Marine fish egg hydration is aquaporin-mediated

The oocytes of marine pelagic teleosts swell abruptly by hydration during the reinitiation of meiosis (oocyte maturation) before ovulation. This process renders the eggs and early embryos buoyant in sea water, allowing their survival and dispersal in the ocean (1, 2). The major osmotic effectors for water uptake are free amino acids derived from the proteolysis of the oocyte major yolk proteins (3), but the membrane-associated mechanisms involved remain unknown. The time course of oocyte hydration in vitro in the marine teleost gilthead sea bream (Sparus aurata) revealed a progressive swelling of the oocyte, which appeared to accelerate within È2 hours after the complete proteolysis of yolk proteins (fig. S1). To investigate whether mo-lecular water channels or aquaporins (AQPs) were involved in this process, we performed reverse transcription–polymerase chain reaction (RT-PCR) to obtain AQP cDNAs from ovarian tissue. We isolated a cDNA that had a deduced amino acid sequence with six potential trans-membrane domains and two asparagine-proline-alanine motifs, the signature of the AQP superfamily (4). Heterologous expression in Xenopus laevis oocytes revealed that the encoded protein, like mammalian aquaporin-1 (AQP1) (5), was selectively permeable for water, and that its permeability could be inhibited by HgCl 2 and recovered by b