NOW YOU SEE ME, NOW YOU DON?T! SELECTION STRATEGIES EMPLOYED BY THE OOCYTE TO BE CORRECTLY FERTILIZED

GUIDOBALDI, HA

Buenos Aires

Congreso; Reunión Conjunta de Sociedades de Biociencia; 2017

Sociedad Argentina de Biología

Polyspermy is the fusion of two or more spermatozoa with the same oocyte. In some organisms, like birds, it is necessary for a normal fertilization and embryo development, and it is known as physiological polyspermy. However, in many taxa, including mammal?s polyspermy results in the death of the embryo, and is called pathological. From an evolutionary point of view, the latter is expensive for females since: i) it reduces the fitness by the loss of viable embryos, ii) the energy investment destined to block the polyspermy could be invested in producing more oocytes, and iii) it can prevent fertilization if the mechanism of blocking is too efficient. Hence females will be under a selection pressure to optimize their response mechanism to reduce the risk of polyspermy. Meanwhile, males compete with others in a race for fertilization. And the selection pressure, promotes the production of many spermatozoa highly efficient to fertilize the oocyte. Being, the pathological polyspermy a collateral damage of that sperm competition. In this context of gender conflicts, communication between gametes, mediated by chemotaxis, arises as a coordinating mechanism that could reconcile the differences of interests between males and females. In the 90?s it was discovered that mammalian spermatozoa could be attracted by substances released during ovulation. The characterization of the sperm chemoattraction mechanism showed that only a subpopulation of capacitated spermatozoa (those in optimum physiological conditions for fertilization) could be chemoattracted by substances released by cumulus cells that surrounds the oocyte (like progesterone or CRISP1). These substances, can diffuse passively along the cumulus and beyond, forming a concentration gradient of attractant that orients the spermatozoa inside the oviduct towards the oocyte. However, once fertilization occurs, this attraction system remains active keeping the oocyte ?visible? for those spermatozoa seeking to fertilize. Up to date, the polyspermy blocking mechanisms characterized in mammals (removal of binding proteins or mechanical blockage at the level of the zona pellucida) take several minutes to hours to be effective, and there is no rapid blockage mechanism characterized yet. Recently, we have discovered that capacitated spermatozoa can be chemorepelled, also by a progesterone concentration gradient but in the presence of zinc in the culture medium. This is an interesting discovery, since the oocyte stores zinc in micro-vesicles next to the plasmatic membrane. And, immediately after fertilization, zinc is released outside of the oocyte by exocytosis. Hence, the zinc released could convert the progesterone attraction gradient into a repulsion one to ?hide? the oocyte from the sight of the capacitated spermatozoa and reorient them away from the fertilized oocyte. Then, the combination of chemoattraction and chemorepulsion, would allow to regulate the fertilization compensating the interests of both genders. Chemoattraction would allow sperm competition, but once fertilized the oocyte, chemorepulsion could discourage it immediately, preserving the normal development of the embryo. The combination of both mechanisms, optimizes the energy invested by the oocyte to blockade the polyspermy and guarantees the normal fertilization.