Discovering the cause and mitigating the effect of reduced reproductive capacity in laboratory-bred X. laevis.

Xenopus laevis has been used as a key vertebrate model organism for cell and developmental biology for more than 60 years. It is used by hundreds of laboratories across the globe. Adults were supplied to laboratories from the wild, but more recently all research frogs in Europe have been required by law to be bred by registered breeders. Although these animals are robust in captivity, their breeding fitness, which is critical for most of their use, has in recent years become variable and there is evidence that it is decreasing. Mitigating this decrease is important because it will save animals (reducing the number needed), time and resource for research. Potential reasons for the decreased reproductive productivity include inbreeding or environmental changes. We have access to male gametes from a number of populations taken from the wild into captivity for ecological studies and access to tadpoles from the major Xenopus breeding colonies worldwide. We will sequence animals from the colonies to understand to what extent inbreeding is found across the community, for example it might be expected that it is greater in Europe, due to the import restrictions. We will use the male gametes from the wild to test whether outbreeding can restore reproductive fitness (froglets for this experiment are already growing due to the time taken to reach maturity). In terms of environment, a clear difference between wild and captive animals can be seen from our preliminary studies, which have shown that the microbiome of captive frogs is very different from those in the wild, either from indigenous or invasive populations. By bringing together biologists studying wild and captive Xenopus, a microbiologist and a bioinformatician we will test whether modifying the captive frog microbiome can improve breeding fitness without introducing health problems. The physical environment in which the frogs are kept has also changed over the period; they used to be kept in larger, still water tanks with two or three water changes weekly but now most are in smaller tanks with recirculating water. We will compare females kept in the traditional and recirculating systems directly for their embryo numbers and quality. If outbreeding, microbiome modification or altered housing can enhance productivity then a reduction (3Rs) in the number of Xenopus needed to provide embryos will result. This will also improve the cost-effectiveness of the model and reduce the workload in laboratories. The European Xenopus Resource centre is very strongly positioned to disseminate improved practices identified in this project to the research community. In addition, the data will increase the understanding of basic relationships between reproductive fitness and the gut microbiome, discover the relationships between the world's captive X. laevis and provide robust data on housing and welfare.