Evolution of karyotype, sex chromosomes, and meiosis in mygalomorph spiders (Araneae: Mygalomorphae)

Spiders divide into three clades: Mesothelae, Mygalomorphae, and Araneomorphae. Although over 700 species of 64 families have already been investigated, most of these species belong to the advanced clade Araneomorphae, while cytogenetics of mygalomorphs remains largerly unknown. Here, we investigated karyotype diversity, sex chromosomes and meiosis in 31 representatives belonging in 13 families of Mygalomorphae. Their chromosome number ranges from 2n = 14 to 2n =128 which is the highest number found in spiders so far. The ancenstral sex chromosome system of spiders, X1X20, is uncommon in mygalomorps. Instead, they often show multiple X chromosomes, in males reaching up to 13 X chromosomes (Macrothele gigas), the highest sex chromosome number ever found. On the other hand, the XY system is rather rare and occurs in some representatives of Avicularioidea. Sex chromosomes also vary in their size and centromeric positions and they originated via the chromosome duplications, fissions, X–X, and X-autosome fusions. Additionally, spiders possess a homomorphic sex chromosome pair which is often inactivated in male meiosis.