Abstract
Seminal fluids are known to have a variety of effects on rival sperm, but in externally fertilizing species it is still unclear what effects seminal fluid can induce under sperm competition. We recorded sperm activity from natural ejaculates (including own seminal fluid) of an external fertilizer, the Arctic charr (Salvelinus alpinus), after activation either in water (the natural medium for milt dilution), in a dilution of water and own seminal fluid or in a dilution of water and seminal fluid of a foreign male. When activation occurred in own or foreign dilutions of seminal fluids, sperm maintained higher velocities than when activated in water only. Yet, velocity did not differ depending on whether sperm was activated in own or foreign seminal fluid solutions. More important, approximately 25% fewer sperm cells were initially activated in own seminal fluid than in foreign seminal fluid or water, indicating that activation is under close control of own seminal fluid only. Our results document that the presence of foreign seminal fluid under sperm competition do not have apparent effect on sperm velocity. Yet, the large inhibitory effect on initial activation of sperm cells seen in own, but not in foreign dilutions of seminal fluids (and water) suggests an individual specific recognition mechanism exerted by something in the seminal fluid on own, but not foreign sperm cells. The importance of this extrasomatic sperm recognition for the outcome of sperm competitions is discussed.
Highlights
Species experiencing sperm competition, i.e., competition between ejaculates of two or more males for the fertilization of a limited number of ova (Parker, 1970) may be selected to adjust reproductive behavior, morphology, physiology, and ejaculate characteristics according to the likelihood and intensity of such competition (Møller and Birkhead, 1989; Parker, 1990; Simmons, 2001; Parker and Pizzari, 2010)
There is no difference between foreign and own seminal fluid on sperm velocity, sperm of unmanipulated ejaculates activated in seminal fluid dilutions maintain higher velocity through the activity period than sperm activated in water only
The initial (i.e., 10 s after activation) number of motile cells is higher when activated in a solution of foreign seminal fluid than when activated in own seminal fluid, yet this initial effect of foreign seminal fluid on sperm motility is not different from that of water
Summary
I.e., competition between ejaculates of two or more males for the fertilization of a limited number of ova (Parker, 1970) may be selected to adjust reproductive behavior, morphology, physiology, and ejaculate characteristics according to the likelihood and intensity of such competition (Møller and Birkhead, 1989; Parker, 1990; Simmons, 2001; Parker and Pizzari, 2010). Effects and compositional adaptations of seminal fluid chemistry in internal fertilizers may be influenced by potential rivals in sperm competition and by the selective environment brought upon the ejaculate by the female in her reproductive tract. The adaptive landscape for the seminal fluids of species with internal fertilization is quite complex (see, e.g., Holman, 2009), making these species difficult models for studies on the specific effects of seminal fluids on sperm competitions. External fertilizing species represent models with a relatively uniform environment for fertilizations, they often show high intensity of sperm competition and their reproduction is often mimicked in vitro
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