Abstract -- 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 hybridogenetic
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 selection for a single
hemiclone acting as a broadly adapted "general-purpose" genotype, or of
interclonal selection, which maintains multiple hemiclones that each are
relatively narrowly adapted and perform differently across environments,
that is, the Frozen Niche Variation model. We tested these competing
hypotheses using 1000-L outdoor artificial ponds to rear tadpoles of the
parental species (Rana lessonae [LL] and Rana ridibunda [RR]) alone, and
each of three hemiclones of Rana esculenta (GUT1, GUT2, GUT3) alone, and in
mixed hemiclonal populations from hatching to metamorphosis. Tadpoles of
three coexisting hemiclones from a single natural population (near
GM-|tighausen, Switzerland) were reared in both two- and three-way mixtures
in equal total numbers at high and low density. For each species and
hemiclone, the proportion of tadples metamorphosing decreased as the
density of tadpoles increased, with the threee hemiclones spanning the
range of values exhibited by the two parental species. LL and GUT1 tadpoles
produced the highest proportion of metamorphs, whereas tadpoles of RR
produced the fewest metamorphs at both densities. GUT1 tadpoles also
produced the largest metamorphs at low density, GUT2 and GUT3 tadpoles
produced smaller metamorphs than did GUT1 tadpoles at the low density, but
the three hemiclones 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 was also 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 results were that three
genotypes (GUT1, GUT2, and RR) maintained rank order and increased days to
metamorphosis from low to high density, whereas 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 metamorphosing for GUT1 and GUT2 at
both densities, but only at the low density for GUT3 tadpoles. Results from
this experiment are incompatible with the General-Purpose Genotype model as
a global explanation of hybrid abundance in these frogs. Alternatively, the
Frozen Niche Variation prediction of general performance superiority of
clonal mixtures relative to single clone populations is strongly supported.
The data confirm that fitness advantages of hemiclones change, depending on
the environment, such that in temporally and spatially heterogeneous
habitats like ponds, frequency-dependent selection among hemiclones may
promote coexistence in hemiclonal assemblages. Yet, differential dispersal
or colonization ability and historical factors affecting hemiclone
distribution may also be important in shaping patterns of clonal
coexistence.
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