Cryopreservation of Xenopus Sperm and In Vitro Fertilization Using Frozen Sperm Samples

Transitioning from a research protocol to a scalable applied pathway for Xenopus laevis sperm cryopreservation at a national stock center: The effect of cryoprotectants

Sperm cryopreservation is a critical tool for safeguarding and managing valuable genetic resources. Protocols for cryopreservation of Xenopus laevis sperm were available but lacking sperm quality evaluation and scalability and the outcomes were inconsistent. The goal of this study was to begin developing a center-level cryopreservation pathway for this species by integrating French straws as containers that would facilitate germplasm repository development. The objectives were to analyze the effect of: (1) three sperm concentrations (33, 50, and 100 × 106 sperm/mL) on post-thaw fertilization, (2) three final concentrations (2.5%, 5%, and 10%) of dimethyl sulfoxide, methanol, and dimethylformamide (DMFA) on sperm membrane integrity of fresh and frozen samples, (3) two concentrations (5% and 10%) of DMFA with and without 5% sucrose at four cooling rates (5, 10, 20, and 40°C/min) on sperm membrane integrity and motility, and (4) egg exposure to different concentrations of DMFA on fertilization. Few differences in sperm viability were found among fresh samples incubated in cryoprotectants, but thawed samples frozen in methanol or DMFA presented higher membrane integrity. Samples frozen in 10% DMFA at 20°C/min showed higher membrane integrity (60 ± 7%) than other DMFA concentrations and cooling rates, and the same total motility (30 ± 7%) as at 10°C/min. Higher DMFA concentrations (10%-13%) were detrimental for embryo development compared to lower concentrations (<6%). This study provided a reliable protocol for sperm cryopreservation in Xenopus laevis to yield an application pathway with potential for high throughput that can be used as a roadmap for work with other species.

Genetics and Gene Editing Methods in Xenopus laevis and Xenopus tropicalis

Our understanding of biological systems has for many years been heavily influenced by experimental approaches that exploit genetic methods. These include gain-of-function experiments that overexpress transgenes or ectopically express injected RNA and loss-of-function experiments that knock out genes or knock down RNAs. Here, we review how these methods have been applied in Xenopus frogs and introduce a variety of protocols for genetic manipulation of Xenopus laevis and Xenopus tropicalis.

Automated multi-sample DNA extraction for genotyping live Xenopus embryos

BACKGROUND

Xenopus frogs are used extensively for modeling genetic diseases owing to characteristics such as the abundance of eggs combined with their large size, allowing easy manipulation, and rapid external embryo development enabling the examination of cellular and phenotypic alterations throughout embryogenesis. However, genotyping of mutant animals is currently done either as part of a large group, requiring many embryos, or late in development with welfare effects. Therefore, we adapted the Zebrafish Embryonic Genotyper for rapid genomic DNA extraction from Xenopus tropicalis and Xenopus laevis at early stages.

RESULTS

Sufficient and good quality DNA was extracted as early as the Nieuwkoop and Faber stage 19 and, importantly, no detrimental effects of the extraction process on the subsequent tadpole development, behavior, or morphology were observed. Amplicons of up to 800 bp were successfully amplified and used for further analyses such as gel electrophoresis, T7 endonuclease I assay and Sanger sequencing.

CONCLUSION

This method allows rapid genotyping during the early stages of Xenopus development, which enables safe identification of mutants. These can be analyzed at early developmental stages or selected for raising without the need for invasive genotyping later, with resource savings and ethical gains in line with the 3Rs principles.