Divergent testis allometry in two subspecies of the bushcricket Poecilimon veluchianus

Static allometry is frequently used as a metric of sexual selection, with traits under strong selection often expected to exhibit positive allometry. Although this appears largely to be true for pre-copulatory traits, such as weapons and ornaments, the allometry of post-copulatory sexually selected traits has seldom been investigated. Here, we explore the evolution of testis allometry between two bushcricket subspecies that have recently diverged in body size and yet still exhibit similar relative investment in testis size. By comparing Poecilimon veluchianus veluchianus with the smaller subspecies P. v. minor, we tested whether this pattern results from conserved isometry or from divergence in either the slope or the intercept of the allometric equation. Based on measurements of 246 males sampled from the wild during the mating season, we found that the similar relative testis size of the two subspecies was not the result of conserved isometric testis scaling and thus not simply explained by recent divergence in body size. Instead, both subspecies exhibited highly similar and positive allometric slopes for testis size (allometric coefficient = 1.4) but differed significantly in the intercept of these slopes. This result hints at an evolutionary constraint conserving the allometric slope between the subspecies but appears to indicate a greater evolvability of the allometric intercept.