COMPETITION AMONG TADPOLES OF COEXISTING HEMICLONES OF HYBRIDOGENETICRANA-ESCULENTA - SUPPORT FOR THE FROZEN NICHE VARIATION MODEL

Vertebrate animals reproducing without genetic recombination typically are hybrids, which have large ranges, are locally abundant, and live in disturbed or harsh habitats. This holds for the hemiclonal hybridog enetic frog Rana esculenta: it is widespread in Europe and commonly is found in disturbed habitats such as gravel pits. We hypothesize that its widespread occurrence may either be the result of natural selectio n for a single hemiclone acting as a broadly adapted ''general-purpose '' genotype, or of interclonal selection, which maintains multiple hem iclones that each are relatively narrowly adapted and perform differen tly across environments, that is, the Frozen Niche Variation model. We tested these competing hypotheses using 1000-L outdoor artificial pon ds to rear tadpoles of the parental species (Rana lessonae [LL] and Ra na ridibunda [RR]) alone, and each of three hemiclones of Rana esculen ta (GUT1, GUT2, GUT3) alone, and in mixed hemiclonal populations from hatching to metamorphosis. Tadpoles of three coexisting hemiclones fro m, a single natural population (near Gutighausen, Switzerland) were re ared in both two-and three-way mixtures in equal total numbers at high and low density. For each species and hemiclone, the proportion of ta dpoles metamorphosing decreased as the density of tadpoles increased, with the three hemiclones spanning the range of values exhibited by th e two parental species. LL and GUT1 tadpoles produced the highest prop ortion of metamorphs, where;ls tadpoles of RR produced the fewest meta morphs at both densities. GUT1 tadpoles also produced the largest meta morphs at low density, GUT2 and GUT3 tadpoles produced smaller metamor phs than did GUT1 tadpoles at the low density, but the three hemiclone s did not differ from each other at high density. The parental species (LL and RR) were: intermediate in metamorphic size to the hemiclones at low density, but all genotypes converged on a similar size at high density. Length of the larval period also was affected by density, but its effect was dependent on genotype. GUT1 tadpoles had the shortest larval period at the low density, but larval period was longer and not different between GUT1, GUT3, and LL at high density. RR tadpoles had the longest larval period at both densities. The most dramatic: resul ts were that three genotypes (GUT1, GUT2, and RR) maintained rank orde r and increased days to metamorphosis from low to high density, wherea s two genotypes (GUT3 and LL) changed rank order and decreased days to metamorphosis from low to high density. Mixtures of hemiclones in two -and three-way combinations facilitated the proportion of tadpoles met amorphosing for GUT1 and GUT2 at both densities, but only at the low d ensity for GUT3 tadpoles. Results from this experiment are incompatibl e with the General-Purpose Genotype model as a global explanation of h ybrid abundance in these frogs. Alternatively, the Frozen Niche Variat ion prediction of general performance superiority of clonal mixtures r elative to single clone populations is strongly supported. The data co nfirm that fitness advantages of hemiclones change, depending on the e nvironment, such that in temporally and spatially heterogeneous habita ts like ponds, frequency-dependent selection among hemiclones may prom ote coexistence in hemiclonal assemblages. Yet, differential dispersal or colonization ability and historical factors affecting hemiclone di stribution may also be important in shaping patterns of clonal coexist ence.