Evidence for tactically antagonistic selection on body size and the
sword in a wild population of the swordtail fish, Xiphophorus
multilineatus
Abstract
Intralocus conflict has been well documented between the sexes, but much
less is known about the potential for this genetic conflict in other
polymorphisms, such as alternative reproductive tactics. Here we
investigate two of three criteria necessary for demonstrating intralocus
tactical conflict: the ARTs have different phenotypic optima for a
shared trait, and one or both ARTs are not at their phenotypic optima
for a shared trait. We address these two criteria in Xiphophorus
multilineatus, a live-bearing freshwater fish that has two male
alternative reproductive tactics, a behaviorally fixed courter male and
a behaviorally plastic sneaker male that switches between courtship and
force-copulatory behavior. We used measures of reproductive success from
a wild population to estimate selection gradients on three tactically
dimorphic traits involved in sexual selection: body size, body shape,
and sword length. We present evidence that both body size and sword
length are experiencing tactically antagonistic selection, providing
evidence for both criteria. Additionally, selection on body shape in
sneaker males appears to be buffered due to behavioral plasticity. Our
study provides novel insight from a wild population into the role that
intralocus tactical conflict can play in constraining ARTs from reaching
their respective phenotypic optima despite tactical dimorphism.