# Metabolic Consequences of Hybridization in European Water Frogs (Pelophylax esculentus complex)

## TL;DR

This study explores how hybridization and cell size affect the metabolism of tadpoles in European water frogs under different oxygen and temperature conditions.

## Contribution

The study provides new insights into how hybrid and polyploid genotypes influence metabolic performance and environmental adaptation in amphibians.

## Key findings

- Triploid tadpoles, especially LLR, are more vulnerable to oxygen limitation in hypoxic conditions.
- RR tadpoles with introgressed P. lessonae mtDNA show lower metabolic rates due to mitochondrial dysfunction.
- Triploids prefer cooler climates due to greater oxygen limitations under hypoxia.

## Abstract

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 (Pelophylax esculentus complex). Hybrids (
P. esculentus
), originating from the primary hybridization between
P. lessonae
(genotype LL) and
P. ridibundus
(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
P. lessonae
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.

Our study links hybridization, ploidy, and cell size to metabolic performance under varying oxygen and temperature conditions in an amphibian larval model. By revealing physiological constraints tied to genotype and mitonuclear mismatch, it provides mechanistic insights into how hybrid and polyploid organisms may respond to environmental change.