Data from: Delineating the roles of males and females in sperm competition

Disentangling the relative roles of males, females and their interactive
effects on competitive fertilization success remains a challenge in sperm
competition. In this study, we apply a novel experimental framework to an
ideally suited externally fertilizing model system in order to delineate
these roles. We focus on the chinook salmon, Oncorhynchus tshawytscha, a
species in which ovarian fluid (OF) has been implicated as a potential
arbiter of cryptic female choice for genetically compatible mates. We
evaluated this predicted sexually selected function of OF using a series
of factorial competitive fertilization trials. Our design involved a
series of 10 factorial crosses, each involving two ‘focal’ rival males
whose sperm competed against those from a single ‘standardized’
(non-focal) rival for a genetically uniform set of eggs in the presence of
OF from two focal females. This design enabled us to attribute variation
in competitive fertilization success among focal males, females (OF) and
their interacting effects, while controlling for variation attributable to
differences in the sperm competitive ability of rival males, and
male-by-female genotypic interactions. Using this experimental framework,
we found that variation in sperm competitiveness could be attributed
exclusively to differences in the sperm competitive ability of focal
males, a conclusion supported by subsequent analyses revealing that
variation in sperm swimming velocity predicts paternity success. Together,
these findings provide evidence that variation in paternity success can be
attributed to intrinsic differences in the sperm competitive ability of
rival males, and reveal that sperm swimming velocity is a key target of
sexual selection.