Concepts of spider karyotype evolution are based mostly on advanced and most diversified clade, the entelegyne lineage of araneomorph spiders. Hence the typical spider karyotype is supposed to consist exclusively of acrocentric chromosomes including the multiple X chromosomes. However, our data show considerable diversity of chromosome morphology and sex chromosome systems in basal clades of araneomorphs. Karyotypes of basal araneomorphs consist of holocentric (superfamily Dysderoidea) or normal chromosomes with localized centromere. In males of basal araneomorphs the prophase of first meiotic division includes a long diffuse stage. Multiple X chromosomes are less common in basal clades. The sex chromosome system of many families includes a Y chromosome or nucleolus organizer region that occurs rarely in the entelegyne spiders. A derived X(1)X(2)Y system with an achiasmatic sex-chromosome pairing during meiosis was found in the families Drymusidae, Hypochilidae, Filistatidae, Sicariidae, and Pholcidae. This suggests a monophyletic origin of the families. In some lineages the X(1)X(2)Y system converted into an X0 system, as found in some pholcids, or into an XY system, which is typical for the family Diguetidae. The remarkable karyotype and sex chromosome system diversity allows us to distinguish four evolutionary lineages of basal araneomorphs and hypothesize about the ancestral karyotype of araneomorphs.

Spider diversity is partitioned into three primary clades, namely Mesothelae, Mygalomorphae, and Araneomorphae. Mygalomorph cytogenetics is largely unknown. Our study revealed a remarkable karyotype diversity of mygalomorphs. Unlike araneomorphs, they show no general trend towards a decrease of 2n, as the chromosome number was reduced in some lineages and increased in others. A biarmed karyotype is a symplesiomorphy of mygalomorphs and araneomorphs. Male meiosis of some mygalomorphs is achiasmatic, or includes the diffuse stage. The sex chromosome system X1X20, which is supposedly ancestral in spiders, is uncommon in mygalomorphs. Many mygalomorphs exhibit more than two (and up to 13) X chromosomes in males. The evolution of X chromosomes proceeded via the duplication of chromosomes, fissions, X–X, and X‐autosome fusions. Spiders also exhibit a homomorphic sex chromosome pair. In the germline of mygalomorph males these chromosomes are often deactivated; their deactivation and pairing is initiated already at spermatogonia. Remarkably, pairing of sex chromosomes in mygalomorph females is also initiated at gonial cells. Some mygalomorphs have two sex chromosome pairs. The second pair presumably arose in early‐diverging mygalomorphs, probably via genome duplication. The unique behaviour of spider sex chromosomes in the germline may promote meiotic pairing of homologous sex chromosomes and structural differentiation of their duplicates, as well as the establishment of polyploid genomes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 377–408.

Background Despite progress in genomic analysis of spiders, their chromosome evolution is not satisfactorily understood. Most information on spider chromosomes concerns the most diversified clade, entelegyne araneomorphs. Other clades are far less studied. Our study focused on haplogyne araneomorphs, which are remarkable for their unusual sex chromosome systems and for the co-evolution of sex chromosomes and nucleolus organizer regions (NORs); some haplogynes exhibit holokinetic chromosomes. To trace the karyotype evolution of haplogynes on the family level, we analysed the number and morphology of chromosomes, sex chromosomes, NORs, and meiosis in pholcids, which are among the most diverse haplogyne families. The evolution of spider NORs is largely unknown. Results Our study is based on an extensive set of species representing all major pholcid clades. Pholcids exhibit a low 2n and predominance of biarmed chromosomes, which are typical haplogyne features. Sex chromosomes and NOR patterns of pholcids are diversified. We revealed six sex chromosome systems in pholcids (X0, XY, X1X20, X1X2X30, X1X2Y, and X1X2X3X4Y). The number of NOR loci ranges from one to nine. In some clades, NORs are also found on sex chromosomes. Conclusions The evolution of cytogenetic characters was largely derived from character mapping on a recently published molecular phylogeny of the family. Based on an extensive set of species and mapping of their characters, numerous conclusions regarding the karyotype evolution of pholcids and spiders can be drawn. Our results suggest frequent autosome–autosome and autosome–sex chromosome rearrangements during pholcid evolution. Such events have previously been attributed to the reproductive isolation of species. The peculiar X1X2Y system is probably ancestral for haplogynes. Chromosomes of the X1X2Y system differ considerably in their pattern of evolution. In some pholcid clades, the X1X2Y system has transformed into the X1X20 or XY systems, and subsequently into the X0 system. The X1X2X30 system of Smeringopus pallidus probably arose from the X1X20 system by an X chromosome fission. The X1X2X3X4Y system of Kambiwa probably evolved from the X1X2Y system by integration of a chromosome pair. Nucleolus organizer regions have frequently expanded on sex chromosomes, most probably by ectopic recombination. Our data suggest the involvement of sex chromosome-linked NORs in achiasmatic pairing.

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Abstract. The aim of this study was to characterize karyotypes of central European spiders of the genera Arctosa, Tricca, and Xerolycosa (Lycosidae) with respect to the diploid chromosome number, chromosome morphology, and sex chromosomes. Karyotype data are reported for eleven species, six of them for the first time. For selected species the pattern in the distributions of the constitutive heterochromatin and the nucleolar organizer regions (NORs) was determined. The silver staining technique for detecting NORs of lycosid spiders was standardized. The male karyotype consisted of 2n = 28 (Arctosa and Tricca) or 2n = 22 (Xerolycosa) acrocentric chromosomes. The sex chromosome system was X1X20 in all species. The sex chromosomes of T. lutetiana and X. nemoralis showed unusual behaviour during late diplotene, namely temporary extension due to decondensation. C-banding technique revealed a small amount of constitutive heterochromatin at the centromeric region of the chromosomes. Two pairs of autosomes bore terminal NORs. Differences in karyotypes among Arctosa species indicate that the evolution of the karyotype in this genus involved autosome translocations and size changes in the sex chromosomes. Based on published results and those recorded in this study it is suggested that the ancestral male karyotype of the superfamily Lycosoidea consisted of 28 acrocentric chromosomes.

Spiders are an ancient and extremely diverse animal order. They show a considerable diversity of genome sizes, karyotypes and sex chromosomes, which makes them promising models to analyse the evolution of these traits. Our study is focused on the evolution of the genome and chromosomes in haplogyne spiders with holokinetic chromosomes. Although holokinetic chromosomes in spiders were discovered a long time ago, information on their distribution and evolution in these arthropods is very limited. Here we show that holokinetic chromosomes are an autapomorphy of the superfamily Dysderoidea. According to our hypothesis, the karyotype of ancestral Dysderoidea comprised three autosome pairs and a single X chromosome. The subsequent evolution has frequently included inverted meiosis of the sex chromosome and an increase of 2 n . We demonstrate that caponiids, a sister clade to Dysderoidea, have enormous genomes and high diploid and sex chromosome numbers. This pattern suggests a polyploid event in the ancestors of caponiids. Holokinetic chromosomes could have arisen by subsequent multiple chromosome fusions and a considerable reduction of the genome size. We propose that spider sex chromosomes probably do not pose a major barrier to polyploidy due to specific mechanisms that promote the integration of sex chromosome copies into the genome.

Abstract. This paper is a review containing new original results on the finite order variational sequence and its different representations with emphasis on applications in the theory of variational symmetries and conservation laws in physics.

Compared with araneomorph spiders, karyotypes of the spider infraorder Mygalomorphae are nearly unknown. In this study we investigated karyotypes of European species of the genus Atypus (Atypidae). The male karyotype of A. muralis and A. piceus comprises 41 chromosomes, whereas female complements contain 42 chromosomes. On the other hand, both sexes of A. affinis possess 14 chromosomes only. It is the lowest diploid number found in mygalomorph spiders so far. Furthermore, obtained data suggest X0 sex chromosome system in A. piceus, A. muralis and neo-XY system in A. affinis. Karyotypes of all three Atypus species are composed of biarmed chromosomes only. Thus they differ significantly from the karyotype of A. karschi, the only other species of this genus studied so far. Its karyotype was reported to be composed of acrocentric chromosomes and possesses X(1)X(2)0 sex chromosome system. All this shows that unlike in most genera of araneomorph spiders, mygalomorphs of the genus Atypus exhibit unusual diversity in the number, morphology of chromosomes, and the sex chromosome system. Considering high number of chromosomes being plesiomorphic character in spiders, then karyotypes of A. muralis and A. piceus represent ancestral situation and that of A. affinis being derived by multiple fusions. Karyotype differences in Atypus correspond with morphological differences, namely the number of segments of the posterior lateral spinnerets. Thus in contrast to published hypothesis, the 3-segmented posterior lateral spinnerets of A. affinis should present a derived state.