Král J, Kořínková T, Forman M, Krkavcová L
Laboratory of Arachnid Cytogenetics, Department of Genetics and Microbiology, Faculty of Science, Charles University in Prague, Czech Republic.
Cytogenet Genome Res. 2011;133(1):43-66. doi: 10.1159/000323497. Epub 2011 Jan 29.
A characteristic feature of spider karyotypes is the predominance of unusual multiple X chromosomes. To elucidate the evolution of spider sex chromosomes, their meiotic behavior was analyzed in 2 major clades of opisthothele spiders, namely, the entelegyne araneomorphs and the mygalomorphs. Our data support the predominance of X(1)X(2)0 systems in entelegynes, while rare X(1)X(2)X(3)X(4)0 systems were revealed in the tuberculote mygalomorphs. The spider species studied exhibited a considerable diversity of achiasmate sex chromosome pairing in male meiosis. The end-to-end pairing of sex chromosomes found in mygalomorphs was gradually replaced by the parallel attachment of sex chromosomes in entelegynes. The observed association of male X univalents with a centrosome at the first meiotic division may ensure the univalents' segregation. Spider meiotic sex chromosomes also showed other unique traits, namely, association with a chromosome pair in males and inactivation in females. Analysis of these traits supports the hypothesis that the multiple X chromosomes of spiders originated by duplications. In contrast to the homogametic sex of other animals, the homologous sex chromosomes of spider females were already paired at premeiotic interphase and were inactivated until prophase I. Furthermore, the sex chromosome pairs exhibited an end-to-end association during these stages. We suggest that the specific behavior of the female sex chromosomes may have evolved to avoid the negative effects of duplicated X chromosomes on female meiosis. The chromosome ends that ensure the association of sex chromosome pairs during meiosis may contain information for discriminating between homologous and homeologous X chromosomes and thus act to promote homologous pairing. The meiotic behavior of 4 X chromosome pairs in mygalomorph females, namely, the formation of 2 associations, each composed of 2 pairs with similar structure, suggests that the mygalomorph X(1)X(2)X(3)X(4)0 system originated by the duplication of the X(1)X(2)0 system via nondisjunctions or polyploidization.
蜘蛛核型的一个显著特征是异常多X染色体占主导。为阐明蜘蛛性染色体的进化，在2个主要的后纺亚目蜘蛛类群中分析了它们的减数分裂行为，这2个类群分别是新蛛亚目和原蛛亚目。我们的数据支持新蛛亚目以X(1)X(2)0系统为主，而在瘤状原蛛亚目中发现了罕见的X(1)X(2)X(3)X(4)0系统。所研究的蜘蛛物种在雄性减数分裂中表现出相当多样的无交叉性染色体配对。在原蛛亚目中发现的性染色体端对端配对逐渐被新蛛亚目中性染色体的平行附着所取代。在第一次减数分裂时观察到的雄性X单价体与中心体的关联可能确保了单价体的分离。蜘蛛减数分裂性染色体还表现出其他独特特征，即在雄性中与一对染色体关联以及在雌性中失活。对这些特征的分析支持了蜘蛛的多个X染色体起源于重复的假说。与其他动物的同配性别不同，蜘蛛雌性的同源性染色体在减数分裂前的间期就已配对，并在减数分裂前期I之前失活。此外，性染色体对在这些阶段表现出端对端的关联。我们认为雌性性染色体的特定行为可能已经进化，以避免重复的X染色体对雌性减数分裂产生负面影响。在减数分裂期间确保性染色体对关联的染色体末端可能包含区分同源和异源X染色体的信息，从而起到促进同源配对的作用。原蛛亚目雌性中4对X染色体的减数分裂行为，即形成2个联合体，每个联合体由2对结构相似的染色体组成，表明原蛛亚目的X(1)X(2)X(3)X(4)0系统起源于通过不分离或多倍体化对X(1)X(2)0系统的重复。
