In the present preliminary study we report an investigation on taxonomic status of individuals displaying diagnostic traits of intermediate values between the morphologically typical forms of Athous haemorrhoidalis (Fabricius 1801) and A. vittatus (Gmelin 1790) species occurring sympatrically. The presence of the "intermediate form" invoked the hypothesis of interspecific hybridization between A. haemorrhoidalis and A. vittatus species. An alternative explanation is that the "intermediate form" comprises individuals that are morphological variants of either of the species, without interspecific gene flow. We used partial sequences of two mitochondrial genes coding for the cytochrome oxidase subunit I (cox1) and 16S ribosomal RNA (rrnL), respectively. Additionally, we examined the variability of five polymorphic allozyme loci: 6Pgd, Idh, Mdh, Pgm and Gpi. Altogether, 28 A. haemorrhoidalis, 7 A. vittatus and 8 "intermediate form" individuals were included into the study. Phylogenetic analyses produced the trees of a consistent topology with high statistical support regardless of the method used. The specimens signified as the "intermediate form" constituted a monophyletic group with individuals of A. vittatus species. Empirical data confirmed that the reproductive isolation between the species studied is complete. No haplotype was shared between A. haemorrhoidalis and A. vittatus in sympatry. Similar results were obtained using the analysis of allozyme data. Especially at Idh and 6Pgd loci, alleles characteristic for A. vittatus and the "intermediate form" were not found in A. haemorrhoidalis, despite the larger sample examined. This suggests that the "intermediate form" is a phenotypic variant of A. vittatus without the signature of introgression.
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