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.

The dataset consists of counts of the number of spermatocysts that stained positively with an antibody that recognizes Serine 10 phosphohistone H3, which indicates both mitotic and meiotic divisions. The images were collected by photographing stained testis tubules from adult male Narnia femorata that had either been autotomized at the fourth instar or control males that were handled, but not autotomized. The positively stained spermatocysts were counted in either the region of the testis tubule where mitotic, transit amplification divisions were occuring (zone 1) or in the region of the testis tubule where meiosis is occuring (zone 2). Counts were performed blind respective to treatment.