Different expression og glycoconjugates during oogenesis in wild and captive bluefin tuna (Thunnus thinnus L.)

Bluefin tuna, one of the most important species in the fishery industry, is subject to intense fishing pressure which has raised concerns for stock survival. The decreased abundance and high commerciai value make it an obvious candidates for aquaculture, for which a knowledge of reproductive biology is fundamental. During oogenesis important morphological and molecular changes occurs. Glycoconjugates costitute a class of molecules involved in numerous biological functions, including the formation of cortical vesicles and zona radiata, endocytosis of yolk precursors and yolk storage. Although investigations of tuna reproductive biology have recently increased, data conceming glycoconjugates in the ovarian follicle are poor (Sarasquete et al., 2002). In an artificial environment, such as the domestication system, some form of reproductive dysfunction has been observed in teleosts (Zohar & Mylonas, 2001). The aim of this study was to compare the oligosaccharide sequences of glycoconjugates in developing ovarian follicles of wild and captive bluefm tuna by means lectin histochemistry. Adult bluefin tuna (99-140 Kg) were caught during lune in South Thyrrenhian Sea, or sacrificed in the experimental cages of Puerto de Mazarron (Spain). Fragments of ovaries were fixed in Bouin's solution for 12-24 hours and embedded in paraffin wax. Sections (4 um thick) were stained using two biotinilated lectins (MAH and SNA) and eight HRP-labeled lectins (PNA, DBA, HPA, Con A, WGA, GSA I-B4, UEA I, LTA). There were no obvious ovaries morphological differences between the oaries from the wild and captive tuna. The main differences in the lectin-binding pattern between the two samples were observed from lipid stage follicles until late vitellogenic stage follicles. Lipid stage oocytes (0 110-220 nm) from captive tuna showed granules positive to MAH, SNA, KOH-sialidase (s)- PNA, DBA, SBA, Con A, WGA, and GSA 1-B4, as well as intranuclear affmity for MAH, SNA, PNA, SBA, and UEA I. Follicular cells of vitellogenic follicles (size 220-500 um) showed reactivity with KOH-sialidase-PNA, GSA 1-B4, and LTA in wild specimens but not in captive ones; zona radiata displayed lectin-binding sites only in thè outer layer, which stained with MAH and Con A in both the samples, but only reacted with SNA, KOH-sialidase-PNA, UEA I, and LTA in wild tuna. The cortical ooplasm from captive specimens was characteristically marked with SNA, WGA, and UEA I. Lastly, thè yolk globules evidenced reactivity with MAA, Con A and LTA in thè oocytes from wild specimens. The observed difference in thè oligosaccharide sequences of ovarìan follicles from thè two bluefin tuna groups may results from domestication-induced stress on captive animals, which can affect both the nervous system and the endocrine system.