Morphofunctional characterization and ploidy levels of cells in the bivalve digestive gland, with special reference to somatic polyploidy

This study investigates the histological organization and ploidy levels of the digestive gland cells in 29 species of marine, brackish water, and freshwater bivalves belonging to 5 subclasses. In all species studied, the digestive gland epithelium consists of two types of differentiated cells: digestive and basophilic. The nuclei of digestive cells contain the diploid quantity of DNA. Basophilic (secretory) cells often remain diploid also; however, in a number of species all or some of the cells showed an increase in the quantity of DNA per nucleus up to 4c. Tetraploidy of basophilic cells in several species of the subclass Anomalodesmata seems to be due to carnivory. In other species, no apparent correlation was found between selective polyploidy of basophilic cells and environmental conditions or biology traits of the bivalves. Additionally, there was no relationship between the occurrence of polyploid cells and the lifespan of the investigated species. In the bivalve mollusks, somatic polyploidy appears to be an adaptation; it is neither a tissue growth strategy nor a component of the cytodifferentiation program.