Marine Biology (1996) 126:77-89 9 Springer-Verlag 1996 A. T. Mikhailov 9 M. Torrado 9 J. M6ndez M. J. Lopez Annual cycle of expression of connective tissue polypeptide markers in the mantle of the mussel Mytilus galloprovincialis Received: 23 February 1996/Accepted: 22 March 1996 Abstract We suggest that gonad development in the mantle tissue of the bivalve mollusc Mytilus gaIlo- provincialis Lmk. is an example of epithelial/mesen- chymal interactions (i.e. soma/germline interactions) and morphogenesis in the adult state. According to this concept, the aim of the present study was to use bio- chemical and immunochemical methods for identifying and characterizing the mantle cell polypeptide markers whose expression is seasonally and morphogenetically regulated. We showed for the first time that M. gallo- provincialis mantle, of both males and females, contains polypeptides (with an apparent mol. wt of 45 to 53 kDa) specific for connective tissue ("mantle connect- ive tissue polypeptides"; MCTPs). Electrophoretic, im- munoblotting and immunofluorescent experiments demonstrated that MCTPs are primarily localized in the adipogranular (ADG) cells, and their expression in the mantle is seasonally regulated. There is a positive correlation between MCTP expression and connective tissue volume in the mantle. MCTPs are overexpressed during the rest period, when the mantle consists of connective tissue mainly, whereas mature gonads con- tain only trace amounts of MCTPs. Moreover, there is a temporal correlation between the onset and decrease of MCTP expression and the appearance and disap- pearance of the ADG cells in the mantle. MCTP local- ization in the mantle tissue should not be associated with the ADG cells only, because positive immuno- fluorescence was also detected in follicle membranes Communicated by O. Kinne, Oldendorf/Luhe A. T. Mikhailov (I~) . M. Torrado 9 J. M6ndez 9 M. J. L6pez Institute of Sciences of the Health and Department of Cell and Molecular Biology, University of La Corufia, Campus de Zapateira s/n, E-15071 La Corufia, Spain A. T. Mikhailov Koltzov Institute of Developmental Biology, Russian Academy of Sciences, Moscow, Russia (but not in germ cells) and superficial mantle epi- thelium. Using immunoblotting and immunofluores- cence, MCTPs were found in the connective tissue of the mantle, posterior adductor muscle and visceral mass, but not gills, foot or hepatopancreas. Possible mechanisms by which MCTPs could participate in the annual processes of mantle gonad/connective tissue development and involution are discussed. Introduction In Mytilus galloprovincialis and M. edulis, paired mantle lobes are the main organs where gonad devel- opment occurs, although a small amount of the genital tissue can also differentiate in the visceral mass and mesosoma (Pieters et al. 1980; Seed and Suchanek 1992). The mytilid gonad is not a permanently differen- tiated organ, but consists of gonoducts that invade the mantle somatic tissue during each annual reproductive cycle. In the mantle, two main somatic cell types are found: adipogranular (ADG) cells and vesicular con- nective tissue (VCT) cells (Lubet 1959; Lowe et al. 1982; Gabbott 1983; Pipe 1987a, b). These cells form a con- nective tissue network in which development and mor- phogenesis of the genital tissue proceed. In addition to these types of mantle connective tissue cells, the somatic support for gamete differentiation is also provided by epithelial cells of the acini (follicle cells in females and Sertoli-like cells in males) and gonoducts. The connective tissue: gonad tissue ratio is variable and depends on the stage of the annual cycle (Seed 1969; Lowe et al. 1982, 1994; Pipe 1987a, b; Mathieu 1987; Peek et al. 1989; Shcherban and Abolmasova 1991; Mikhailov et al. 1995; Ogueta et al. 1995; Villalba 1995). In general, mantle connective tissue displays an annual cycle pattern which is opposite to that of the gonad tissue (see Fig. 1). In Mytilus galloprovincialis collected in Galicia (NW Spain) in autumn, connective tissue occupies most of the mantle volume and the