Abstract

The evolutionary factors affecting testis size are well documented, with sperm
competition being of major importance. However, the factors affecting sperm
length are not well understood; there are no clear theoretical predictions and
the empirical evidence is inconsistent. Recently, maternal effects have been
implicated in sperm length variation, a finding that may offer insights into its
evolution. We investigated potential proximate and microevolutionary factors
influencing testis and sperm size in the bruchid beetle Callosobruchus maculatus
using a combined approach of an artificial evolution experiment over 90
generations and an environmental effects study. We found that while
polyandry seems to select for larger testes, it had no detectable effect on
sperm length. Furthermore, population density, a proximate indicator of
sperm competition risk, was not significantly associated with sperm length or
testis size variation. However, there were strong maternal effects influencing sperm length.