Abstract
Whether the ovarian fluid (OF) represents a selective environment influencing cryptic female choice was tested using an external fertilizer experiencing intense sperm competition and large effects of OF on sperm swimming behavior—the Arctic charr (Salvelinus alpinus). We physically separated the OF from the eggs of reproductively active females and reintroduced either their own OF or fluid from another female to the eggs. The eggs were then fertilized in vitro in a replicated split‐brood design with sperm from two males under synchronized sperm competition trials, while also measuring sperm velocity of the individual males in the individual OFs. We found large effects of males, but no effect of females (i.e., eggs) on paternity, determined from microsatellites. More important, we found no effect of OF treatments on the relative paternity of the two competing males in each pair. This experimental setup does not provide support for the hypothesis that OF plays an important role as medium for cryptic female choice in charr. Power analyses revealed that our sample size is large enough to detect medium‐sized changes in relative paternity (medium‐sized effect sizes), but not large enough to detect small changes in relative paternity. More studies are needed before a conclusion can be drawn about OF's potential influence on paternity under sperm competition—even in charr.
Highlights
Sperm competition occurs when ejaculates from different males compete over fertilizing eggs either inside the females’ reproductive tract or externally to the female’s body (Birkhead & Møller, 1998; Parker, 1970; Pizzari & Parker, 1998; Simmons, 2001; Stockley, Gage, Parker, & Møller, 1997)
Females of external fertilizers might not be regarded as only provid‐ ing an arena for sperm competition, as they may actively dis‐ criminate among which of the males involved in sperm competition is allowed to fertilize her eggs through cryptic choice (Birkhead, 1998; Eberhard, 1996; Olsson, Shine, Madsen, Gullberg, & Tegelstrom, 1996; Thornhill, 1983; Zeh & Zeh, 1996)
Our main finding is that exchange of ovarian fluid (OF) between egg batches from different females did not affect the males’ relative fertilization success under sperm competition
Summary
Sperm competition occurs when ejaculates from different males compete over fertilizing eggs either inside the females’ reproductive tract or externally to the female’s body (Birkhead & Møller, 1998; Parker, 1970; Pizzari & Parker, 1998; Simmons, 2001; Stockley, Gage, Parker, & Møller, 1997). This is the case for highly polyandrous broadcast spawners (Firman, Gasparini, Manier, & Pizzari, 2017), as for example in species with lek‐like mat‐ ing systems In some such species, cryptic female control under sperm competition has the potential to generate large differences in offspring survival (Rudolfsen, Figenschou, Folstad, Nordeide, & Søreng, 2005; Wedekind, Muller, & Spicher, 2001). An intraspecific study on Chinook salmon (O. tshawytscha) documented no overall effect of OF on pa‐ ternity success and no evidence for male–female interactions on pa‐ ternity (Evans et al, 2013) This is surprising, as positive associations between sperm velocity in OF and both fertilization success and em‐ bryo survival have been reported in the same species (Rosengrave et al, 2016). If OF acts as a medium for cryptic female choice, we predicted that our experimental exchange of OF between eggs from the two females would influence paternity
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