Sex-dependent carboxylesterase expression in the reproductive system of bivalve molluscs: an approach to substrate-specific detection of male associated polypeptide (MAP) after SDS-electrophoretic separation of crude gonad extracts

Esterase-like and fibronectin-like polypeptides share similar sex-cell-biased patterns in the gonad of hermaphroditic and gonochoric species of bivalve mollusks

The mechanism of sexualization of the tubular gonad in seawater bivalves is unknown, and no information regarding the genes involved in this process is yet available, except for the identification of esterase (Est)-like "male-associated polypeptide" in the male gonad of Mytilus galloprovincialis. Our present work reveals distinct protein profiles specific for the testicular or ovarian portion of the ovotestis of Pecten maximus. Two proteins exhibiting testis- or ovary-dependent enrichment in the ovotestis have been identified and partially characterized as Est-like and fibronectin (Fn)-like polypeptides, respectively. Immunofluorescence has demonstrated a close association between the localization of these polypeptides and the gonad tubule network and interstitial stroma of the ovotestis of P. maximus. We also present evidence of Est-like and Fn-like protein enrichment, respectively, in testicular and ovarian tissue in hermaphroditic, sex-reversal, and gonochoric species of seawater bivalves. Together, the results (1) strongly suggest that sex-cell-biased expression of Est-like and Fn-like polypeptides in gonad tissue is a widespread phenomenon among bivalve mollusks, despite the high diversification of their sexual patterns, (2) confirm and expand our previous demonstration of sex-biased protein expression in M. galloprovincialis, and (3) indicate a direct link between germ cell differentiation and sexual specialization of the bivalve somatic gonad.

Mussel MAP, a major gonad-duct esterase-like protein, is released into sea water as a dual constituent of the seminal fluid and the spermatozoon

Our interest in the comparative analysis of male reproductive-tract esterases in different animal groups has led us to undertake a detailed study of the Mytilus galloprovincialis male-associated polypeptide (MAP) throughout the mussel gonad-duct tract and at spawning. The results of this work indicate that MAP is a major protein in M. galloprovincialis semen, with dual presence in both sperm cells and cell-free seminal fluid. Shortly after spawning, the released sperm mass is subdivided in diffused cloudy-like and thread-shaped 'clots', in which a soluble-phase MAP may persist as long as the clots keep their compact form. Additional experiments involving the incubation of spawned spermatozoa at increasing Triton X-100 concentrations demonstrated that MAP is also strongly associated with sperm cells. These results were further validated by immunofluorescent staining, which revealed that MAP is localized in the mid-piece region of spawned spermatozoa. This unexpected finding raises the possibility that MAP may play a role in sperm fertility in bivalves. Using whole-mount histology and micromanipulation techniques, we studied the structural patterning of the mantle gonad-duct network and assessed the sampling of luminal contents from the ducts. Of particular interest is the observation that MAP content in the luminal fluid increases from the lumen of the spermatogenic tubules to that of the collecting gonad ducts, where MAP is detected at a very high concentration. These high levels may lead to a significant presence of MAP in semen and consequently to a prolonged survival of sperm spawned at sea. In addition, data related to the potential structural similarity between mussel MAP and esterase S of the Drosophila virilis ejaculatory bulb are presented and discussed. Finally, we show that the 64kDa protein of human semen reveals positive cross-reactivity with antibodies directed against Mytilus MAP and Drosophila esterase S. Taken together, the results reveal mussel MAP as the only esterase-like protein described so far whose distribution in the gonad and semen can be specifically associated with maturation, transport, emission and survival of spermatozoa outside.