Insights into the Karyotype Evolution of Charinidae, the Early-Diverging Clade of Whip Spiders (Arachnida: Amblypygi).

Abstract

Simple SummaryWhip spiders (Amblypygi) are spectacular arachnids characterized by powerful raptorial pedipalps and exceptionally elongated forelegs. Although cytogenetic data from amblypygids might be, given their phylogenetic position, important for the reconstruction of arachnid ka-ryotype evolution, cytogenetics of this order is still largely understudied. Here, we applied conventional and molecular cytogenetics to describe the karyotype patterns in Charinidae—the family placed almost at the base of amblypygid phylogeny, thereby providing insights into the ancestral amblypygid karyotype traits. We surveyed four Charinus and five Sarax species and found a wide range of diploid chromosome numbers (2n) in both genera. Representatives with lower 2n possessed generally a higher proportion of biarmed (metacentric and submetacentric) chromosomes. Our results indicate the important roles of chromosome fusions and pericentric inversions in the karyotype differentiation of Charinidae, similarly to that suggested previously for neoamblypygids. Our data, gathered from C-banding, fluorescent banding, and chromosomal mapping of ribosomal DNA and telomeric repeats, bring evidence for the action of these rearrangements and suggest the general trajectory towards 2n reduction from ancestral high chromosome counts. However, we cannot rule out the contribution of chromosome fissions. Whip spiders (Amblypygi) represent an ancient order of tetrapulmonate arachnids with a low diversity. Their cytogenetic data are confined to only a few reports. Here, we analyzed the family Charinidae, a lineage almost at the base of the amblypygids, providing an insight into the ancestral traits and basic trajectories of amblypygid karyotype evolution. We performed Giemsa staining, selected banding techniques, and detected 18S ribosomal DNA and telomeric repeats by fluorescence in situ hybridization in four Charinus and five Sarax species. Both genera exhibit a wide range of diploid chromosome numbers (2n = 42–76 and 22–74 for Charinus and Sarax, respectively). The 2n reduction was accompanied by an increase of proportion of biarmed elements. We further revealed a single NOR site (probably an ancestral condition for charinids), the presence of a (TTAGG)n telomeric motif localized mostly at the chromosome ends, and an absence of heteromorphic sex chromosomes. Our data collectively suggest a high pace of karyotype repatterning in amblypygids, with probably a high ancestral 2n and its subsequent gradual reduction by fusions, and the action of pericentric inversions, similarly to what has been proposed for neoamblypygids. The possible contribution of fissions to charinid karyotype repatterning, however, cannot be fully ruled out.