The order Araneae is among the most diverse groups of animals on earth, including 42,473 taxonomically described species. Nevertheless, less than 2% of the spider species were characterized from the cytogenetic viewpoint. Most cytogenetic analyses in spiders has been performed using standard staining techniques, which revealed an extraordinary variation in diploid number (2nB=7 to 2nB=110), and types of sex chromosome system (X0, XY, X1X2Y, X1X20, X1X2X30, X1X2X3X40, XnYn) within this group. In relation to specific chromosome sites, such as the nucleolar organizer regions (NOR), only 30 species were investigated through of the use of silver impregnation. Despite the scarcity of NOR information, some authors hypothesized that in basal groups of spiders the NORs were localized on autosomes and sex chromosomes while in derivative groups this region occurred only on autosomes. However, the description of NORs on sex chromosome of species belonging to derived spider family has raised certain questions: if NORs on autosomes and sex chromosomes is a synapomorphy of basal groups of spiders or it also constitutes a shared character among derived clades; 2) if change in the NORs distribution pattern is related to differentiation in the sex chromosome system; 3) if there is a positive correlation between the diploid number of species and the number of NORs; 4) if variability in location and number of NORs could represents the mechanism of interspecific karyotype differentiation in some families of spiders that possess highly conserved chromosome features. Taking into account these information, the purpose of this work is to investigate the evolution of ribosomal DNA genes in Araneae. The chromosomal analyses will be accomplished with conventional and molecular cytogenetic techniques; moreover, species of distinct families of Mygalomorphae and Araneomorphae will be examined to perform a widespread sampling within the order and to obtain detail data about the karyotype evolution in spiders, emphasizing the NORs.
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