Mini Review

Cryopreservation of gametes in Wildlife: A Mini Review

Megha B Ambalkar and Chaitanya H Pawshe*

¹Department of Animal Reproduction, Gynecology & Obstetrics, India

²Post Graduate Institute of Veterinary & Animal Sciences, India

Submission: December 12,2020;Published: February 22, 2021

*Corresponding author: CH Pawshe, Post Graduate Institute of Veterinary & Animal Sciences, Akola, India

How to cite this article: Megha B A, Chaitanya H P. Cryopreservation of gametes in Wildlife: A Mini Review. JOJ Wildl Biodivers. 2021: 3(3): 555619. DOI: 10.19080/JOJWB.2021.03.555619

Introduction

Animal conservation aims at understanding and sustaining biodiversity because the loss of even a single species can affect the functioning of entire ecosystems [1]. The International Union for Conservation of Nature (IUCN) estimates that 25% of mammals, 12% of birds, 20% of reptiles, 30% of amphibians, 20% of fishes, 30% of invertebrates, and 55% of plant species are threatened with extinction (IUCN, 2014). Many of these wild species populations are small and fragmented in their habitat with little or no genetic exchange, which increases homozygosity and inbreeding which leads to a bad adaptive capacity to environmental changes and fertility problems [2]. For the conservation of endangered species, past 20-year major progresses in wildlife reproductive science have been made with the help of non-invasive endocrine monitoring (measuring fecal or urine steroid metabolites) to either a study reproductive parameter such as ovarian cyclicity or seasonality of testicular activity, monitoring pregnancies and evaluate the stress though cortisol level or design the best protocols to enhance fertility or induce ovulation Schwarzen berger Brown, Monfort.

Conservation can also be enhanced with assisted reproductive techniques (ART) and these approaches have been widely helped over the past decades for enhancing breeding management and sustaining small populations of rare species Holt and Lloyd. Besides the techniques of artificial insemination (AI), embryo transfer (ET), and in vitro fertilization (IVF), a wide range of methods and tools have been developed [2,3]. Among these critical tools, germplasm cryopreservation also has played a key role in establishing biorepositories for capturing extant genomic diversity [4].

Recently, the natural calamities such as Amazon fire and Australian bushfire destroyed the many endangered species in Brazil and Australia. Most of the endangered species were burnt out and this species were affecting the whole ecosystem. For maintaining the whole ecosystem, the preservation of the even single species is important. Cryopreservation of the gametes and embryo is important to conserve the endangered species and to maintain the cycle of whole ecosystem. Cryopreservation of gametes and embryos is a technique of choice for bio-banking the genetic potential of desired animals. This technique permits long-term storage with logistically acceptable if the cryopreserved materials would need to be transferred. Two cryopreservation techniques that have been frequently used for cryopreservation of gametes and embryos are slow freezing and vitrification.

Slow freezing

Cryopreservation requires an optimal freezing rate principally to balance the intra- and extra-cellular water and cryoprotectant(s). Slow freezing in which relatively low concentration of cryoprotectant is used showing little toxicity to cell or tissue. As cryoprotectant is added to cells it results in initial cellular dehydration followed by return to isotonic volume with the permeation of cryoprotectant and water. Generally, cells are cooled slowly using a controlled rate freezing machine which allows samples to be cooled at various rates.

Vitrification

More recently, the process of vitrification has shown to be more successful for cryopreservation of gametes. It is a process that uses very high rates of cooling, so fast that water is solidifies without crystallization, “like glass.” Extremely high concentration of cryoprotectant do not crystallized when cooled even if it is done slowly. Cryoprotectant solutions are toxic to cells at very high concentration. Solute toxicity is major drawback of using vitrification for preservation even with high cooling rates. To reduce toxicity concentration of cryoprotectant can be lowered as long as cooling is fast to preclude ice formation.

Genome Resource Banking (GRB)

Germplasm cryopreservation and Genome resource banking (GRB) refers to the collection, processing, storage, and use of germplasms (sperm, eggs, embryos, ovarian, and testicular tissues) and other biomaterials (blood products and DNA samples) that can be used for understanding and sustaining biodiversity. If it is used properly in association with ART, GRB have the potential to slow down the loss of gene diversity in captive populations by reintroducing original genetic material (without removing genetically valuable individuals from the wild) and decrease the interval between generations [5,6]. Fertility preservation strategies using cryopreservation have huge potential for helping sustain and protect rare and endangered species. However, widescale applications currently are difficult because of the significant physiological variations among species and a complete lack of fundamental knowledge in germplasm cryobiology. The best examples can be found within the conservation programs of the giant pandas and black-footed ferrets [6].

Semen freezing

Semen cryopreservation represents the widest effort, with live births reported after AI. Physico-chemical properties related to cryo-resistance differ between species by tolerances to glycerol concentrations in the extender: 5% in cattle, no more than 4% in deer, 3% in pigs, 1.75% in mice, 6% in chinchillas, and large differences observed in marsupials [4,7]. Recent progress in vertebrates have been reviewed [6] and include original studies on endangered gazelles and Iberian lynx [8,9]. Sperm processing challenges also are illustrated from amphibian to fish studies. Generally, these types of spermatozoa remain immotile in seminal plasma until released into the environment. In case of frogs, spermatozoa are actually excreted in the urine (pH = 7.5; 85 mos mol/L) that, in turn, naturally activates motility due to a lower osmolarity than is present in testicular tissue [10]. A similar phenomenon occurs in fish spermatozoa (mostly from salmonids, sturgeons, carp, turbot, halibut, and cod) that are initially immotile in seminal plasma but then are activated by fresh or saltwater [7]. Spermatozoa from some fish species maintain motility for < 1 min whereas others retain this function for several days. As a result of these characteristics, amphibian and fish protocols generally focus on processing and storing inactivated cells by collecting samples into a buffered saline solution that duplicates the original seminal plasma environment. In endangered amphibians, excellent progress has been made in developing sperm cryopreservation methods for anuran species with embryos or more advanced offspring generated from frozen sperm in several species, including sperm successfully cryopreserved after non-invasive collection by hormonal induction [10]. Recently, there has been some success with cryopreserving sperm cells from a variety of coral species (in vitro production of larvae). Based on that, GRB have been established to help offset these threats to the Great Barrier Reef and other areas Hagedorn and Spindler.

Oocyte Cryopreservation

Oocyte freezing remains challenging and unsuccessful in wild species and will require more research before becoming a standard procedure [6]. Despite extensive efforts conducted in different wild mammals, not a single individual has been produced from a frozen–thawed egg. In amphibian and fish, the potential for cryopreservation of the female is also challenging, with no offspring reported to date from cryopreserved oocytes. Egg size and structure and yolk composition appear to create technical barriers to cryopreservation.

Gonadal Tissue Preservation

As an alternative to fully grown gametes, gonadal tissue preservation has become a promising option in vertebrates [4]. In amphibians, the direct cryopreservation of immature ovarian follicles holds promise but would need to be combined with procedures such as xeno-transplantation to generate mature, ovulated oocytes. Cryopreservation of primordial germ cells also holds promise but would likely need to be combined with the generation of chimeras to obtain adults that can produce viable gametes Clulow [11]. This approach also seems to be the future for birds and fish ART [6].

Conclusion

Cryopreservation of gametes is very important for conservation of germplasm of endangered species for balancing of ecosystem. For future generation, conservation of germplasm of wildlife is very important otherwise wildlife animals are kept as a clay model or as pictures in the museum.

References

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