Journal of Experimental Biology - Latest Issue

  • Extracellular and intracellular digestion in bivalves, studied by magnetic resonance imaging with a contrast reagent

    ABSTRACT
    We investigated extracellular and intracellular digestion in bivalves, employing magnetic resonance imaging (MRI). Ruditapes philippinarum clams and Mytilus galloprovincialis mussels were incubated in seawater containing a contrast reagent [gadolinium-diethylenetriamine pentaacetic acid (GdDTPA)] at 20°C. The digestive systems, from the esophagus to the rectum, were visualized at a high signal intensity by the T1-weighted MRI. The crystalline style of the clam was also identified, which turned counterclockwise when viewed from a ventral–posterior position at a rate of 16 revolutions min−1. Determined using the T1 relaxation rate, the uptake and excretion rates of the GdDTPA in the mussel's digestive glands were 2.9 and 0.25 day−1, respectively, indicating that intracellular digestion in the gland acinar cells is slower than extracellular digestion. These results demonstrate that MRI with contrast reagents is useful to study the activity of the digestive system in bivalves, and that this technique could be applied to study other invertebrates.

  • Dynamic reciprocal morphological changes in insect hosts and bacterial symbionts

    ABSTRACT
    Symbiotic interactions, central to most life on Earth, are interwoven associations that vary in intimacy and duration. Some of the most well-known examples of symbioses occur between animals and gut bacteria. These associations lead to physiological integration of host and symbionts. The diversity of microbes within animal hosts can make studying them technically challenging. Thus, most science heavily focuses on the animal side of symbioses, limiting study of the microbial symbionts to characterization of their genetic and functional diversity. These limitations are minimized in Heteropteran insects that have specialized midguts that separately house single symbiont species away from ingested food. These insect–bacteria associations allow us to address fundamental questions regarding how both hosts and symbionts change to establish a cooperative relationship. In this study, through ex vivo and in vivo observations of cellular behaviors, we explore concurrent structural and cellular dynamics in both the squash bug host (Anasa tristis) and its Caballeronia zhejiangensis symbionts during the initiation of symbiosis. We elucidate how C. zhejiangensis is sequestered within a specialized symbiotic organ within the A. tristis midgut, how the symbiont uses active motility to reach the symbiotic organ, how symbionts colonize host crypts within the organ and how host crypt morphogenesis progresses during the initiation of symbiotic interactions. Our findings provide insight into how dynamic cellular activity and morphological development reciprocally change in both host and symbiont as they establish symbiotic interactions.

  • Evolutionary insights into gut acidification: invertebrate-like mechanisms in the basal vertebrate hagfish

    ABSTRACT
    Acidification is a key component of digestion throughout metazoans. The gut digestive fluid of many invertebrates is acidified by the vesicular-type H+-ATPase (VHA). In contrast, vertebrates generate acidic gut fluids using the gastric H+/K+-ATPase (HKA), an evolutionary innovation linked with the appearance of a true stomach that greatly improves digestion, absorption and immune function. Hagfishes are the most basal extant vertebrates, and their mechanism of digestive acidification remains unclear. Herein, we report that the stomachless Pacific hagfish (Eptatretus stoutii) acidify their gut using the VHA, and searches of E. stoutii gut transcriptomes and the genome of a closely related hagfish species (E. burgerii) indicate they lack HKA, consistent with its emergence following the 2R whole-genome duplication. Immunostaining revealed prominent VHA presence in the apical membrane of enterocytes and sub-apical expression of both VHA and soluble adenylyl cyclase. Interestingly, akin to vertebrates, VHA was also observed in immature pancreatic-like zymogen granules and was noticeably absent from the mature granules. Furthermore, isolated gut sacs from fed hagfish demonstrate increased VHA-dependent luminal H+ secretion that is stimulated by the cAMP pathway. Overall, these results suggest that the hagfish gut shares the trait of VHA-dependent acidification with invertebrates, while simultaneously performing some roles of the pancreas and intestine of gnathostomes.

  • Accounting for the role of the gastro-intestinal tract in the ammonia and urea nitrogen dynamics of freshwater rainbow trout on long-term satiation feeding

    ABSTRACT
    The contribution of the gut to the ingestion, production, absorption and excretion of the extra ammonia and urea nitrogen (urea-N) associated with feeding (‘exogenous’ fraction) has received limited attention. Analysis of commercial pellet food revealed appreciable concentrations of ammonia and urea-N. Long-term satiation feeding increased whole-trout ammonia and urea-N excretion rates by 2.5-fold above fasting levels. Blood was sampled from the dorsal aorta, posterior, mid- and anterior sub-intestinal veins, as well as the hepatic portal vein in situ. Ammonia, urea-N and fluid flux rates were measured in vitro using novel gut sac preparations filled with native chyme. The sacs maintained the extreme physico-chemical conditions of the lumen seen in vivo. Overall, these results confirmed our hypothesis that the stomach, and anterior intestine and pyloric caecae regions play important roles in ammonia and urea-N production and/or absorption. There was a very high rate of urea-N production in the anterior intestine and pyloric caecae, whereas the posterior intestine dominated for ammonia synthesis. The stomach was the major site of ammonia absorption, and the anterior intestine and pyloric caecae region dominated for urea-N absorption. Model calculations indicated that over 50% of the exogenous ammonia and urea-N excretion associated with satiation feeding was produced in the anaerobic gut. This challenges standard metabolic theory used in fuel-use calculations. The novel gut sac preparations gained fluid during incubation, especially in the anterior intestine and pyloric caecae, owing to marked hyperosmolality in the chyme. Thus, satiation feeding with commercial pellets is beneficial to the water balance of freshwater trout.