Karyotypes, Sex Chromosomes, and Meiotic Division in Spiders

Abstract

From the cytogenetic point of view, spiders are the best studied arachnid order. In spite of this, karyotype data are still available only for less than 2 % of spider species. These data indicate a considerable diversity of diploid numbers, chromosome morphology, and sex chromosome systems in some spider lineages. Most spiders exhibit a standard chromosome structure except for the superfamily Dysderoidea that has holokinetic chromosomes (a derived type of chromosomes without centromere). The unusual multiple sex chromosomes of spiders have received more attention than any other aspect of their cytogenetics. Most species exhibit the so-called X₁X₂0 system with X₁X₂ males and X₁X₁X₂X₂ females. This sex chromosome determination, considered ancestral for spiders, has an unclear origin. Recently found unusual sex chromosome behaviour at meiosis of female spiders may represent a system acting to restrict pairing and recombination to homologous X chromosomes and support the hypothesis on the origin of multiple X chromosomes by duplications. Despite its evolutionary stability, the X₁X₂0 system has been transformed in some lineages by X–X fusions, by X chromosome duplications, or via sex chromosome–autosome rearrangements. Finally, some spiders also exhibit modifications of meiotic division, e.g. the bipolarisation of prophase I nucleus in males of entelegyne spiders: bivalents form two groups at opposite sides of the nucleus. Prior to formation of the metaphase plate, each bivalent is encased in a double membrane tube, which extends nearly to the spindle poles.