Range-wide validation of reduced locomotor endurance in unisexual Ambystoma salamanders

Locomotor endurance is a critical physiological trait dictating terrestrial dispersal and metapopulation connectivity, especially in amphibians. The unisexual Ambystoma complex is an ancient, all-female polyploid lineage that reproduces via kleptogenesis. This unique reproductive mode creates an evolutionary mismatch between a conserved mitochondrial genome and divergent nuclear subgenomes that are taken from sympatric, sexual species. This provides a compelling system for testing the physiological limits of polyploidy and how mitonuclear dissonance and subgenome composition influence phenotypes. Previous locomotor assessments of this lineage demonstrated that polyploid salamanders display reduced locomotor endurance compared to sexual species. To overcome previous limitations in geographic sampling and sample size, we conducted standardized treadmill endurance trials on a broad geographic sampling of 102 salamanders, comparing the performance of triploid unisexual biotypes (LJJ and LLJ) directly to their sexually reproducing parental species (A. jeffersonianum and A. laterale). We found that reproductive mode significantly dictated endurance performance. Sexual species broadly outperformed unisexuals, driven by A. jeffersonianum exceeding all unisexual biotypes and A. laterale outperforming LJJ unisexuals. Within the unisexual cohort, subgenome dosage did not influence performance: LJJ individuals and LLJ individuals had similar performance.. We propose that the aerobic capacity of unisexual salamanders is limited, potentially due to mitonuclear mismatch or the biophysical constraints of increased cellular volume, but not due to the contributions of individual subgenomes. This endurance deficit is likely to restrict unisexual salamanders dispersal capabilities, leaving populations uniquely vulnerable to ongoing habitat fragmentation compared to more mobile, sexual species while potentially preventing unisexuals from driving sexual hosts locally extinct.