Cryopreservation of Sperm from an Endangered Snake with Tests of Post-Thaw Incubation in Caffeine

Simple Summary

Cryopreservation of sperm from reptiles to aid the recovery of endangered species continues to be a challenge. In this study, we tested the cryoperformance of a cryoprotective agent (CPA) mixture to cryopreserve sperm from the endangered Louisiana pinesnake (Pituophis ruthveni). The mixture contained Lake’s buffer with 10% N,N-dimethyl formamide (DMF), 2% methanol, 5% clarified egg yolk, (v/v% final concentration) and was tested against 16 experimental mixtures containing variable concentrations and mixtures of diluents, extenders, CPAs, and additives. In addition, we investigated the effects of post-thaw incubation on sperm motility in TL HEPES supplemented with 10% fetal bovine serum (H10) alone or supplemented with caffeine. We found that the majority of our test additives did not significantly improve the post-thaw motility or viability of sperm. The best performing experimental CPA mixture contained Lake’s buffer with 10% DMF, 2% methanol, and 5% clarified egg yolk with the addition of 5 mg/mL bovine serum albumin (BSA), and post-thaw incubation in both H10 and H10 with caffeine showed improved forward motility. Cryopreservation of sperm from the Louisiana pinesnake improved with the addition of BSA to our base CPA mixture, and post-thaw incubation in H10 improved with caffeine.

Abstract

Cryopreservation of sperm to preserve the genetic diversity of declining populations is a promising technique to aid in the recovery of endangered species such as the Louisiana pinesnake (Pituophis ruthveni). However, this technique has been performed on only a handful of snake species and with limited success. Here, we tested a cryoprotective agent (CPA) mixture containing Lake’s buffer with 10% N,N-dimethyl formamide (DMF), 2% methanol, 5% clarified egg yolk, (v/v% final concentration) against 16 other CPA-treatment mixtures. These contained either Lake’s buffer or TEST egg yolk buffer as the base diluent with a penetrating or non-penetrating CPA on the post-thaw recovery of sperm motility and viability. We also investigated the effect of post-thaw incubation treatment in TL HEPES supplemented with 10% fetal bovine serum (H10) alone or with caffeine on post-thaw motility parameters. Sperm from 16 Louisiana pinesnakes was cryopreserved, and the effectiveness of the CPA treatment mixtures and post-thaw treatments was determined based on measurements of sperm motility and viability. Sperm cryopreservation significantly reduced initial post-thaw sperm quality for all of the extender treatments. Viability of sperm was best maintained when cryopreserved in an CPA treatment mixture containing Lake’s buffer with 10% DMF, 2% methanol, and 5% clarified egg yolk with the addition of 5 mg/mL bovine serum albumin (BSA). For several extender mixtures a similar percent of post-thaw motility was observed, but no forward motility returned in any post-thaw samples prior to incubation in dilution treatments. Following incubation in both post-thaw treatments, the percent of forward motility and the index of forward progressive movement improved significantly. Post-thaw dilution with H10 containing caffeine improved motility parameters over H10 alone, suggesting further investigation of post-thaw treatment in caffeine could be beneficial. Although, cryopreservation of sperm from the Louisiana pinesnake continues to present a challenge, post-thaw dilution and the addition of BSA to CPA mixtures provides areas for improving cryopreservation methods for this endangered species.

Reptile assisted reproductive technologies: can ART help conserve 300million years of evolution by preserving extant reptile biodiversity?

Biodiversity loss is the greatest environmental problem threatening ecosystem, animal, and human health. Anthropogenic induced changes to climate, habitat, disease, species distributions, poaching, and unsustainable trade have accelerated extinction rates in all vertebrates, including reptiles. Preventing reptile extinctions will require humans to acknowledge these losses and develop ex situ and in situ plans to preserve them. Assisted reproductive technologies (ART) are management tools used to protect numerous vertebrate taxa; however, progress in developing ART for reptiles has lagged. Creating functional and sustainable reptile ART will strengthen our conservation capacity by capturing genetic material from select individuals to overcome natural or manmade boundaries. Utilising short-term gamete storage and genome resource banking, in conjunction with timed artificial insemination (AI) or ex ovo incubation, could lead to profound advances in reptile conservation, mitigating the loss of reptile biodiversity. In this article, we review ART reptile research completed since the 1970s. Topics include AI, hormonal control of reproduction, gamete collection, gamete storage, and genome resource banking. Additionally, we review the potential application of advanced reproductive methodologies, including in vitro/ex ovo fertilisation, intracytoplasmic sperm injection, cloning (somatic cell nuclear transfer), and genetic editing.

Sperm cryopreservation in the Burmese python Python bivittatus as a model for endangered snakes

Burmese pythons Python bivittatus captured in the Florida Everglades as part of an invasive species monitoring program served as a model for the development of sperm cryopreservation protocols for endangered snakes. Spermatozoa were collected from the vas deferens and initial motility, plasma membrane integrity and acrosome integrity were recorded before cryopreservation. Spermatozoa were extended in TES and Tris (TEST) yolk buffer with glycerol (GLY) or dimethyl sulfoxide (DMSO) concentrations of 8%, 12% or 16%, or combinations of GLY and DMSO with final concentrations of 4%:4%, 6%:6% or 8%:8%, and frozen at a rate of 0.3°C min-1. Sperm frozen in combinations of GLY and DMSO exhibited greater post-thaw motility and plasma membrane integrity than those frozen in GLY or DMSO alone. All DMSO and GLY:DMSO treatments preserved a greater proportion of intact acrosomes than GLY alone. To determine the best overall cryopreservation protocol for this species, a sperm quality index was calculated, giving equal weight to each of the three measured indicators of cryosurvival. This analysis revealed that Burmese python spermatozoa frozen in 6% GLY:6% DMSO or 4% GLY:4% DMSO exhibited the highest post-thaw viability. This study represents the first comparative, comprehensive attempt to develop a sperm cryopreservation protocol for any snake species.

Toxicity of cryoprotective agents to semen from two closely related snake species: The endangered Louisiana pinesnake (Pituophis ruthveni) and bullsnake (Pituophis cantenifer)

Cryopreservation of sperm is an important tool for the conservation of threatened species. Many species of reptile are under considerable threat of extinction and there has been limited investigation of sperm cryopreservation in this taxonomic group. We performed a comparative test of toxicity to sperm of six commonly used cryoprotective agents (CPAs) at three concentrations (5%, 10%, 20%) from the Louisiana pinesnake, Pituophis ruthveni (n = 11), and the closely related bullsnake, Pituophis cantenifer (n = 8). Our objective was to determine the general toxicity of CPAs for cryopreservation in snakes and the cryoprotective ability of CPAs for sperm from the endangered Louisiana pinesnake. We conducted three experiments to: 1) evaluate the short-term in vitro toxicity of common CPAs in two closely related snake species, 2) determine the effectiveness of cryoprotectants for freezing and thawing semen in the Louisiana pinesnake, and 3) test the possible reduction in toxic effects of individual CPAs on semen of the Louisiana pinesnake by combining two of them. We used measures of motility including total motility, forward motility, and forward progressive motility index to characterize toxic effects and cryoprotective ability of each CPA. The results of our three experiments provide several important findings: 1) sperm of the bullsnake and Louisiana pinesnake responded differently to CPAs, 2) few CPAs provided any cryoprotection, as measured by percent recovered motility, in Louisiana pinesnakes, and 3) using mixtures of CPAs did not reduce toxicity relative to the best performing CPA on its own. Motility was best maintained at a concentration of 5% for CPAs tested; however, cryoprotection was best achieved with glycerol at 20% followed by DMA and DMF at 10%. These results provide further insight into the challenges faced by researchers attempting to cryopreserve sperm from snakes. Further comparative studies are required to determine the generality of cryopreservation methods in reptiles and suggest caution should be taken when developing cryopreservation protocols across species, particularly in snakes. All CPAs tested in this study were permeating CPAs and showed a significant acute toxic effect on motility at concentrations that provided cryoprotection. Future work in snakes might consider additional avenues of cryoprotection and combinations of multiple approaches.

Challenges in the development of sperm cryopreservation protocols for snakes

Snake populations are declining worldwide, but research devoted to the development of sperm cryopreservation techniques for this taxon is very limited. Spermatozoa were collected postmortem from snakes of four squamate families (Elapidae, Colubridae, Viperidae and Pythonidae). Viability assessment was performed before and after cryopreservation. Spermatozoa were extended in TES and Tris (TEST) yolk buffer with 12% dimethylsulfoxide (DMSO) or 12% glycerol and frozen at a rate of 0.3°Cmin-1 . The sperm quality index (SQI), representing three viability parameters (motility, plasma membrane and acrosome integrity), was determined. Despite some species differences, glycerol was a more effective cryoprotectant for Colubridae, whereas DMSO provided greater cryoprotection for spermatozoa of members of the other three families.

DETERMINING THE SAFETY OF REPEATED ELECTROEJACULATIONS IN VEILED CHAMELEONS CHAMAELEO CALYPTRATUS

Investigation into antemortem semen collection methods in reptiles is required to counteract anthropogenic induced reptile biodiversity loss and to further develop assisted reproductive technologies in reptiles. The purpose of this study was to evaluate whether repeated electroejaculation under anesthesia could be done safely in the veiled chameleon (Chamaeleo calyptratus). Electroejaculation under anesthesia (alfaxalone 15 mg/kg intravenously) was performed once weekly over a 4-wk period in 12 Chamaeleo calyptratus. External vent appearance and cloacoscopy was performed prior to and after electroejaculation to assess the effect of the electroejaculation on the external and internal anatomy. Hematological and chemistry values were evaluated prior to the study and afterwards. Semen collection was successful in 50% of the electroejaculation events. Cloacoscopy images scores were significantly higher between the pre- and postelectroejaculation events for each week. No significant difference was observed between study week 1 prior to electroejaculation and study week 4 after electroejaculation in vent external morphology (P = 0.157), and internal cloacal scoring (P = 0.058). A significant increase in the white blood cell counts (P = 0.015), absolute heterophil counts (P = 0.02), absolute monocyte counts (P = 0.003), and albumin (P = 0.038) was observed between the pre- and postejaculation sampling periods. Aspartate aminotransferase (P = 0.001) and potassium (P = 0.036) decreased significantly between the pre- and postsampling periods. Vent image scores were significantly higher (P = 0.046) between the pre- and postsampling images collected during week 1; however, there were no significant differences noted between pre- and postsample images for weeks 2 (P= 0.063), 3(P= 0.083), or 4 (P= 0.083). Serial electroejaculation procedures under alfaxalone anesthesia can be performed safely in veiled chameleons. Although changes were observed in the complete blood count and biochemistry results, these changes were determined to not be clinically significant. The results of this study demonstrate that antemortem assisted reproductive techniques such as electroejaculation can be safely used repeatedly.