BACKGROUND/OBJECTIVES: Arachnids are a megadiverse arthropod group. The present study investigated the chromosomes of pedipalpid tetrapulmonates (orders Amblypygi, Thelyphonida, Schizomida) and two arachnid orders of uncertain phylogenetic placement, Ricinulei and Solifugae, to reconstruct their karyotype evolution. Except for amblypygids, the cytogenetics of these arachnid orders was almost unknown prior to the present study. METHODS: Chromosomes were investigated using methods of standard (Giemsa-stained preparations, banding techniques) and molecular cytogenetics (fluorescence in situ hybridization, comparative genomic hybridization). RESULTS AND CONCLUSIONS: New data for 38 species, combined with previously published data, suggest that ancestral arachnids possessed low to moderate 2n (22-40), monocentric chromosomes, one nucleolus organizer region (NOR), low levels of heterochromatin and recombinations, and no or homomorphic sex chromosomes. Karyotypes of Pedipalpi and Solifugae diversified via centric fusions, pericentric inversions, and changes in the pattern of NORs and, in solifuges, also through tandem fusions. Some solifuges display an enormous amount of constitutive heterochromatin and high NOR number. It is hypothesized that the common ancestor of amblypygids, thelyphonids, and spiders exhibited a homomorphic XY system, and that telomeric heterochromatin and NORs were involved in the evolution of amblypygid sex chromosomes. The new findings support the Cephalosomata clade (acariforms, palpigrades, and solifuges). Hypotheses concerning the origin of acariform holocentric chromosomes are presented. Unlike current phylogenetic hypotheses, the results suggest a sister relationship between Schizomida and a clade comprising other tetrapulmonates as well as a polyploidization in the common ancestor of the clade comprising Araneae, Amblypygi, and Thelyphonida.

Departamento de Ecología y Recursos Naturales Facultad de Ciencias Universidad Nacional Autónoma de México México City 04510 Mexico

Department of Genetics and Microbiology Faculty of Science Charles University Viničná 5 128 44 Prague Czech Republic

Department of Molecular Biology and Genetics Institute of Entomology Biology Centre CAS Branišovská 31 370 05 České Budějovice Czech Republic

Department of Molecular Cytogenetics Institute of Plant Molecular Biology Biology Centre CAS Branišovská 31 370 05 České Budějovice Czech Republic

Department of Zoology and Entomology University of Pretoria Pretoria 0083 South Africa

Department of Zoology Denver Museum of Nature and Science 2001 Colorado Blvd Denver CO 80205 USA

Department of Zoology Faculty of Science Charles University Viničná 7 128 44 Prague Czech Republic

Ditsong National Museum of Natural History 432 Paul Kruger Street Pretoria 0001 South Africa

Division of Crop Genetics and Breeding Crop Research Institute Drnovská 507 73 161 00 Prague Czech Republic

Division of Invertebrate Zoology American Museum of Natural History Central Park West at 79th Street New York NY 10024 USA

Faculty of Science University of South Bohemia Branišovská 1760 370 05 České Budějovice Czech Republic

Institute of Animal Physiology and Genetics AS CR Rumburská 89 277 21 Liběchov Czech Republic

Institute of Physiotherapy and Selected Medical Disciplines Faculty of Health and Social Sciences University of South Bohemia J Boreckého 1167 27 370 11 České Budějovice Czech Republic

Na Perštýně 2 110 00 Prague Czech Republic

Senckenberg Research Institute Arachnology Mertonstrasse 17 21 60325 Frankfurt Germany

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