Male-associated polypeptide (MAP) expression in different compartments of the reproductive system of the mussel Mytilus galloprovincialis: immunocytochemical and western blot study

Mytilus mussels are characterized by annually repeated reproduction which is associated with subsequent growth, morphogenesis, breakdown and redevelopment of the gonad and reproductive tract into mantle mesenchyme. We present a description of the expression of the male-associated polypeptide (MAP; see Mikhailov et al. 1995) in different compartments of the male reproductive system as well as in mantle gonad-supporting tissue. MAP is expressed in both gonad and mantle structures in dynamic patterns that show a substantial overlap in terms of dependence on the stage of gonad development/involution. In general, the total MAP concentration directly correlates with the volume of gonad tubule/duct structures but inversely correlates with mantle connective tissue cell fraction. A maximum of MAP expression is reached in the fully ripe male gonad. MAP is localized around gonad tubules/ducts, in the gonoduct epithelium, membranes of follicle-like structures as well as in the extracellular fiber-like structures of the mantle. However, we also demonstrate unique sites of MAP accumulation in the lumen of gonad follicle-like tubules and in ductal fluid. The latter is characterized by a very high MAP concentration. MAP is also detected in sperm-containing cell suspension obtained by gonad biopsy which we interpret as a result of the adsorption of MAP on mature spermatozoa. The results obtained should be taken into consideration in the interpretation of possible MAP functions since they seem to point to MAP as a major component of ductal (seminal) fluid of the male reproductive tract. It is likely that MAP is able to complement the processes of sperm terminal differentiation and maturation. In addition, we demonstrate that the male-predominant character of MAP expression is restricted by gonad-containing tissues (i.e., mantle and visceral mass) only, although the polypeptide is also detected in other somatic organs in both males and females.