Multivariate analysis of male mating success on morphometric traits and chromosome dosage in the grasshopper Sinipta dalmani.

Sinipta dalmani is a South American grasshopper that is chromosomally polymorphic for a pericentric inversion in the M4 pair. The inversion affects morphology, showing a negative average effect on body size. In the present work, male mating success was analysed in competition cages and possible correlations examined between this fitness component, body size and inversion polymorphism. The outcome of the study revealed that mated males were larger and had lower inversion dosage than unmated ones. The analysis of standardized selection differentials (S') demonstrated that directional selection operated on chromosome dosage and 3rd tibia, 3rd femur, thorax and tegmina lengths. The analysis of karyotype frequencies showed that mated males had a higher frequency of the standard homozygote karyotype. The analysis of selection gradient (beta') and the transformed logistic regression (alpha) showed an important effect of selection on 3rd tibia length. Identical conclusions were obtained when linear and logistic multiple regression were performed on the first three components of PCA analyses. Our results indicate that differences in some body size-related traits are determined by the karyotype, and that of these length of 3rd tibia provides the greatest contribution to variation in fitness. The selective effects detected on chromosome dosage and the other morphological traits may be considered as indirect ones caused by correlated effects.