Effect of cryopreservation on fish sperm subpopulations.

The evaluation of the motility data obtained with a CASA system, applying a Two-Step Cluster analysis, identified in seabream sperm 3 different sperm subpopulations that correlated differently with embryo hatching rates. Hence, we designed an experiment to understand the effect of the application of different cryopreservation protocols in these sperm motility-based subpopulations. We analyzed Sparus aurata frozen/thawed semen motility 15, 30, 45 and 60s after activation, using CASA software. Different protocols were applied for cryopreservation: three different cryoprotectants (Dimethyl Sulfoxide (Me(2)SO), Ethylene Glycol (EG) and Propylene Glycol (PG)) each at two different concentrations and two packaging volumes (0.5ml straws, and 1.8ml cryovials) were tested. Different freezing rates were evaluated corresponding to 1, 2, 3, 4 and 8cm above the liquid nitrogen surface for the straws and 1, 2 and 4cm for the cryovials. Motility parameters rendered by CASA were treated with a Two-Step Cluster analysis. Three different subpopulations were obtained: SP1 - slow non-linear spermatozoa, SP2 - slow linear spermatozoa and SP3 - fast linear spermatozoa. We considered SP3 as the subpopulation of interest and focused further analyses on it. Generally, SP3 was the best represented subpopulation 15s after activation and was also the one showing a greater decrease in time, being the least represented after 60s. According to the applied univariate general linear model, samples frozen in straws with 5% Me(2)SO and in cryovials with 10% Me(2)SO at 2 and 1cm from the LN(2,) respectively, produced the best results (closer to the control). Clustering analysis allowed the detection of fish sperm subpopulations according to their motility pattern and showed that sperm composition in terms of subpopulations was differentially affected by the cryopreservation protocol depending on the cryoprotectant used, freezing rates and packaging systems.