Carleton J. Phillips

Ultrastructural diversity in chiropteran salivary glands

Mammalian salivary glands, which producea huge variety of glycoproteins and mucosubstances, as well as engaging in intense transportof electrolytes, have an obvious appeal for morphologists, physiologists, and biochemists. Despite the versatility of these organs, studies onsalivary glands have been restricted largely toa few commonly available laboratory animals.Comparative studies, though still relativelysparse, have shown that salivary gland histologyis generally similar throughout the class Mammalia, but that both the acinar and duct cellsexhibit considerable ultrastructural variationbetween species [1–3]. To illustrate this morphological variability, the present chapter surveysultrastructural aspects of the parotid, submandibular, and accessory submandibular salivaryglands in bats, order Chiroptera.

Interspecific Comparisons of Immunohistochemical Localization of Retinal Neurotransmitters in Four Species of Bats

Four ecologically distinctive Neotropical bat species of the family Phyllostomidae were collected and their retinae surveyed immunohistochemically for the presence of neurotransmitter candidates: glucagon, somatostatin, vasoactive intestinal peptide, substance P (SP), methionine enkephalin, serotonin (5-HT) and two enzymes, glutamic acid decarboxylase (GAD) and tyrosine hydroxylase (TOH). In all four species immunoreactivity (IR) to GAD, TOH and SP was found. GAD-IR and SP-IR showed little interspecies variation whereas TOH-IR differed interspecifically in a pattern that matched the systematic relationships and the ecological characteristics of the bats. 5-HT-IR, which has not previously been reported from mammalian retinae, was found in fibers in the inner nuclear layer and in the outer and inner plexiform layers of Macrotus waterhousii, which is a relatively underived insectivorous phyllostomid bat, but was not found in the retinae from frugivorous or nectarivorous species.

Comparative anatomy and general histology of tongues of long-nosed bats (Leptonycteris sanborni and L. nivalis) with reference to infestation of oral mites.

Anatomically and histologically, tongues of two species of long-nosed bats, Leptonycteris sanborni and L. nivalis , are generally similar. The tongue in each species is highly adapted for pollen- and nectar-feeding, which is characteristic of glossophagine bats. Differences in the singly-pointed papillae located along the lateral and dorsolateral surfaces were studied in detail by means of histological techniques and the scanning electron microscope. It was concluded that differences in these papillae may well account for the absence of oral mites ( Radfordiella oricola : Macronyssidae) in the oral cavity of L. sanborni .

Systematic Implications of Comparative Ultrastructure of Secretory Acini in the Submandibular Salivary Gland in Artibeus (Chiroptera: Phyllostomidae)

The principal submandibular salivary gland in four species of the stenodermatine genus Artibeus was examined ultrastructurally to determine the possible intrageneric systematic value of salivary gland morphology. In terms of general architecture, the submandibular glands are similar in Artibeus cinereus , the previously studied A. phaeotis, A. jamaicensis, A. lituratus , and A. concolor . The secretory acini in all five species consist of serous cells that often are capped by a seromucous demilune. Secretory granules in the serous cells are large and uniformly electron-dense. In contrast, secretory granules in the seromucous cells have a complex substructure that varies from species to species. The most highly organized seromucous granules are found in A. cinereus and closely resemble those in A. phaeotis . The secretory granules in Artibeus jamaicensis and A. lituratus differ slightly from these other two species, but are extremely similar to one another. Artibeus concolor differs from all of the other four in having seromucous granules that are ultrastructurally distinctive. Similarities and differences in seromucous secretory granules thus match independently derived genic data, showing that the ultrastructure of salivary gland secretory product can have systematic significance. This also is the first reported instance in which ultrastructural differences in a cell product have been correlated with intrinsic genic differences as opposed to functional state or some other factor.

Unique Salivary Glands in Two Genera of Tropical Microchiropteran Bats: An Example of Evolutionary Convergence in Histology and Histochemistry

Submandibular salivary glands were examined in 38 genera of microchiropteran and megachiropteran bats. In species of two of these, Trachops cirrhosus (a Neotropical phyllostomid) and Megaderma lyra and M. spasma (Old World megadermatids), the histology of the accessory submandibular salivary gland was unique among mammals. In both Trachops and Megaderma this gland consists of large, follicle-like structures rather than the usual secretory endpieces. In both genera the secretory cells (except for a few cells in Trachops ) are completely negative to a variety of histochemical staining procedures for glycoproteins, so the secretory product is unknown; in Trachops scattered or clumped mucous cells in the follicular walls contain acidic and neutral glycoproteins. Given the phylogeny of these genera it is likely that this previously unknown histological arrangement evolved independently in each family and thus represents a remarkable example of histological and histochemical convergence. We postulate that the evolution of this unique salivary gland is related to feeding on frogs.

Mammalian Evolution at the Cellular Level

The development of new techniques in molecular biology has greatly advanced our understanding of the evolutionary and systematic biology of mammals. Before 1965 our comprehension of mammalian evolution and our approach to systematics rested upon our ability—sometimes strictly intuitive—to interpret the significance of interspecific differences in gross anatomic features. Since then, we have been increasingly able to study proteins, chromosomal structure and organization, comparisons of individual genes, gene loci, and nuclear and mitochondrial DNA. Evolutionary and systematic investigations in mammalogy now routinely include biochemistry of proteins as well as gross anatomy, but often neglect everything in between.

Unusual smooth endoplasmic reticulum in submandibular acinar cells of the male round-eared bat, Tonatia sylvicola

A form of crystalloid smooth endoplasmic reticulum (SER) is described in the cytoplasm of seromucous acinar cells in the submandibular salivary gland of male round-eared bats, Tonatia sylvicola. The structure consists of an array of membranous tubules organized in an hexagonal pattern within a latticework of 12-nm filaments running in three planes. At the periphery of the organelle, the tubules are in direct continuity with SER or with transitional elements. Although the function of the organelle is unknown, data are presented that suggest that it is steroidogenic. Given the biology of T. sylvicola, it seems likely that the salivary secretions play some role as a species isolating mechanism or in sexual behavior, or both. Such a salivary gland system has not previously been reported for any mammals.

Comparative retinal pigment epithelium and photoreceptor ultrastructure in nocturnal and fossorial rodents: the Eastern woodrat, Neotoma floridana, and the plains pocket gopher, Geomys bursarius

In fossorial plains pocket gophers ( Geomys bursarius ) the eyes are extremely small and the cells of the retinal pigment epithelium (RPE) have extensive, highly developed apical microvilli and numerous melanolysosomes. The neural retina consists of both cone (up to 25%) and rod photoreceptor cells. The cones often are clustered and surrounded by rods. Ultrastructurally, the cones differ from rods in position and appearance of nuclei, overall shape of cell body, disc membrane, and presence of pedicle-type synaptic terminals. By way of comparison, in nocturnal eastern woodrats ( Neotoma floridana ) the RPE cells have poorly developed apical microvilli and relatively few melanolysosomes. The neural retina consists almost entirely (99%) of abundant and densely packed rod photoreceptors characterized by unusually long outer segments with a high number of discs. Numerous significant quantitative differences between RPE and photoreceptors of Neotoma and Geomys serve to underscore further the dissimilarities of the retinas of these rodents. Although G. bursarius and other species of gophers live in dark or semi-dark burrow systems and have eyes that are reduced in size, the RPE and neural retina is characterized by an intermediate (diurnal-nocturnal) structural pattern suggestive of good visual acuity. The lack of any obvious specialization in RPE or photoreceptor morphology is striking and could imply that the generalized eye in gophers is representative of the eyes found in their ancestors. If so, gophers probably evolved from ground-dwelling diurnal species.

Comparative Histology of Molar Dentitions of Microtus and Clethrionomys, with Comments on Dental Evolution in Microtine Rodents

The molar dentitions of two microtine rodents, Clethrionomys gapperi and Microtus penmylvanicus , were compared histologically. Analysis of cellular and tissue differences between the highly specialized, ever-growing molars of M. pennsylvanicus and the more primitive molars of C. gapped enabled development of a preliminary model of evolution of molars in Microtus . For this model a single mutation eliminating determination of the coronal cervex, thus allowing for continuous production of enamel and dentin, could account for ever-growing molars. At least three preadaptive factors (cementoid buttresses, cementoid covering of mature enamel, and passive closure of the pulp cavity with reparative dentin) have influenced selection of ever-growing molars.

Three patterns of mitochondrial DNA nucleotide divergence in the meadow vole, Microtus pennsylvanicus

SummaryThe DNA sequence was determined for the cytochrome c oxidase II (COII), tRNALys, and ATPase 8 genes from the mitochondrial genome of the meadow vole, Microtus pennsylvanicus. When compared to other rodents, three different patterns of evolutionary divergence were found. Nucleotide variation in tRNALys is concentrated in the TΨC loop. Nucleotide variation in the COII gene in three genera of rodents (Microtus, Mus, Rattus) consists predominantly of transitions in the third base positions of codons. The predicted amino acid sequence in highly conserved (>92% similarity). Analysis of the ATPase 8 gene among four genera (Microtus, Cricetulus, Mus, Rattus) revealed more detectable transversions than transitions, many fixed first and second position mutations, and considerable amino acid divergence. The rate of nucleotide substitution at nonsynonymous sites in the ATPase 8 gene is 10 times the rate in the COII gene. In contrast, the estimated absolute mutation rate as determined by analysis of nucleotide substitutions at fourfold degenerate sites probably is the same for the two genes. The primary sequences of the ATPase 8 and COII peptides are constrained differently, but each peptide is conserved in terms of predicted secondary-level configuration.