Hybridization and introgression between toads with different sex chromosome systems

Area of Science:

• Evolutionary Biology
• Genetics
• Herpetology

Background:

• Sex determination systems are highly labile in vertebrates like amphibians.
• Closely related species often exhibit rapid sex chromosome turnover.
• Understanding hybridization between species with different sex determination is crucial for speciation research.

Purpose of the Study:

• To investigate hybridization and admixture between toad species with different sex determination systems.
• To analyze the role of labile sex chromosomes in speciation.
• To characterize the hybrid zone and introgression patterns between common toads (B. bufo) and spined toads (B. spinosus).

Main Methods:

• Genome-wide marker analysis (RADseq) to characterize the hybrid zone.
• Analysis of adult and sibship data to confirm sex determination systems.
• Species distribution modeling to infer range expansion.

Main Results:

• Common toad (B. bufo) is female heterogametic (ZW), while spined toad (B. spinosus) is male heterogametic (XY).
• A narrow hybrid zone (∼10 km) was identified in southeastern France.
• Asymmetric introgression and widespread cyto-nuclear discordances were observed, challenging Haldane's rule predictions.

Conclusions:

• Common and spined toads can successfully hybridize despite distinct sex determination mechanisms.
• Reproductive barriers persist between these species, potentially influenced by XY and ZW gene interactions.
• Labile sex chromosomes may play a significant role in the speciation process.