Comparative digestive morphology and physiology of five species of Peromyscus under controlled environment and diet

Digestive morphology and physiology differ across animal species, with many comparative studies uncovering relationships between animal ecology or diet, and the morphology and physiology of the gastrointestinal tract. However, many of these studies compare wild-caught animals feeding on uncontrolled diets and compare broadly related taxa. Thus, few studies have disentangled the phenotypic consequences of genetics from those potentially caused by the environment, especially across closely related species that occupy similar ecological niches. Here, we examined differences in digestive morphology and physiology of five closely related species of Peromyscus mice that were captive bred under identical environmental conditions and identical diets for multiple generations. Using phylogenetic generalized least squares (PGLS) of species means to control for body size, we identified a phylogenetic signal in the mass of the foregut and length of the small intestine across species. As proportions of total gut mass, we identified phylogenetic signals in relative foregut and small intestine masses, indicating that the sizes of these structures are more similar among closely related species. Finally, we detected differences in activities of the protease aminopeptidase-N enzyme across species. Overall, we demonstrate fine-scale differences in digestive morphology and physiology among closely related species. Our results suggest that Peromyscus could provide a system for future studies to explore the interplay between natural history, morphology, and physiology (e.g. ecomorphology and ecophysiology), and to investigate the genetic architecture that underlies gut anatomy.

Macronutrient signals for adaptive modulation of intestinal digestive enzymes in two omnivorous Galliformes

According to the adaptive modulation hypothesis, digestive enzyme activities are matched to their respective dietary substrate level so that ingested nutrients are not wasted in excreta due to insufficient digestive capacity, and so membrane space or expenditures building/maintaining the intestinal hydrolytic machinery are not wasted when substrate levels are low. We tested predictions in juvenile northern bobwhites (Colinus virginianus) and juvenile and adult domestic chickens (Gallus gallus domesticus) by feeding them on diets varying in starch, protein, and lipid composition for 7-9 d (bobwhites) or 15 d (chickens). Birds were euthanized, intestinal tissue harvested, and enzyme activities measured in tissue homogenates from proximal, medial and distal small intestine. We found that (1) α-glucosidase (AG; maltase and sucrase) activities were induced by dietary starch in both juvenile and adult chickens but not in northern bobwhites; (2) aminopeptidase-N (APN) activities were induced by dietary protein in both bobwhites and juvenile but not adult chickens; (3) AG activities were suppressed by an increase in dietary lipid in both bobwhites and juvenile but not adult chickens; and (4) APN activities were not suppressed by high dietary lipid in any birds. We review findings from 35 analogous trials in 16 avian species. 100% of avian omnivores modulate at least one enzyme in response to change in dietary substrate level. AG induction by dietary carbohydrate occurs in more members of Galloanserae than in Neoaves, and all omnivorous members of Neoaves tested so far increase APN activity on high dietary protein, whereas fewer of the Galloanserae do.

Diet alters rodent fecal pellet size: implications for paleoecological and demographic studies using fecal dimensions

Measurements of fecal pellet size can provide important information about wild mammals, such as body size and demographic information. Previous studies have not rigorously tested whether diet can confound these measurements. Furthermore, it is unknown whether diet might alter fecal dimensions directly or through changes in animal physiology. Here, we studied three closely related rodent species that differ in natural feeding strategies. Individuals were fed diets that varied in protein and fiber content for 5 weeks. We then measured body size, fecal widths and lengths, and the radius of the large intestine. Diet composition significantly changed fecal widths in all species. High-fiber content significantly increased fecal widths and would cause overestimations of body size if applied to wild feces. Using path analysis, we found that fiber can increase fecal widths both directly and indirectly through increasing the large intestine radius. Protein affected each species differently, suggesting that protein effects vary by species feeding strategy and existing physiology. Overall, diet and large intestine morphology can alter fecal pellet measurements. Studies using fecal measurements therefore must consider these effects in their conclusions.

Rapid and parallel changes in activity and mRNA of intestinal peptidase to match altered dietary protein levels in juvenile house sparrows (Passer domesticus)

Although dietary flexibility in digestive enzyme activity (i.e. reaction rate) is widespread in vertebrates, mechanisms are poorly understood. When laboratory rats are switched to a higher protein diet, the activities of apical intestinal peptidases increase within 15 h, in some cases by rapid increase in enzyme transcription followed by rapid translation and translocation to the intestine's apical, brush-border membrane (BBM). Focusing on aminopeptidase-N (APN), we studied intestinal digestive enzyme flexibility in birds, relying on activity and mRNA data from the same animals. Our model was nestling house sparrows (Passer domesticus), already known to modulate intestinal peptidase activity when switching between lower and higher protein diets. Twenty-four hours after a switch from an adequate, lower protein diet to a higher protein diet, APN activity was increased in both whole intestinal tissue homogenates and in isolated BBM, but not at 12 h post-diet switch. Twenty-four hours after a reverse switch back to the lower protein diet, APN activity was decreased, but not at 12 h post-diet switch. Changes in APN activity in both diet switch experiments were associated with parallel changes in APN mRNA. Although transcriptional changes seem to be an important mechanism underlying dietary modulation of intestinal peptidase in both nestling house sparrows and laboratory rodents, the time course for modulation in nestlings seemed slower (taking approximately twice as long) compared with laboratory rodents. It may be ecologically advantageous if nestlings biochemically restructure their gut in response to a sustained increase in insects and protein intake rather than one or a few lucky insect meals.

Dietary adaptation to high starch involves increased relative abundance of sucrase-isomaltase and its mRNA in nestling house sparrows

Dietary flexibility in digestive enzyme activity is widespread in vertebrates but mechanisms are poorly understood. When laboratory rats are switched to a higher carbohydrate diet, the activities of the apical intestinal α-glucosidases (AGs) increase within 6-12 h, mainly by rapid increase in enzyme transcription, followed by rapid translation and translocation to the intestine's apical, brush-border membrane (BBM). We performed the first unified study of the overall process in birds, relying on activity, proteomic, and transcriptomic data from the same animals. Our avian model was nestling house sparrows (Passer domesticus), which switch naturally from a low-starch insect diet to a higher starch seed diet and in whom the protein sucrase-isomaltase (SI) is responsible for all maltase and sucrase intestinal activities. Twenty-four hours after the switch to a high-starch diet, SI activity was increased but not at 12 h post diet switch. SI was the only hydrolase increased in the BBM, and its relative abundance and activity were positively correlated. Twenty-four hours after a reverse switch back to the lower starch diet, SI activity was decreased but not at 12 h post diet switch. Parallel changes in SI mRNA relative abundance were associated with the changes in SI activity in both diet-switch experiments, but our data also revealed an apparent diurnal rhythm in SI mRNA. This is the first demonstration that birds may rely on rapid increase in abundance of SI and its mRNA when adjusting to high-starch diet. Although the mechanisms underlying dietary induction of intestinal enzymes seem similar in nestling house sparrows and laboratory rodents, the time course for modulation in nestlings seemed half as fast compared with laboratory rodents. Before undertaking modulation, an opportunistic forager facing limited resources might rely on more extensive or prolonged environmental sampling, because the redesign of the intestine's hydrolytic capacity shortly after just one or a few meals of a new substrate might be a costly mistake.

Optimal integration between host physiology and functions of the gut microbiome

Host-associated microbial communities have profound impacts on animal physiological function, especially nutrition and metabolism. The hypothesis of 'symmorphosis', which posits that the physiological systems of animals are regulated precisely to meet, but not exceed, their imposed functional demands, has been used to understand the integration of physiological systems across levels of biological organization. Although this idea has been criticized, it is recognized as having important heuristic value, even as a null hypothesis, and may, therefore, be a useful tool in understanding how hosts evolve in response to the function of their microbiota. Here, through a hologenomic lens, we discuss how the idea of symmorphosis may be applied to host-microbe interactions. Specifically, we consider scenarios in which host physiology may have evolved to collaborate with the microbiota to perform important functions, and, on the other hand, situations in which services have been completely outsourced to the microbiota, resulting in relaxed selection on host pathways. Following this theoretical discussion, we finally suggest strategies by which these currently speculative ideas may be explicitly tested to further our understanding of host evolution in response to their associated microbial communities. This article is part of the theme issue 'The role of the microbiome in host evolution'.