HYBRID DYSFUNCTION IN FIRE-BELLIED TOADS (BOMBINA)

Area of Science:

• Evolutionary Biology
• Speciation Research
• Amphibian Ecology

Background:

• Reproductive isolation mechanisms, including endogenous and exogenous selection, are crucial for speciation.
• The relative importance of these selection modes remains debated, necessitating clear fitness data for hybrid genotypes.
• The Bombina hybrid zone presents a model system to study these processes, though hybrid dysfunction evidence is currently equivocal.

Purpose of the Study:

• To investigate the fitness of hybrid genotypes in the Bombina hybrid zone.
• To determine the impact of hybridization on embryonic and larval development.
• To assess the role of endogenous and exogenous selection in reproductive isolation between Bombina bombina and Bombina variegata.

Main Methods:

• Collection of naturally laid eggs from a transect across the Bombina hybrid zone in Croatia.
• Laboratory-rearing of collected eggs to assess developmental success and mortality.
• Quantification of embryonic and larval mortality rates and morphological abnormalities in hybrid and parental populations.

Main Results:

• Hybrid populations exhibited significantly reduced fitness compared to parental types.
• Central hybrid zone populations showed substantially higher embryonic and larval mortality (up to 20%) versus pure populations (2%).
• Increased frequencies of morphological abnormalities were observed in hybrid offspring, with notable variation in fitness among hybrid genotypes.

Conclusions:

• Hybridization in the Bombina zone results in significant fitness costs, primarily due to increased mortality and developmental issues.
• These findings support the role of endogenous selection (genomic incompatibilities) in driving reproductive isolation.
• Variation in hybrid fitness suggests complex interactions influencing gene flow barriers between these toad species.