Evaluation of carboxylated poly‐l‐lysine for cryopreservation of Labeo rohita sperm

Carboxylated poly-l-lysine (CPLL) is an anti-freeze agent having pronounced non-permeating yet membrane stabilizing cryoprotective capabilities. The objective was to evaluate the CPLL supplementation in extender in terms of post-thaw quality (sperm), total anti-oxidant activity (milt) and fertilization potential of cryopreserved Labeo rohita sperm. For this purpose, male brood fish reared at a fish seed hatchery, Rawal Town Islamabad, Pakistan were captured from different rearing ponds and acclimatized in hatchery ponds for 6 h. The brooder was injected with Ovaprim (0.2 mL/kg), and milt was collected after 8 h in cooled sterilized falcon tubes, maintained at 4°C and evaluated for sperm motility. The milt collected from three brooders (n = 3) was diluted in extenders viz., modified Kurokura-2 extender having 10% methanol (control); experimental extenders with CPLL supplementation at the rate of 0.5%, 1% and 1.5%. Diluted milt was filled in 0.5 mL straws, exposed to liquid nitrogen vapours and cryopreserved. Cryopreserved milt was thawed at 25°C and assessed for post-thaw sperm quality. Sperm motility, motility duration, viability, total anti-oxidant capacity and DNA integrity was significantly higher (p < 0.05) in the extender having 1.5% CPLL than control. To evaluate the fertilization rates, male and female brooders were injected with Ovaprim at 0.2 mL/Kg and 0.5 mL/Kg body weight respectively. Fresh eggs and milt were collected through abdominal stripping. Batches of 10 g of eggs from each female (n = 2) were fertilized with one straw, each from frozen sperm with KE + methanol (control), KE + methanol + 1.5% CPLL and 50 μL fresh milt (negative control). After 1.5 h of fertilization, eggs were collected from all jars and a total of 200 eggs were counted. The fertilized eggs appeared clear and transparent while unfertilized eggs looked opaque with disintegrated nuclei. Sperm fertilization rate (%) was higher (p < 0.05) in extender KE + methanol + 1.5% CPLL (78.7 ± 0.5) compared to control (KE + methanol) (52.0 ± 0.4) however, it was lower compared to that of negative control, the fresh milt (85.2 ± 0.6). In conclusion, supplementation of carboxylated poly-l-lysine (1.5%) to modified Kurokura-2 extender having 10% methanol improves post-thaw motility, motility duration, viability, DNA integrity, anti-oxidant capacity (milt) and fertilizing ability of cryopreserved L. rohita sperm.