Plasticity and patterns of evolution in mammalian salivary glands: comparative immunohistochemistry of lysozyme in...

Salivary gland plasticity was a significant adaptive feature in the mammalian radiation. This plasticity is reflected in remarkable and well-documented interspecific phenotypic variation in gland ultrastructure and in the chemical components of saliva. However, comparative data are still too sparse for determination of evolutionary trends that combine phenotypic patterns with evolutionary history and the actual secretory products. Although our theoretical approach assumes that natural selection has taken advantage of salivary gland plasticity in gene regulation, gland development, and secretory cell organelles and processes, it still is difficult to delineate the biological roles of secretory products in the context of ecological adaptation. In the present investigation we used immunohistochemical techniques and a polyclonal antiserum against lysozyme to compare the parotid and principal submandibular glands in a set of 12 species of microchiropteran bats. With this data set we used comparative methods and phylogenetic trees to develop the foundations for testable hypotheses about the molecular genetics and adaptive significance of lysozyme production in bats. By comparing immunohistochemical results with ultrastructure, lysozyme-like immunoreactivity was associated with serous secretory granules in parotid gland acinar calls, parotid gland intercalated duct cells, and submandibular gland demilune cells. Lysozyme production in submandibular gland demilune cells marks a point of evolutionary divergence between three families of insectivorous bats and four families composed of species with diverse diets (ranging from carnivory to nectarivory). In terms of diet, lysozyme-like immunoreactivity corresponded most strongly with feeding on hard-bodied insects, leading to the hypothesis that lysozyme serves as an important chitinase in bat saliva.