Factors influencing the composition of mixed populations of a hemiclonal hybrid and its sexual host


B.Hellriegel Zoological Museum, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland. Tel.: 41-1-6354974; fax: 41-1-6356826; e-mail: barhell@zool.unizh.ch


Hemiclonal/hybridogenetic hybrids combine demographic superiority of asexuals and genetic diversity of sexuals, but their need for backcrossing with a parental species tightly couples them with this sexual host. How can systems like this persist in ecological and evolutionary time? Two discrete-time mathematical models describing the complex life cycle and mating system of hybridogenetic waterfrogs (Rana esculenta) identified four factors and their interactions as important. Although female mating preferences, in combination with differences in fecundity, determine species coexistence, differences in larval competitiveness seem to be more important for the hybrid‘s actual frequency. However, coexistence is possible even when host and hybrid are equally fecund and competitive. Dispersal and competition interact in their influence on species composition, but ecological and reproductive dispersal has opposing effects. In ecological terms our results explain the remarkable stability of observed species ratios over time within natural hybridogenetic populations, and indicate why the species composition can vary so widely between localities. In evolutionary terms they explain the old age of these and other hybridogenetic systems. They also suggest interesting consequences for other tightly coupled systems.