239 INFLUENCE OF BREED PREDOMINANCE ON MATURATION COMPETENCE, FERTILIZATION, AND DEVELOPMENT IN VITRO OF BOVINE OOCYTES

Abstract

The purpose of this research was to compare the competence of bovine oocytes of different breed predominance (Bos taurus v. Bos indicus) to mature and to be fertilized. This was done through the collection, selection, maturation, and fertilization of oocytes from slaughtered cows, predominantly either B. taurus or B. indicus. Only cows that were at least 5/8 B. taurus or 5/8 B. indicus, according to a series of phenotypic characteristics, such as the presence of a hump, dewlap, length of the ears, and others, were selected. To obtain cumulus–oocyte complexes, ovarian follicles (3 to 10 mm in diameter) were aspirated, and only oocytes with 2 or more layers of cumulus cells, an intact zona pellucida, and a homogeneous granular cytoplasm were selected. After selection, oocyte maturation [in vitro maturation (IVM)] and IVF were done. Frozen–thawed semen was used from one Brahman bull (B. indicus). For the evaluation of IVM as for IVF, oocytes were fixed for approximately 24 h at 4°C in a solution of ethanol : acetic acid (3 : 1). They were then stained with 1% acetic orcein. A chi-squared test was performed for all reported rates. The rate of maturation of oocytes from cows with a predominant B. indicus phenotype was 66.93%, whereas cows with a B. taurus phenotype reached 43.10% (P < 0.001). Regarding the fertilization rate, predominantly B. indicus females had 43.68% of oocytes normally fertilized and 41.74% of oocytes were abnormally penetrated. This category included polyspermic and asynchronic (abnormally developed pronucleus) oocytes. In cows with B. taurus predominance, 31.96% of oocytes were normally penetrated and 46.39% were abnormally penetrated by spermatozoa (no significant differences were found). The rate of non-fertilized oocytes was significantly different (P < 0.05) among B. indicus and B. taurus oocytes (6.79 and 17.52%, respectively). A small and nonsignificant proportion of degenerated oocytes resulted in both groups (7.79% for B. indicus and 4.14% for B. taurus). The cleavage rate was not different among phenotypic groups (36.12 and 32.30%, respectively, for B. indicus and B. taurus). In conclusion, the present results indicate that oocytes from predominantly B. indicus cows were more competent than oocytes from cows with a predominance of the B. taurus breed. Nonetheless, this superiority was not evident in terms of cleavage rates. Semen from other B. indicus and B. taurus breeds must be tested to clarify any breed interactions.