264 MITOCHONDRIAL DNA COPY NUMBER IN OOCYTES OF PREPUBERTAL AND CYCLIC GILTS

Abstract

In many domestic species (pig, cow, sheep), oocytes from prepubertal females show impaired quality when compared with those from adult animals. Incomplete cytoplasmic maturation is thought to be the main factor responsible for reduced developmental competence of embryos derived from prepubertal oocytes. The status of ooplasm maturation is also reflected by the copy number of mitochondrial DNA (mtDNA). Because replication of mtDNA ceases when oocytes reach their final size and occurs again at the blastocyst stage, the mtDNA copy number is a proved marker of oocyte quality in the pig (El Shourbagy et al. 2006 Reproduction 131, 233–245). The number of mtDNA copies in the grown oocyte is crucial to sustain the first embryonic divisions. To increase the rate of good-quality blastocysts, oocytes of domestic animals have been evaluated by the brilliant cresyl blue test (BCB). According to El Shourbagy et al. (2006), more competent BCB+ oocytes possess higher copy number of mtDNA (on average 222 446) than do their BCB– counterparts (115 352). However, there are no published data on the variation in mtDNA copy number in oocytes derived from ovaries of prepubertal (NCL) and cyclic (CL) gilts. Ovaries of NCL and CL gilts were collected in a local slaughterhouse. Cumulus–oocyte complexes (COC) were aspirated from nonatretic follicles 2 to 6 mm in diameter and evaluated morphologically. Only COC with a proper morphology were subjected to the BCB test. A group of non-BCB-treated COC served as control. Four groups of COC were collected: BCB+ (CL, NCL) and control (CL, NCL). Follicular cells attached to oocytes were removed by pipetting, and completely denuded gametes were individually frozen in liquid nitrogen. Analysis of the mtDNA copy number included isolation of the total DNA followed by amplification of the Cytochrome b (CYTB) gene by real-time PCR (one copy per one mitochondrial genome). Differences in mtDNA copy number among experimental groups were evaluated by Student’s t-test. To date, 30 BCB+ oocytes have been analysed individually (15 CL and 15 NCL). The analysed parameter varied in a wide range from 79 852 to 522 712 copies in CL oocytes and from 52 270 to 287 852 copies in NCL oocytes. Oocytes from cyclic gilts contained significantly more mtDNA copies (on average 267 524) than did gametes of prepubertal females (179 339; P < 0.05). The data on the mtDNA copy number in the control oocytes are currently under investigation. The preliminary results indicate that impaired oocytes quality of prepubertal gilts may be also attributed to the reduced copy number of mtDNA.