Heterozygous fitness effects of clonally transmitted genomes in waterfrogs

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The European waterfrog Rana esculenta (RL-genotype) is a natural hybrid between R. ridibunda (RR) and R. lessonae (LL) and reproduces by hybridogenesis, i.e. it eliminates the L-genome from the germline and produces gametes only containing the clonally transmitted R-genome. Because of the lack of recombination, R-genomes are prone to accumulate spontaneous deleterious mutations. The homozygous effects of such mutations become evident in matings between hybrids: their offspring possess two clonal R-genomes and are generally inviable. However, the evolutionary fate of R. esculenta mainly depends on the heterozygous effects of mutations on the R-genome. These effects may be hidden in the hybrid R. esculenta because it has been shown to benefit from spontaneous heterosis. To uncouple clonal inheritance from hybridity, I crossed R. esculenta with R. ridibunda to produce nonhybrid offspring with one clonal and one sexual R-genome, and compared their survival and larval performance with normal, sexually produced R. ridibunda tadpoles. Because environmental stress can enhance the negative effects of mutation accumulation, I measured the performance at high and low food levels. There was no indication that tadpoles with a clonal genome performed worse at either food level, suggesting that at least in the larval stage, R. esculenta benefits from heterosis without incurring any costs because of heterozygous effects of deleterious mutations on the clonally transmitted R-genome.

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