26 Season affects cryotolerance of in vitro-produced buffalo embryos

Abstract

Buffaloes are tendentially short-day breeders, and seasonality is one of the main factors affecting the feasibility of ovum pickup and in vitro embryo production technology in this species. An improvement of oocyte developmental competence during decreasing daylight months was previously reported in Italian Mediterranean buffalo (Di Francesco et al. 2011 Anim. Reprod. Sci. 123, 48-53). The aim of this work was to evaluate whether season also affects embryo quality and cryotolerance. Abattoir-derived buffalo cumulus-oocyte complexes were collected during the breeding season, characterised by decreasing daylight length (n=349 over 6 replicates), and the non-breeding season, characterised by increasing daylight length (n=770 over 12 replicates). Buffalo cumulus-oocyte complexes were in vitro matured, fertilized, and cultured according to standard procedures (Di Francesco et al. 2011 Anim. Reprod. Sci. 123, 48-53). The embryos obtained by the end of culture (i.e. on Day 7 post-IVF) were scored for quality and developmental stage, and the percentages of total transferable embryos (tight morulae and blastocysts) were recorded. Embryos (n=107 and 110 in the breeding and non-breeding seasons, respectively) were vitrified by cryotop in 16.5% ethylene glycol, 16.5% dimethyl sulfoxide, and 0.5M sucrose (Boccia et al. 2013 Ital. J. Anim. Sci. 12, 492-496). Warming was carried out by plunging the cryotop strip into a 0.25M sucrose solution and transferring the embryos into 0.15M sucrose for 5min. Embryos were then washed and cultured in SOF for 24h to evaluate post-culture viability. The resistance to cryopreservation was evaluated by assessing the survival rate, on the basis of morphological criteria, and development rate (i.e. the percentage of embryos that resumed their development and reached a more advanced developmental stage) after 24h post-warming culture. Data were analysed by Student’s t-test. Both cleavage (82.8±4.3v. 73.1±1.7 in the breeding and non-breeding seasons, respectively; P<0.05) and blastocyst (32.9±3.5v. 18.3±1.7 in the breeding and non-breeding seasons, respectively; P<0.01) rates increased during the breeding season, confirming previous observations. Due to the different efficiency, a higher number of replicates was required during the non-breeding season to obtain an equal number of embryos. In addition, a seasonal effect was recorded on embryo quality, indicated by poorer cryotolerance of in vitro-produced buffalo embryos during the non-breeding season. Indeed, both survival (94.6±2.7% and 74.0±5.5% in the breeding and non-breeding seasons, respectively; P<0.01) and development (67.3±7.6% and 40.0±7.2% in the breeding and non-breeding seasons, respectively; P<0.01) rates of vitrified blastocysts decreased after 24h post-warming culture in the non-breeding season. These findings suggest that the reduced developmental competence of buffalo oocytes during the non-breeding season may also lead to lower blastocyst quality. This is in contrast to the evidence in cattle that embryo quality is mainly determined by culture conditions, whereas blastocyst production depends on oocyte quality.