147 Pre-incubation of bovine sperm used for IVF accelerates the developmental kinetics of the resulting embryos and possibly their sex ratio

Abstract

The sex ratio of newborn calves and embryos produced in vivo is ~1:1. However, numerous studies on bovine in vitro-produced embryos suggest that the sex ratio may differ from 1:1 and that the rate of development may be influenced by the sex of the embryo under certain culture conditions. The duration of sperm-oocyte interaction and sperm pre-incubation also affect the sex ratio of bovine embryos produced in vitro. It is well documented that in vitro male embryos reach the more advanced stages earlier than do their female counterparts. Selection of developmentally more advanced embryos in anticipation that they have a greater developmental capacity may be one of the underlying causes of the disproportionate number of males among offspring born after transfer of in vitro-produced embryos. The aim of the present study is to test whether a pre-incubation of sperm before IVF might improve the developmental rates and also influence the sex ratio of the resulting embryos. Bovine cumulus-oocyte complexes were recovered from abattoir-derived ovaries by the slicing method. After 24 h of maturation, fertilization was realised using a standard protocol. Prior to IVF, sperm cells from 2 different bulls were treated as follows: sperm within group A were pre-incubated in IVF medium for one hour. This step was omitted for sperm in group B (control). After 19 h of co-culture of COC and sperm, presumptive zygotes were cultured in SOFaa for a period of 7 days. Cleavage and developmental rates were recorded at Day 3 and 7 (Day 0 = IVF). Day 7 blastocysts from all groups were sexed using bovine and Y chromosome-specific primers. Data were analysed by ANOVA. As shown in Table 1, sperm pre-incubation did not affect the cleavage and developmental rates for the individual bull (P > 0.05). On average, at Day 7 of development a higher number of blastocysts was determined when embryos had been produced from pre-incubated sperm (P ≤ 0.05). This held true for both bulls. The shift in favour of male embryos was detectable in all groups of embryos, with a drastic one for bull 1 after sperm pre-incubation. In conclusion, sperm pre-incubation accelerated embryo development and possibly enhanced the proportion of male embryos, which was already shifted toward males.

Table 1.  Developmental rates, developmental kinetics and sex ratio of embryos after sperm pre-incubation before IVF (mean ± standard deviation)