Fixation of deleterious mutations in clonal lineages: evidence from hybridogenetic frogs

The hemiclonal waterfrog Rana esculenta (RL genotype), a bisexual hybrid between R. ridibunda (RR) and R. lessonae (LL), eliminates the L genome from its germline and clonally transmits the R genome (hybridogenesis). Matings between hybrids produce R. ridibunda offspring, but they generally die at an early larval stage. Mortality may be due to fixed recessive deleterious mutations in the clonally inherited R genomes that were either acquired through the advance of Muller's ratchet or else frozen in these genomes at hemiclone formation. From this hypothesis results a straightforward prediction: Matings between different hemiclones, that is, between R. esculenta possessing different R genomes of independent origin, should produce viable R. ridibunda offspring because it is unlikely that different clonal lineages have become fixed for the same mutations. I tested this prediction by comparing survival and larval performance of tadpoles from within- and between-population crossings using R. esculenta from Seseglio (Se) in southern, Alpnach (Al) in central, and Elliker Auen (El) in northern Switzerland, respectively. Se is isolated from the other populations by the Alps. Enzyme electrophoresis revealed that parents from Se belonged to a single hemiclone that was different from all hemiclones found north of the Alps. Parents from Al also belonged to one hemiclone, but parents from El belonged to three hemiclones, one of which was indistinguishable from the one in Al. Rana esculenta from Se produced inviable tadpoles when crossed with other hybrids of their own population, but when crossed with R. esculenta from Al and El, tadpoles successfully completed metamorphosis, supporting the hypothesis I tested. Within-population crosses from Al were also inviable, but some within-population crosses from El, where three hemiclones were present, produced viable offspring. Only part of the crosses between Al and El were viable, but there was no consistent relationship between hemiclone combination and tadpole survival. When backcrossed with the parental species R. ridibunda, hybrids from all source populations produced viable offspring. Performance of these tadpoles with a sexual and a clonal genome was comparable to that of normal, sexually produced R. ridibunda tadpoles. Thus, in the heterozygous state, the deleterious mutations on the clonal R genomes did not appear to reduce tadpole fitness.