Viability assessment of honey bee, Apis mellifera, sperm using dual fluorescent staining

Since the development of instrumental insemination of honey bee (Apis mellifera) queens in the 1930s, there has been interest in the evaluation and in vitro storage of semen. Several fluorescent stains, when used in combination, have been effectively used to assess sperm viability in mammalian and avian species. Our objectives were to test two combinations of living:dead fluorescent stains, SYBR-14 with propidium iodide (PI), or Calcein-AM with PI, and validate the use of these probes with honey bee sperm. SYBR-14 is a nuclear stain producing green fluorescence of the DNA in living sperm, Calcein-AM is a membrane-permeant esterase substrate staining entire sperm green, and PI is a traditional dead cell stain giving a contrasting red color. Both living stains fluoresced bee sperm, but the SYBR-14:PI produced a clearer distinction between the living and dead sperm. A graduated series of known living:dead sperm proportions was used to validate the accuracy of the stains for determining sperm viability in honey bees.