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Abstract

Abstract.-Two hybridizing species of newts, Triturus cristatus and T. marmoratus, with overlapping distributions show a parapatric distribution when surveyed in detail. The factors that govern the distribution of cristatus vs. marmoratus in the departement (province) of Mayenne in western France are identified as forestation and relief. The parapatric hybrid zone running through Mayenne is narrow but widens to approximately 20 km in an area with mixed habitat. In this area most breeding sites are shared and F, hybrids form about 4% of the total population.Analysis of survey data collected about 30 years previously also shows an essentially parapatric distribution. Comparison ofpast and present distribution maps reveals that cristatus has superseded marmoratus over large areas in the south ofMayenne. An area where marmoratus replaced cristatus also exists, but it is more limited in size.Gene flow between cristatus and marmoratus is analyzed using 10 diagnostic genetic markers [9 protein loci and mitochondrial (mt) DNA]. In syntopic populations nuclear gene flow is bidirectional with a mean frequency ofintrogressed alleles (j) of 0.3%. In allotopic populations of cristatus and marmoratus gene flow is present in areas of species replacement if= 0.3%), while gene flow appears to be absent in those areas that have been continuously occupied by a single species. At the biogeographic level, the presence or absence of introgression is paralleled by the persistence or absence, respectively, of pockets of cristatus-marmoratus syntopy.All F, hybrids possess the cristatus type mtDNA. This may be due to asymmetric interspecific mate choice and would explain the observed absence of introgression of the maternally inherited mtDNA genome in areas where cristatus replaced marmoratus.The cristatus-marmoratus hybrid zone bears characteristics ofboth the clinal (parapatric) hybrid zone model and the mosaic hybrid zone model. Such a mixed model-for which we propose the term "reticulate hybrid zone" -can be appreciated only if studied over a two-dimensional geographic area and also through time. Hybrid zones between parapatric species pose numerous interesting evolutionary questions. A more thorough understanding of the dynamics of these zones has come about in recent years through a variety of genetic data including allozyme loci, ribosomal genes, and the mitochondrial genome (Hewitt, 1988; Harrison, 1990). Many hybrid zones can be classified as tension zones (Key, 1968) having arisen through secondary contact of partly differentiated populations. The genetic structure ofthese has been described by diffusion models, where zone width is determined by the opposing forces ofmigration and disruptive selection, which tend to broaden and narrow the zone, respectively (Barton and Hewitt, 1985). A transect across a zone under this model is expected to reveal smooth concordant sigmoidal transitions (clines) for the genetic characters examined. Selection acting on a locus can change cline shape and position relative to other clines, as well a...