Zoological Institute and Zoological Museum, University of Zürich (Switzerland)
Ecological explanations for the maintenance of sexual reproduction have assumed that the increased genetic diversity of progeny of sexuals compared to clonals is beneficial in heterogeneous environments. Although a single clone may be quite specific in its resource requirements, and narrowly distributed in nature, most clonal populations have been found to consist of multiple clones. Thus, interclonal selection may produce an assemblage of clones adapted to a wide range of environmental conditions. Using a hemiclonal system of European water frogs, we show that performance traits closely related to adult fitness can vary significantly among clones in an adaptive manner. Rana esculenta are natural hybrids between Rana ridibunda and Rana lessonae that reproduce by hybridogenesis: they produce haploid gametes, whether eggs or sperm, that contain one unrecombined ridibunda genome, the lessonae genome being lost in the germ line prior to meiosis; hybridity is restored by matings with the sexual host parental species Rana lessonae. We present data from a laboratory experiment where individual hybrid tadpoles (Rana esculenta produced sexually (Poz; F1 hybrids Rana ridibunda x Rana lessonae) and hemiclonally (Gut1, Gut2, Gut3, Hel1; natural Rana esculenta lineages from two Swiss sites)) were reared at two food levels, and from an artificial pond experiment where populations of sexually (Poz) and clonally produced tadpoles (hemiclones Gut1, Gut2, Hel1) were reared in competition with the sexual host Rana lessonae at high and low larval densities. We found that performance traits such as the larval growth rate, duration of larval period, and size at metamorphosis all were affected by the type of clone (ridibunda haplotype). Growth rates of two natural hemiclones of tadpoles (Gut1, Gut3) were similar to that of the sexually produced F1 tadpoles (Poz), but greater than that of the other two hemiclones (Gut2, Hel1). Length of larval period was also significantly affected by clone type. Sexually produced F1 tadpoles (Poz) had the shortest larval period and tadpoles of the hemiclone Gut2 had the longest, with those of the hemiclones Gut1, Gut3, and Hel1 being intermediate. Gut2 tadpoles metamorphosed at the largest body sizes, whereas Poz tadpoles were the smallest and Gut1, Gut3 and Hel1 tadpoles had intermediate sizes. Sexually produced F1 hybrid tadpoles (Poz) performed most poorly when raised together with the parental species Rana lessonae at both high and low density, whereas all clonally produced tadpoles (Gut1, Gut2, Hel1) performed equally well. The observed differences between coexisting Rana esculenta hemiclones (Gut1, Gut2) in larval performance traits are of similar magnitude as those found between hybridogenetic Rana esculenta and its sexual host parental species Rana lessonae in earlier experiments.