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When females mate promiscuously, sperm from rival males compete within the female reproductive tract to fertilize ova. Sperm competition is a powerful selective force that has shaped sexual behavior, sperm production, and sperm morphology. However, nothing is known about the influence of sperm competition on fertilization-related processes, because it has been assumed that sperm competition only involves a race to reach the site of fertilization. We compared four closely related rodent species with different levels of sperm competition to examine whether there are differences in the proportion of spermatozoa that become ready to interact with the ovum ('capacitated') and in the proportion of spermatozoa that experience the acrosome reaction in response to a natural stimulant. Our results show that differences between species in levels of sperm competition were associated with the proportion of spermatozoa that undergo capacitation and with the proportion of spermatozoa that respond to progesterone, an ovum-associated signal. Sperm competition thus favors a larger population of spermatozoa that are competent to fertilize, and spermatozoa that are more sensitive to the signals emitted by the ovum and that may penetrate the ova vestments more rapidly. These results suggest that, contrary to previous assumptions, competition between spermatozoa from rival males continues at the site of fertilization. These findings may have further evolutionary implications because the enhanced competitiveness of spermatozoa during fertilization may increase the risk of polyspermy to females. This could lead to antagonistic coevolution between the sexes and may contribute to the explanation of the rapid divergence observed in fertilization-related traits. sperm function | capacitation | acrosome reaction fertilization | antagonistic coevolution

Exocytosis in mammalian spermatozoa (the acrosome reaction) is a process essential for fertilization. Both progresterone and zona pellucida induce exocytosis in spermatozoa, which may encounter both during penetration of the oocyte's vestments. When mouse spermatozoa were exposed first to progresterone and then to zona pellucida, exocytosis was enhanced to a greater degree than that seen when the agonists were presented together or in the inverse order, which suggests that the steroid exerts a priming effect. Progesterone similarly primed the generation of intracellular messengers evoked by zona pellucida. The effects triggered by progesterone were mimicked by γ-aminobutyric acid (GABA) and were blocked by bicuculline, which indicates that the steroid acts on a GAB[A.sub.A] receptor., At fertilization, the spermatozoon undergoes exocytosis in response to an oocytederived signal or signals (1). However, two conflicting views exist regarding the agonist responsible for triggering exocytosis. The generally accepted [...]

A study compares 25 species of snakes with different levels of sperm competition to investigate whether sperm competition and reproduction mode of oviparity or viviparity influence their sperm dimensions and structure. Findings indicate that sperm competition and reproductive mode influence sperm components differently in different taxa of snakes, since their structure and function are different in the taxa.

The study hypothesizes that the degree of gametic incompatibility in reciprocal crosses is linked with levels of sperm competition as this selective factor supports increased sperm competitiveness and ovum defensiveness. Results suggest sperm competition is associated with degree of gametic isolation and of asymmetries between closely related species.

The effect of inbreeding depression on parasite infection in three species of endangered Gazelles is studied. The studies suggest that high levels of inbreeding may make individuals more susceptible to parasitism, even under favorable environmental conditions and hence this factor should be taken in to account by those managing endangered species.

Abstract: We investigated whether GABA activates phospholipase A2 (PLA2) during acrosomal exocytosis, and if the MEK-ERK1/2 pathway modulates PLA2 activation initiated by GABA, progesterone or zona pellucida (ZP). In guinea pig spermatozoa prelabelled with [14C]arachidonic acid or [14C]choline chloride, GABA stimulated a decrease in phosphatidylcholine (PC), and release of arachidonic acid and lysoPC, during exocytosis. These lipid changes are indicative of PLA2 activation and appear essential for exocytosis since inclusion of aristolochic acid (a PLA2 inhibitor) abrogated them, along with exocytosis. GABA activation of PLA2 seems to be mediated, at least in part, by diacylglycerol (DAG) and protein kinase C since inclusion of the DAG kinase inhibitor R59022 enhanced PLA2 activity and exocytosis stimulated by GABA, whereas exposure to staurosporine decreased both. GABA-, progesterone- and ZP-induced release of arachidonic acid and exocytosis were prevented by U0126 and PD98059 (MEK inhibitors). Taken together, our results suggest that PLA2 plays a fundamental role in agonist-stimulated exocytosis and that MEK-ERK1/2 are involved in PLA2 regulation during this process. [Copyright &y& Elsevier]