Evolutionary regularities of development of somatic polyploidy in salivary glands of gastropod mollusks: V. Subclasses Opisthobranchia and Pulmonata

Salivary glands of 25 species of Opisthobranchia and Pulmonata gastropod mollusks were studied with the use of histochemical methods and cytophotometry of DNA in cellular nuclei. In the secretory epithelium, cells of three main types are identified: granular cells (with granular glycoprotein inclusions), mucocytes-I (containing sulphatized acid mucopolysaccharides), and mucocytes-II (containing neutral and acid nonsulphatized polysaccharides and protein), as well as epithelial ciliated cells and cells of ducts. It is shown that the salivary gland secretory cells of all studied mollusk species are polyploidized, but to different degree. In most species, the maximal degree of polyploidy estimated by the DNA content amounts to 64–128c. Giant polyploidy—up to 4096c—is revealed in salivary gland cells of Tritonia diomedea. The functional properties due to the peculiarities of nutrition of different molluscan species and phylogenetic tendencies of development of somatic polyploidy in the class of Gastropoda are discussed. The high degree of obligatory polyploidization revealed in salivary gland cells of the opisthobranchian and pulmonate mollusks is considered as a peculiar cytological arogenesis as compared with allogenic, facultative, and slight manifestations of polyploidy in prosobranchian gastropod mollusks. The probable causes of such differences are due to the euthyneural type of organization of the central nervous system and to the giant neuronal polyploidy in opisthobranchian and pulmonate mollusks. The causes, mechanisms, and significance of such correlations are so far unclear.