144 HEAT TOLERANCE OF DEMI-EMBRYOS FROM A TROPICAL-ADAPTED BOS TAURUS BREED

Abstract

Long-time selection of autochthonous Bos taurus breeds in a tropical environment is known to result in some physiological changes, including gamete thermotolerance. The aim of this study was to evaluate the resistance to heat shock (HS) of demi-embryos from tropical-adapted B. taurus cattle. The experiment was carried out in a tropical wet climate, at the experimental station of Instituto de Zootecnia (latitude 22°46′S; longitude 47°17′W) from November to February (average maximum temperature = 30.0 ± 0.8°C, and average absolute precipitation = 153.1 ± 78.8 mm³). Embryos (morulae) from 37 superovulated Caracu (a local B. taurus breed) cows were collected by uterine flushing on Day 6 after AI. Sixty-two grade I morulae were split with a metal microblade coupled to a micromanipulator, and each demi-embryo was placed separately in a 35-μL drop of culture medium [SOF supplemented with modified Eagle medium amino acids, sodium citrate, myo-inositol (SOFaaci), and 10% FCS] under mineral oil in a humidified atmosphere of 5% CO₂, at 38.5°C for 24 h (control) or at 41°C for 12 h and a further 12 h at 38.5°C (HS). Halves of morulae that developed to the blastocyst stage (showing a central cavity) after a 24-h culture were transferred to recipient heifers previously synchronized with the donors. Pregnancy was monitored by ultrasound scanning 40 days after transfer, and data were analysed by chi-square test. There was a significant difference in the development to the blastocyst stage between control (44/62, 71.0%) and HS (30/62, 48.4%) demi-embryos (P < 0.01). Pregnancy rate obtained from control (14/44, 31.8%) and HS (8/30, 26.7%) blastocysts was not different (P > 0.05). We conclude that HS affected embryonic development from the morulae to blastocyst stage of cultivated demi-embryos, although fertility of blastocysts that survived to HS was not further compromised. Further studies are necessary to elucidate whether Caracu embryos are more thermotolerant than other tropically sensitive B. taurus breeds.