In vivo and in vitro induction of germinal vesicle breakdown in a freshwater bivalve, the zebra mussel Dreissena polymorpha (Pallas)

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Abstract
Oocyte maturation and germinal vesicle breakdown (GVBD) was induced in zebra mussel (Dreissena polymorpha) oocytes by in vivo and in vitro application of serotonin (5‐hydroxytryptamine, 5‐HT), and in vitro application of 8‐hydroxydipropylaminotetralin hydrobromide (8‐OH‐DPAT, a 5‐HT1A receptor agonist). Oocytes initiated GVBD approximately 30 minutes after exposure to 5‐HT (10−3 M) at 23°C, and by 40–50 minutes after exposure, most oocytes lacked a germinal vesicle. An exposure time to 5‐HT as brief as five to ten minutes was required to trigger the maturation process, which terminates in spawning of fertilizable oocytes in nearly all mussels. But, with an exposure time of less than five minutes, spawning was reduced by application of 10−4 M methiothepin (a potent blocker of 5‐HT‐induced spawning in zebra mussels). Thus, the sequence of oocyte maturation events in zebra mussels was determined. Oocytes are arrested at the germinal vesicle stage (prophase I) within the ovary. 5‐HT reinitiates the maturation process, including GVBD and spawning of metaphase I oocytes, which are further arrested until fertilization. To our knowledge this is the first demonstration of oocyte maturation induction by serotonergic ligands in a freshwater bivalve.