The Synthesis of Vitellogenin in the Mussel Mytilus Galloprovincialis: An Interdisciplinary Approach

Vitellogenins (VTGs) represent the most important precursors of vitellins (Vns), the egg yolk proteins utilized by oviparous and ovoviviparous animals to guarantee the embryo development. In mature females, VTGs are synthesized in different districts under the control of sexual hormones, as 17β-estradiol, and transferred to the growing oocytes via the bloodstream. In the oocytes, VTGs are processed by cathepsins into Vns that are then stored into the yolk platelets. Differently, in immature females and in the males VTGs are not synthesized, but their synthesis may be activated by xeno-estrogens, an heterogeneous group of environmental contaminants that modify the endocrine reproductive system with consequent production of molecules not present physiologically, as VTG. For this reason, VTG production, in immature females and males, is an useful biomarker to study the health of aquatic and terrestrial ecosystems. Till now, the VTG was utilized as biomarker mostly in fish, and more recently in mussels too. Such organisms indeed have acquired a prominent role as sentinel species in aquatic pollution-monitoring programs. Differently from fish, the knowledge of VTG synthesis and its endocrine control in the mussels is scant. At this regard we carried out an interdisciplinary investigation to identify the site/s of VTG synthesis in the mussel Mytilus galloprovincialis, that may represent a useful sentinel species in the Mediterranean sea. In this organism we have investigated two districts, the ovary and the digestive gland, that are the most probably sites of VTG synthesis in the mussels. Results of RT-PCR, in situ hybridization, electron-microscopy and immunohistochemistry experiments, taken together, provide evidence that VTG synthesis occurs only in the ovary of females, and particularly both in the oocyte (autosynthesis) and follicle and adipogranular cells (eterosynthesis). The possibility that in the mussel Mytilus galloprovincialis VTG may be used as biomarker in ecotoxicology is also discussed.