For Male Caenorhabditis elegans, Sperm Activation Is a “Just-in-Time” Event

In the game of evolutionary fitness, males must maximize the chance that their sperm successfully fertilize oocytes. Speciesspecific strategies include making more, bigger, or faster sperm, or producing seminal fluid that does more than serve as a vehicle for sperm transfer [1]. Importantly, seminal fluid components not only modulate sperm function and promote their competitiveness and long-term viability, but also initiate various physiological changes within the female such as increasing her rate of ovulation and decreasing her receptivity to other males [2–4]. Adding to this complexity, ejaculate is often generated in a series of compositionally distinct spurts resulting from the sequential emptying of various sexual glands [2,5]. Yet how most seminal proteins function, particularly in the context of sperm activation (Figure 1A), remains unclear. Even less is known about how reproductive fitness is maximized when both sexes (males and hermaphrodites) produce sperm. Certainly male and hermaphrodite Caenorhabditis elegans sperm face distinct challenges. In hermaphrodites, sperm are produced first, before their gonad switches over to exclusively producing oocytes [6]. Hermaphrodite sperm activate to form motile spermatozoa as they are physically pushed into the spermatheca by ovulating oocytes. Once there, they wait for fertilization opportunities as individual oocytes enter the spermatheca in an assembly-line fashion. Typically, every sperm gets an oocyte, and their only challenge is to remain in the spermatheca as the newly fertilized oocytes squeeze through the spermatheca on their way to the uterus [7]. In contrast, the larger male sperm are stored in a quiescent state within the seminal vesicle and only activate to form motile spermatozoa during the process of ejaculation. After insemination, male sperm must migrate from the vulva to the spermatheca and then out-compete the hermaphrodite’s own sperm [8,9]. Three lines of evidence suggest that C. elegans sperm activation is regulated in a sex-specific manner: I) Molecular genetic studies identified components of a sperm-specific SPE-8 signal transduction cascade that function in both male and hermaphrodite sperm but are only essential for the activation of the hermaphrod i te ’ s own sperm (Figure 1B) [10–14]. Although mutant hermaphrodites are self-sterile, their sperm can be trans-activated by seminal fluid from either wildtype or spe-8 males. Conversely, mutant males are fertile, but their sperm activates abnormally in response to in vitro activation by the protease Pronase. Together, these results suggest that although the sperm activators are expressed in a sex-specific manner, both male and hermaphrodite sperm retain the capacity to respond to either activator.