Males have the ability to compete for fertilizations through both
pre-copulatory and post-copulatory intrasexual competition.
Pre-copulatory competition has selected for large weapons and other
adaptations to maximize access to females and mating opportunities while
post-copulatory competition has resulted in ejaculate adaptations to
maximize fertilization success. Negative associations between these
strategies support the hypothesis that there is a trade-off between
success at pre- and post-copulatory mating success. Recently, this
trade-off has been demonstrated with experimental manipulation. Male
leaf-footed cactus bugs, Narnia femorata, that lose a weapon by autotomy
during development invest instead in large testes. While evolutionary
outcomes of the trade-offs between pre- and post-copulatory strategies
have been identified, less work has been done to identify proximate
mechanisms by which the trade-off might occur, perhaps because the
systems in which the trade-offs have been investigated are not ones that
have the molecular tools required for exploring mechanism. Here we
applied knowledge from a related model species for which we have
developmental knowledge and molecular tools, the milkweed bug Oncopeltus
fasciatus, to investigate the proximate mechanism by which autotomized
N. femorata males developed larger testes. Autotomized males had
evidence of a higher rate of transit amplification divisions in the
spermatogonia, which would result in greater sperm numbers.
Identification of mechanisms underlying a trade-off can help our
understanding of the direction and constraints on evolutionary
trajectories and thus the evolutionary potential under multiple forms of
selection.