Hybridization in natural populations of ground beetles (Coleoptera, Carabidae)

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Abstract
We investigated 13 populations of the carabid subgenus Chrysocarabus in the Spanish mountains Sierra de Urbasa and Sierra de Andia. In some areas there were up to 16% hybrids between Carabus lineatus and Carabus splendens. Hybrids are best distinguished as individuals having combinations of morphological characters that are diagnostic for both species. Multivariate discriminant analysis showed a gap between the parental species, but hybrids were either C. lineatus-like or C. splendens-like. Experimentally produced hybrids lie in the same range as natural hybrids. Electrophoretic studies have interpretable results for eight enzymes, but only one sex-linked locus (G-6-pd) was fixed for alternative alleles in the parental species, allowing identification of only female hybrids. Analysis of proteolytic enzymes from the gut revealed one fixed difference for chymotrypsin, but the locus for this enzyme is sex-linked. Trypsin appears polymorphic at several loci and is not useful for identifying hybrids. The geographical range of overlap and hybridization is large when compared with dispersal abilities of C. lineatus and C. splendens. We suggest that the hybrid zone is broken up into a number of subzones defined by differences in habitat structure and related to specific ecological requirements of the parental species. Within each of these zones, theory developed from study of narrow hybrid zones may apply.

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