiani body is formed in the primary oocytes. It consists of the Golgi apparatus, endoplasmic reticulum (ER), and numerous mitochondria aggregated with germ plasm, but its form may differ among animals. Hypothetically, during oogenesis oocytes become adapted to future development in two different environments depending on internal or external fertilization. We aimed to investigate, using light and transmission electron microscopy, the development of the Balbiani body during oogenesis in representatives of Osteoglossiformes, one of the most basal Teleostei groups. We analyzed the structure of oogonia and primary oocytes in the internally fertilizing butterflyfish Pantodon buchholzi and the externally fertilizing Osteoglossum bicirrhosum and Arapaima gigas to compare formation of the Balbiani body in relation to modes of fertilization. We demonstrated that the presence of the germ plasm as well as the fusion and fission of mitochondria are the conserved features of the Bb. However, each species exhibited also some peculiar features, including the presence of three types of ooplasm with different electron density and mitochondria-associated membranes in P. buchholzi; annulate lamellae, complexes of the Golgi apparatus, ER network, and lysosome-like bodies in O. bicirrhosum; as well as karmellae and whorls formed by the lamellae of the ER in A. gigas. Moreover, the form of the germ plasm observed in close contact with mitochondria differed between osteoglossiforms, with a "net-like" structure in P. buchholzi, the presence of numerous strings in O. bicirrhosum, and irregular accumulations in A. gigas. These unique features indicate that the extreme diversity of gamete structure observed so far only in the spermatozoa of osteoglossiforms is also characteristic for oocyte development in these basal teleosts. Possible reason of this variability is a period of about 150 million years of independent evolution of the lineages.