Masked Damage: Mutational Load in Hemiclonal Water Frogs

Hemiclonal hybrids of Western Palearctic water frogs of the Rana esculenta complex transmit only one parental genome to their offspring without recombination (hybridogenesis). Such genomes are thus prone to accumulate deleterious mutations. The frog complex is unique among hybridogens in that hemiclonal hybrids occur in both sexes. This provides the opportunity of using experimental crosses to produce offspring possessing two clonal genomes of various origins and thereby study their homozygous and heterozygous effects on fitness. Here we review work that made use of this possibility to assess the evolutionary consequences of clonal inheritance in water frogs. Overall, these studies indicate that clonally transmitted genomes bear a substantial load of fixed deleterious mutations, yet these mutations appear to have minor effects on fitness in the heterozygous state. We also point out potential mechanisms for episodic recombination by which otherwise clonal genomes may be purged of deleterious alleles, and we present evidence for such episodic recombination to occur in natural populations of hybridogenetic frogs. Finally, we provide an outlook on work in progress that exploits the peculiarities of this system to obtain relevant estimates of the frequency of segregating lethal mutations in sexual populations of water frogs