A successful new approach to honeybee semen cryopreservation

Honeybee biodiversity is under massive threat, and improved methods for gamete cryopreservation could be a precious tool for both the in situ- and ex situ-conservation of subspecies and ecotypes. Recent cryoprotocols for drone semen have improved the viability and fertility of frozen-thawed semen by using increased diluent:semen-ratios, but there is still much room for progress. As semen cryopreserved after dilution often appeared hyperactive, we speculated that the disruption of sperm-sperm interactions during dilution and cryopreservation could reduce the fertile lifespan of the cells. We therefore developed protocols to reduce admixture, or abolish it altogether by dialyzing semen against a hypertonic solution of cryoprotectant. Additionally, we tested methods to reduce the cryoprotectant concentration after thawing. Insemination of queens with semen cryopreserved after dialysis yielded 49%, 59% and 79% female (=stemming from fertilized eggs) pupae in three separate experiments, and the numbers of sperm found in the spermathecae of the queens were significantly higher than those previously reported. Post-thaw dilution and reconcentration of semen for cryoprotectant removal reduced fertility, but sizeable proportions of female brood were still produced. Workers stemming from cryopreserved semen did not differ from bees stemming from untreated semen with regard to indicators of fluctuating asymmetry, but were slightly heavier. Cryopreservation after dialysis tended to increase the proportion of cells with DNA-nicks, as measured by the TUNEL-assay, but this increase appears small when compared to the baseline variations of this indicator. Overall, we conclude that cryoprotectant-addition through dialysis can improve the quality of cryopreserved drone semen. Testing of offspring for vitality and genetic integrity should continue.