Unusual karyotype diversity in the European spiders of the genus Atypus (Araneae: Atypidae)

Compared with araneomorph spiders, karyotypes of the spider infraorder Mygalomorphae are nearly unknown. In this study we investigated karyotypes of European species of the genus Atypus (Atypidae). The male karyotype of A. muralis and A. piceus comprises 41 chromosomes, whereas female complements contain 42 chromosomes. On the other hand, both sexes of A. affinis possess 14 chromosomes only. It is the lowest diploid number found in mygalomorph spiders so far. Furthermore, obtained data suggest X0 sex chromosome system in A. piceus, A. muralis and neo-XY system in A. affinis. Karyotypes of all three Atypus species are composed of biarmed chromosomes only. Thus they differ significantly from the karyotype of A. karschi, the only other species of this genus studied so far. Its karyotype was reported to be composed of acrocentric chromosomes and possesses X(1)X(2)0 sex chromosome system. All this shows that unlike in most genera of araneomorph spiders, mygalomorphs of the genus Atypus exhibit unusual diversity in the number, morphology of chromosomes, and the sex chromosome system. Considering high number of chromosomes being plesiomorphic character in spiders, then karyotypes of A. muralis and A. piceus represent ancestral situation and that of A. affinis being derived by multiple fusions. Karyotype differences in Atypus correspond with morphological differences, namely the number of segments of the posterior lateral spinnerets. Thus in contrast to published hypothesis, the 3-segmented posterior lateral spinnerets of A. affinis should present a derived state.