Femoral bone structure and mechanics at the edge and core of an expanding population of the invasive frog Xenopus laevis.

Understanding how living tissues respond to changes in their mechanical environment is a key question in evolutionary biology. Invasive species provide an ideal model for this as they are often transplanted between environments that differ drastically in their ecological and environmental context. Spatial sorting, the name given to the phenomenon driving differences between individuals at the core and edge of an expanding range, has been demonstrated to impact the morphology and physiology of Xenopus laevis from the invasive French population. Here, we combined a structural analysis using micro-CT scanning and a functional analysis by testing the mechanical properties of the femur to test whether the increased dispersal at the range edge drives differences in bone morphology and function. Our results show significant differences in the inner structure of the femur as well as bone material properties, with frogs from the centre of the range having more robust and resistant bones. This is suggestive of an energy allocation trade-off between locomotion and investment in bone formation, or alternatively, may point to selection for fast locomotion at the range edge. Overall, our results provide insights on the growth of the long bones and the formation of trabecular bone in frogs.

Investigations on Xenopus laevis body composition and feeding behavior in a laboratory setting.

The African clawed frog, Xenopus laevis, has been used as a laboratory animal for decades in many research areas. However, there is a lack of knowledge about the nutritional physiology of this amphibian species and the feeding regimen is not standardized. The aim of the present study was to get more insights into the nutrient metabolism and feeding behavior of the frogs. In Trial 1, adult female X. laevis were fed either a Xenopus diet or a fish feed. After 4 weeks, they were euthanized, weighed, measured for morphometrics and dissected for organ weights and whole-body nutrient analysis. There were no significant differences between the diet groups regarding the allometric data and nutrient contents. The ovary was the major determinant of body weight. Body fat content increased with body weight as indicator of energy reserves. In Trial 2, 40 adult female frogs were monitored with a specifically developed digital tracking system to generate heat-maps of their activity before and up to 25 min after a meal. Three diets (floating, sinking, floating & sinking) were used. The main feed intake activity was fanning the feed into the mouth, peaking until 20 min after the meal. The different swimming characteristics of the diets thereby influenced the activity of the animals. Our dataset helps to adjust the feeding needs to the physical composition and also to meet the natural behavioral patterns of feed intake as a prerequisite of animal wellbeing and animal welfare in a laboratory setting.

Regeneration from three cellular sources and ectopic mini-retina formation upon neurotoxic retinal degeneration in Xenopus.

Regenerative abilities are not evenly distributed across the animal kingdom. The underlying modalities are also highly variable. Retinal repair can involve the mobilization of different cellular sources, including ciliary marginal zone (CMZ) stem cells, the retinal pigmented epithelium (RPE), or Müller glia. To investigate whether the magnitude of retinal damage influences the regeneration modality of the Xenopus retina, we developed a model based on cobalt chloride (CoCl2 ) intraocular injection, allowing for a dose-dependent control of cell death extent. Analyses in Xenopus laevis revealed that limited CoCl2 -mediated neurotoxicity only triggers cone loss and results in a few Müller cells reentering the cell cycle. Severe CoCl2 -induced retinal degeneration not only potentializes Müller cell proliferation but also enhances CMZ activity and unexpectedly triggers RPE reprogramming. Surprisingly, reprogrammed RPE self-organizes into an ectopic mini-retina-like structure laid on top of the original retina. It is thus likely that the injury paradigm determines the awakening of different stem-like cell populations. We further show that these cellular sources exhibit distinct neurogenic capacities without any bias towards lost cells. This is particularly striking for Müller glia, which regenerates several types of neurons, but not cones, the most affected cell type. Finally, we found that X. tropicalis also has the ability to recruit Müller cells and reprogram its RPE following CoCl2 -induced damage, whereas only CMZ involvement was reported in previously examined degenerative models. Altogether, these findings highlight the critical role of the injury paradigm and reveal that three cellular sources can be reactivated in the very same degenerative model.

Optical Estimation of Bioelectric Patterns in Living Embryos.

Fluorescent lifetime imaging (FLIM) is a powerful tool for visualizing physiological parameters in vivo. We present here a 3-dye strategy for mapping bioelectric patterns in living Xenopus laevis embryos leveraging the quantitative power of fluorescent lifetime imaging. We discuss a general strategy for disentangling physiological artifacts from true bioelectric signals, a method for dye delivery via transcardial injection, and how to visualize and interpret the fluorescent lifetime of the dyes in vivo.