Abstract
European water frog hybrids Rana esculenta reproduce hemiclonally, by hybridogenesis: In the germ line they exclude the genome of the parental
species Rana lessonae and produce haploid, unrecombined gametes with a genome of the parental species Rana ridibunda. These hybrids coexist
with and depend as sexual parasites on the host parental species R. lessonae (the L-E population system); matings with R. lessonae restore somatic
hybridity in each generation of R. esculenta. We investigated 15 L-E system populations in northern Switzerland, which is outside R. ridibunda
native range. Frequency of hybrids in samples varied from 8% in marsh ponds to 100% in gravel pits and forest ponds. Clonal diversity (variation
among R. ridibunda genomes of hybrids), detected by six protein electrophoretic marker loci, revealed a total of eight hemiclones and locally
ranged from uniclonal populations in southern parts of the survey region to six coexisting hemiclones in the north. All alleles distinguishing
hemiclones occur commonly in the nearest native R. ridibunda populations of east-central Europe; the most probable source of clonal diversity in
our samples is multiple clone formation by primary hybridizations in the sympatry area of R. ridibunda and R. lessonae and subsequent dispersal
of hemiclonal lineages. A positive correlation between amount of clonal diversity and hybrid frequency, predicted by the Frozen Niche Variation
(FNV) model (each hemiclone is characterized by a relatively narrow niche, coexistence is possible through niche partitioning), was not found;
this contrasts with hemiclonally reproducing fish hybrids (Poeciliopsis). Historical factors, such as availability of different colonizing hemiclones
may be strong enough to override the signal from operation of the FNV.
Hotz, H