Spermatocysts stained positively with anti-pHH3 antibody

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. Males of this species can lose their hind limbs, their
primary weapon in male-male competition, by autotomy during development.
Weaponless males 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 more spermatocytes and thus 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.