XXXY sex chromosomes in males of the jumping spider genus Pellenes (Araneae: Salticidae)

Observations of male meiosis and female chromosome number indicate that eight species of Pellenes have the X₁X₂O male, X₁X₁X₂X₂ female sex chromosome system typical of salticids, four species have an X₁X₂X₃Y male, X₁X₁X₂X₃X₃X₃ female system, and one species has both X₁X₂O and X₁X₂X₃Y males. This is the first report of a Y chromosome in spiders. It is hypothesized that the X₁X₂X₂Y system was derived from an X₁X₂O system by a tandem X-autosome fusion which yielded the X₂ and a centric autosome-autosome fusion which yielded the Y. Data on heteropycnosis, chiasmata, segregation, chromosome number and arm length support this hypothesis. The distribution of the X₁X₂X₃Y system within the genus is phylogenetically confusing and suggests that the two sex chromosome systems have been maintained together as a polymorphism in some lineages for long periods of time or that there have been repeated derivations of the X₁X₂X₃Y or X₁X₂O systems.