Thesis title: Influenza dei cambiamenti climatici sulla stagionalità della pecora sarda: indagine ultrastrutturale sulla qualità dell'ovocita
Seasonality of reproduction, a common feature in sheep of temperate latitudes, has been recognized for a very long time. Sheep are domesticated animals, which in temperate latitudes still remain seasonal breeders (Misztal et al., 2002). In general, the annual breeding season in Sarda sheep is controlled by photoperiod: long days inhibit and short days stimulate sexual activity (Mara et al., 2013). Then the season of high sexual activity in Sarda sheep is represented by autumn. The reproductive seasonality of domestic animals is often manipulated to have extended reproductive periods for commercial purposes related to the production of milk and meat. In Sardinia, an extension of the sheep breeding season is finalized to distribute lambing along the year to ensure milk production for several additional months as well as to satisfy the meat market demand. This is achieved using the out-of-breeding mating strategy, which consists of interrupting sexual promiscuity using of the male effect and exogenous hormones for oestrus cycle control. In this way the lambing calendar is modified in order to obtain 30–40-day-old lambs (weighing 8–10 kg) during the Easter and Christmas celebrations. However, and despite their wide and frequent use, these strategies may result in decreased fertility (Serra et al., 2021). Although the reproductive performance of grazing cattle is lower in summer compared to winter, the effect of season on oocyte developmental competence has not been thoroughly examined (Baez et al., 2021). In the present study we evaluated the effects of season, in terms of climate conditions (temperature and humidity), on oocyte quality. Oocytes were collected during two different period: group 1, Winter 00 (January- March) (W00) and group 2, Summer Non Gravidic (May-June and July) (SNG), and their quality was investigated through morphological and ultrastructural evaluation by light and transmission electron microscopy (LM and TEM), to evidence possible alterations. In addition, we wanted to make a further assessment of the effect of the gravidic corpus luteum on oocyte quality by comparing oocytes collected during the summer period in non-pregnant and pregnant ewes: the above-mentioned group 2, Summer Non Gravidic (SNG), and group 3, Summer Gravidic (SG), respectively. By LM and TEM, oocytes of group 1 (W00) and group 2 (SNG) evidenced a round shape with a thin perivitelline space surrounded by a continuous zona pellucida; microvilli covered the oolemmal surface. The ooplasm presented numerous mitochondria clusters, clear vacuoles and highly electron-dense lipid droplets that in group 1 oocytes appeared more abundant and mostly located in the cortical ooplasm. On the contrary, group 3 (SG) oocytes showed clear differences from group 2 (SNG) oocytes; the former were no longer perfectly spherical in shape, but irregular and surrounded by a much larger perivitelline space with the presence of spikes of the oolemma attached patchily to the zona pellucida. Vescicless also appeared more abundant in group 3 oocytes, when compared to group 2. In conclusion, our data showed very few differences between the first two groups examined (W00 and SNG) but greater differences emerged between the groups SNG and SG. These preliminary findings could be useful to better investigate how ovine oocyte quality could be influenced by environmental conditions and by season. In addition, these results stimulate further research and deep investigation on the effects of the gravidic corpus luteum on oocyte quality.