In this study, the effect of the steps involved in
zona-free somatic cell nuclear transfer (SCNT) on oocyte transcripts was investigated in sheep. To establish the reliable combined electrical-chemical activation for
zona-free oocytes, oocytes were first exposed to an electrical pulse and then treated with 18 chemical activation regimens designed through modifying duration and concentration of
ionomycin and 6-dimethyl aminopurine (6-DMAP), which is routinely used for SCNT. Electrofusion-mediated nuclear transfer significantly reduced transcript abundances of CCNB1, POU5F1, NPM2, GMMN, and
CX43 compared to intact oocytes. Maximum parthenogenetic blastocyst development was obtained when oocytes were submitted to electric pulse and then to (1) 5 μM
ionomycin for 5 or 2.5 min, both followed by 2 h of incubation with
6-DMAP (41.7±1.1, and 42.4±1.4%, respectively), (2) 5 μM
ionomycin for 1 min+6-DMAP for 4 h (43.1±1.4%), and (3) 2.5 μM
ionomycin for 1 min+6-DMAP for 2 h (42.4±1.4%), with significant differences compared to all the other groups. Statistical assessment of interactions between duration and concentration of
ionomycin and duration of
6-DMAP exposure revealed that (1) concentration of
ionomycin may be a more important factor than its duration, (2) both a long exposure period and a low concentration of
ionomycin had marked decreasing effects on parthenogenetic development of
zona-free oocytes, and (3) high duration of exposure to
6-DMAP can reduce parthenogenetic development. Despite an activation preference of parthenogenetic oocytes, a significantly higher rate of cloned blastocyst development was observed when reconstructed oocytes were activated with 5 μM
ionomycin for 5 min rather than 2.5 μM
ionomycin for 1 min (8.8±2.5 vs. 1.25±2.2%). These results suggested that SCNT steps have determining effects on oocyte transcripts and activation preferences of the reconstituted oocytes compared to intact counterparts. In this sense, reconstituted oocytes may need a higher concentration of
ionomycin for a longer period than intact oocytes.