Metabolic Consequences of Hybridization in European Water Frogs ( <i>Pelophylax esculentus</i> complex)

Hybridization influences speciation processes, either slowing or reversing species differentiation due to gene flow and recombination, or else accelerating speciation via adaptive introgression and/or polyploidization. One of many consequences of polyploidization is an increase in cell size associated with genome multiplications. Although cell size is regarded as affecting Darwinian fitness across environmental gradients, particularly due to its effects on oxygen transport, fitness effects of changes in cell size associated with hybridization are not well understood. In this study, we examined the effects of ploidy level, our proxy for cell size, and genotypes on metabolic responses to thermal and oxygen conditions in tadpoles of European water frogs (<i>Pelophylax esculentus</i> complex). Hybrids (<i>P. esculentus</i>), originating from the primary hybridization between <i>P. lessonae</i> (genotype LL) and <i>P. ridibundus</i> (RR), were crossed to produce tadpoles with various genotypes (RR, LR, LLR, LRR) and ploidy levels (diploid, triploid). Our results indicate that triploids, particularly LLR, are most susceptible to oxygen limitation in hypoxic water. Additionally, RR progeny with introgressed <i>P. lessonae</i> mtDNA exhibited the lowest metabolic rates under normoxia, suggesting mitochondrial dysfunction due to mitonuclear incompatibility. The greater oxygen limitation in triploids, particularly under hypoxic conditions, may explain their preference for cooler climates. In a time of rapid environmental change, uncovering the physiological trade-offs associated with hybrid and polyploid genotypes, in connection with cell size changes, is a promising framework for predicting species responses to shifting oxygen and temperature regimes.