Cryopreservation of hormonally induced sperm for the conservation of threatened amphibians with Rana temporaria as a model research species

Abstract

The survival of hundreds of threatened amphibian species is increasingly dependent on conservation breeding programs (CBPs). However, there is an ongoing loss of genetic variation in CBPs for most amphibians, reptiles, birds, and mammals. Low genetic variation results in the failure of CBPs to provide genetically competent individuals for release in supplementation or rehabitation programs. In contrast, in the aquaculture of fish the perpetuation of genetic variation and the production of large numbers of genetically competent individuals for release is accomplished through the cryopreservation of sperm. Successful protocols for the cryopreservation of amphibian sperm from excised testes, and the use of motile frozen then thawed sperm for fertilisation, have been adapted from those used with fish. However, there have been no protocols published for the cryopreservation of amphibian hormonally induced sperm (HIS) that have achieved fertility. We investigated protocols for the cryopreservation of amphibian HIS with the European common frog ( Rana temporaria) as a model research species. We induced spermiation in R. temporaria through the intraperitoneal administration of 50 μg LHRHa and sampled HIS through expression in spermic urine. Highly motile HIS at a concentration of 200 × 10 6/mL was then mixed 1:1 with cryodiluents to form cryosuspensions. Initial studies showed that; 1) concentrations of ∼15 × 10 6/mL of HIS achieve maximum fertilisation, 2) TRIS buffer in cryodiluents did not improve the recovery of sperm after cryopreservation, and 3) high concentrations of DMSO (dimethylsulphoxide) cryoprotectant reduce egg and larval survival. We then compared four optimised cryopreservation protocols for HIS with the final concentrations of cryodiluents in cryosuspensions of; 1) DMSO, (½ Ringer Solution (RS), 10% sucrose, 12% DMSO); 2) DMSO/egg yolk, (½ RS, 10% sucrose, 12% DMSO, 10% egg yolk), 3) DMFA, (½ RS, 10% sucrose, 12% dimethylformamide (DMFA)), and 4) MIS/glycerol, (Motility Inhibiting Saline (MIS), 5% glycerol, 2.5% sucrose, 5% egg yolk). Cryosuspensions were frozen in LN 2 vapour, stored in LN 2, thawed in 40° C water bath, and activated by slow equilibration with 1:3 dilutions of cryosuspensions with 20 mM/L NaCl. Protocol efficacies were assessed through the post-thaw percentage of; 1) sperm motility, 2) sperm membrane integrity, 3) fertilisation, 4) fertilised eggs hatching, and 5) larval survival from fertilised eggs to 7 d. The DMFA cryodiluent proved superior to the DMSO based cryodiluents in recovery of sperm motility and fertility after cryopreservation. MIS/glycerol cryodiluent provided low sperm viability and no fertility. Considering the ease of obtaining HIS from many Rana species, the success of our protocols offer the potential for the perpetuation of the genetic variation of the 42 threatened Rana species and the 193 threatened Ranid species in total.