Evoluion, 47(2), 1993, pp. 510-519
ASYMMETRIC COMPETITION IN MIXED POPULATIONS OF TADPOLES OF THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX

RAYMOND D. SEMLITSCH Institute of Zoology, University of Zurich, CH-8057 Zurich, SWITZERLAND

Abstract.ÐHybridogenetic Rana esculenta tadpoles display tolerance to extreme environmental conditions and fit criteria of the "general-purpose" genotype. A trade-off between generality and competitive ability is assumed to occur in asexual species, but the evidence remains unclear. The purpose of my experiment was to test the competitive ability of hemiclonal hybrid Rana esculenta tadpoles relative to the parental species Rana lessonae. Mixed and single genotype populations of R. esculenta and R. lessonae tadpoles were reared at three densities in artificial ponds. Survival of R. esculenta was higher than for R. Iessonae tadpoles, but did not differ among densities. Body size at metamorphosis was the same between genotypes, but decreased with increasing density. Larval period was not affected by density, but R. esculenta tended to metamorphose earlier than R. Iessonae. Percentage of individuals metamorphosing was higher for R. esculenta at both medium and high densities, but the same as R. Iessonae at the low density. The difference in survival, body size, and larval period between tadpoles reared in single and mixed genotype populations was unaffected by genotype or density. The difference in the percentage of metamorphs, however, was strongly affected. The percentage of hybrids metamorphosing was 9% above the responses of single genotype populations at the highest density. Conversely, the percentage of R. lessonae metamor-phosing was 12% below the responses of single genotype populations at the same density. Hybrid success in this experiment further supports the criterion of a "general-purpose" genotype without assumptions of reduced competitive ability.

Key words.- Amphibian, body size, competition, Rana lessonae, survival, tadpole.