Allometric patterns and phenotypic variation of sexual traits in two sympatric scorpion species with reproductive interference

Abstract

Allometric patterns and phenotypic variation of some traits reveal which selective pressures underlie their evolution. In sexual traits these patterns may fluctuate according to their functional role as well as other factors such as the sexual selection (SS) intensity. Two scorpions species (Urophonius achalensis, U. brachycentrus) with reproductive interference (RI) in a sympatric zone (Córdoba, Argentina) are well suited to explore the relationship between SS intensity (higher under RI) and allometric and phenotypic variation patterns in sexual traits. For this, we compared allometric slopes (estimated by major axis regression) and coefficients of variation (CV) of male multiple traits used in courtship between allometric and sympatric populations. We predicted isometry for traits used in courtship and feeding (‘mixed traits’: pectines, pedipalps, chelicerae, telson), negative allometry for ‘pure traits’ used exclusively in courtship for grasp the female pedipalps (pedipalp apophysis) and positive allometry for a ‘pure traits’ used for female stimulation (telson gland). We found higher CVs for pure traits than for mixed traits. We obtained isometry for mixed traits, evidencing a balance in the underlying selective forces (SS and Natural selection). The telson gland was allometrically positive, with a greater slope in sympatry than in allopatry for one species. Contrary to our prediction, the pedipalp apophysis showed positive allometry. Interestingly, in the smaller species the pectines were allometrically positive, suggesting that the role of scramble competition could be underlying this pattern. We will discuss these results in terms of the asymmetry of SS intensity an RI consequences for these species.