Dehydrated Caenorhabditis elegans Stocks Are Resistant to Multiple Freeze-Thaw Cycles

Use of fine capillaries for cryopreservation of Caenorhabditis elegans by vitrification

Caenorhabditis elegans is an exceptional model organism. More than twenty thousand different strains have been developed, increasing knowledge on countless topics. However, the traditional method to cryopreserve this nematode, based on slow freezing, usually reaches recovery rates of around 35% for the L1 and L2 larval stages. Here, we propose two alternative methods to cryopreserve this nematode based on vitrification that are applicable in common laboratories and allow the selective individual cryopreservation of this organism. These new methods require ultra-high warming rates, which are achieved by employing very thin capillaries as the nematode container, and a very low final concentration of cryoprotectants, which, as compared to slow freezing, reduce toxicity damage. The recovery rate was 98.5% for larvae (L1 - L4) and 84.3% for adults. Given these results, our procedures offer an alternative to cryopreserve this nematode (larvae and adults) with higher recovery rates, avoiding expensive requirements. Indeed, it only needed a container with liquid nitrogen and a warming bath for water at 37 °C. The high performance of this approach has been revealed by preserving the long-term memory and, probably, the connectome of this nematode.

Dry-freezing Steinernema carpocapsae infective juveniles for robust preservation of stocks

Cryopreservation allows strains to be stored, eliminating genetic drift and maintenance costs. Existing cryopreservation methods for the economically-important entomopathogenic nematode Steinernema carpocapsae involve multiple incubation and filtration steps to precondition the animals. The standard protocol for freezing the model organism Caenorhabditis elegans in buffer is simpler, and a recent C. elegans dry-freezing protocol allows stocks to survive multiple freeze-thaws, a possibility during a power failure. Here we report the efficacy of C. elegans cryopreservation protocols adapted for S. carpocapsae . We show that dry freezing with disaccharides, but not glycerol-based or trehalose-DMSO-based freezing buffer, allows reliable recovery of infective juveniles.

Long-term cryostorage does not negatively affect the recovery of Caenorhabditis elegans

Cryostorage of Caenorhabditis elegans nematodes is important to maintain the many lines used for research. The standard method uses 15% of glycerol in M9-Buffer and a cooling rate of 1 °C/min; then worms can be stored in a -80 °C freezer or in liquid nitrogen. The recovery of C. elegans from stocks stored in liquid nitrogen is reported to be in the range of 35-45% and slightly decreases after years of storage. The storage at -80 °C is also considered safe, but the recovery is not as high as in liquid nitrogen. These observations have not been experimentally reported and therefore require verification. In this study, the standard methods were used in a set of experiments to compare the recovery of larvae and adult worms stored at -80 °C or in liquid nitrogen, after short- (a week) or long-term storage (3.5 years). No differences were observed in recovery, either for the time of storage or for the temperature of storage. Recovery of larvae was 32% at -80 °C and 36% in liquid nitrogen after 3.5 yr and that was not significantly different from the 7-d recovery rates. Adult worm recovery was below 5% for all treatments. These results suggest that both methods of storage can be used to successfully store C. elegans larvae for at least 3.5 years.

Cryopreservation of C. elegans and Other Nematodes with Dimethyl Sulfoxide and Trehalose

One of the key attributes that has contributed to the popularity of Caenorhabditis elegans as a model system is its ability to survive freezing. By preserving stocks at ultralow temperature, researchers have been able to generate an unlimited number of strains without the burden of constantly maintaining them. This has facilitated the use of large-scale forward genetic screens and CRISPR-mediated genome editing where large numbers of novel and informative mutants can be generated. Traditionally, C. elegans and other nematodes were frozen using glycerol as a cryoprotectant. While effective, a large majority of animals do not survive a typical freeze-thaw cycle. Here I describe an alternative method based on the popular combination of DMSO and trehalose as a cryoprotectant. This method allows the survival of large numbers of worms and effectively protects most developmental stages including adults.

Cryopreservation of dehydrated Caenorhabditis elegans with multiple recoveries using a granular medium

Ultracold storage is widely used to preserve genetic stocks. Standard cryopreservation methods for the nematode C. elegans are vulnerable to refrigeration failures, which can result in the loss of stock viability due to freeze-thaw damage. In previous work our laboratory developed a method for cryopreserving worms in a dehydrated form that remains viable after multiple freeze-thaw cycles. However, strains preserved in this manner can be recovered only once from each cryopreservation tube. Here we describe a cryopreservation method in which C. elegans are dehydrated in a granular medium (cornmeal) prior to freezing. To recover worms, a small fraction (~1%) of the medium may be removed with the remainder returned to cold storage. Our improved cryopreservation method is not only resistant to refrigeration failures but also greatly increases the number of recoveries per tube compared to current methods.