Evolution of multiple sex chromosomes in the spider genus Malthonica (Araneae: Agelenidae) indicates unique structure of the spider sex chromosome systems

Most spiders exhibit a multiple sex chromosome system, X1X20, whose origin has not been satisfactorily explained. Examination of the sex chromosome systems in the spider genus Malthonica (Agelenidae) revealed considerable diversity in sex chromosome constitution within this group. Besides modes X1X20 (M. silvestris) and X1X2X30 (M. campestris), a neo-X1X2X3X4X5Y system in M. ferruginea was found. Ultrastructural analysis of spread pachytene spermatocytes revealed that the X1X20 and X1X2X30 systems include a pair of homomorphic sex chromosomes. Multiple X chromosomes and the pair exhibit an end-to-end pairing, being connected by attachment plaques. The X1X2X3X4X5Y system of M. ferruginea arose by rearrangement between the homomorphic sex chromosome pair and an autosome. Multiple X chromosomes and the sex chromosome pair do not differ from autosomes in a pattern of constitutive heterochromatin. Ultrastructural data on sex chromosome pairing in other spiders indicate that the homomorphic sex chromosome pair forms an integral part of the spider sex chromosome systems. It is suggested that this pair represents ancestral sex chromosomes of spiders, which generated multiple X chromosomes by non-disjunctions. Structural differentiation of newly formed X chromosomes has been facilitated by heterochromatinization of sex chromosome bivalents observed in prophase I of spider females.