Dynamics of ovulation in the South American plains vizcacha, Lagostomus maximus (Mammalia, Rodentia): Massive ovulation, ovulation during gestation and oocyte quality

WILLIS MIGUEL A.; LEOPARDO NOELIA P.; FRAUNHOFFER NICOLAS A.; VITULLO ALFREDO D.

New York

Encuentro; 2010 Germ Cells Meeting; 2010

Cold Spring Harbor Laboratory

The South American plains vizcacha, Lagostomus maximus , is a hystricognathe rodent that displays the highest ovulation rate recorded for a mammal, reaching up to 800 oocytes per cycle. This massive polyovulation has been classically explained as the result of the highly convoluted ovary increasing the organ surface for ovulation. More recently, we have shown that apoptosis-dependent follicular atresia is suppressed due to the over expression of anti-apoptotic BCL2 gene and little or no expression of pro-apoptotic BAX, leading to little restriction to germ cell proliferation and folliculogenesis. Despite massive ovulation, a few 8 to 12 embryos are implanted and only 2 of them are born after a 155-day long gestation. The remaining embryos are resorbed at mid gestation. Ovulation number has been examined in the early 70´s by Weir who also suggested the possibility of ovulation during gestation, an event recently reinforced by our fortuitous finding of an ovulating female at mid-gestation. We examined during the last two years, natural and induced ovulation rate, ovulation during gestation, and oocyte quality in females throughout the reproductive season from a natural population at Buenos Aires province (ECAS, Villa Elisa). 22 out of 53 females captured at the beginning of the reproductive season, displayed a natural ovulation mean of 154.5 ±18.67, a quite far number from the 800 value mentioned above. It is worth, however, to mention that when animals analyzed by Weir (N= 9) are re-analyzed not taking into account the only female showing 800 eggs, a mean value of 159.9 ±35.48 comparable to our result emerges. Induction of ovulation by the administration of eGC and hCG or PPLE yielded up to 284 eggs. A total of 13 females were trapped at mid-gestation; 10 of them showed ovaries plenty of ovulatory points, however a very low number of eggs were recovered from oviduct flushing. Histological inspection of these ovaries reflected a high proportion of oocyte-retained ovulatory follicles, indicating an active recruitment process rather than a true ovulation. A very poor oocyte quality was evident both in natural and induced ovulation regarding morphological integrity and chromosome segregation. Interestingly, up to 30% parthenogenetic spontaneous activation was found. Our results confirm massive ovulation and highlight the existence of follicle recruitment/ovulation at mid-gestation. Low quality of released eggs could arise from suppression of apoptosis-dependent atresia.