5 Sex Chromosomes and Meiosis in Spiders : A Review

According to Platnick (2011), the order Araneae possesses 110 families, 3,849 genera, and 42,473 species. It is divided into two suborders: Mesothelae, consisting of only one family (Liphistiidae), and Opisthothelae. The latter suborder is divided into two infra-orders: Mygalomorphae, consisting of spiders with paraxial chelicerae, and Araneomorphae, consisting of spiders with diaxial chelicerae. The latter infra-order is divided into the basal clades (Hypochilidae and Austrochiloidea), Haplogynae, and Entelegynae, which includes the majority of extant spiders (Coddington & Levi, 1991) (Fig. 1).

In the first cytogenetic studies in spiders performed by Carnoy (1885), gonads of male or female individuals were imbedded in paraffin, sectioned, and stained with Heidenhain's iron haematoxylin.Chromosome visualisation and interpretation of cytogenetic analyses were difficult to achieve with this method.Decades later, Sharma et al. (1959) and Beçak & Beçak (1960) were the first researchers to obtain spider chromosomes by the aceto-orcein or aceto-carmin squash methods.Pinter & Walters (1971) introduced the use of colchicine solution for cytological preparations of spider testes and ovaries.This solution promotes an increase in the number of cells in mitotic and/or meiotic metaphase, the stage in which chromosomes are most easily visualised and identified.In the same decade, Brum-Zorrila & Cazenave (1974) applied 3:1 methanol:acetic acid as a fixative solution and Giemsa solution as a stain.Matsumoto (1977) pioneered the observation of chromosomes in spider embryos.Embryos are a valuable source of mitotic metaphase cells due to the high rate of cellular division that occurs during embryonic development.There are a number of tissues that can be used in cytogenetic studies of spiders, such as gonads (testes and ovaries), cerebral ganglion, and cultured blood cells, as described by Wang & Yan (2001).Fig. 1.Phylogenetic relationships within major clades of spiders according to Coddington & Levi (1991).The number of families was determined according to Platnick (2011).
The gonads, especially the testes, have been found to be more suitable than other tissues for karyotype analysis in the vast majority of cytogenetic investigations.Analysis of gonads allows both mitotic and meiotic chromosomes to be studied.In addition to data regarding the diploid number, length, and morphology of chromosomes, analyses of chromosomes during meiosis have contributed to the identification of types of sex chromosome systems (SCS) in spiders.This is very important in the case of Araneae due to the diversity of simple or multiple SCS that have been recorded in representatives of this order.Furthermore, in investigations of mitotic cells, it is not possible to recognise certain types of SCS, such as X0 and X 1 X 2 Y, by only taking into account the difference in the diploid number observed in male and female individuals.However, analysis of meiotic cells allows investigation of the behaviour of chromosomes in relation to association, synapsis, recombination, and segregation.These features are indispensable for understanding the origin and evolution of sex chromosomes.

Sex chromosome systems (SCS) in spiders
Currently, there are 678 cytogenetic records in spiders (www.arthropodacytogenetics.bio.br/spiderdatabase).Of these, 456 species (67.3%) have an SCS of the X 1 X 2 0 type; 105 species (15.5%) have an X0 system; 59 species (8.7%) have an X 1 X 2 X 3 0 system; 10 species (1.5%) have an SCS of the X 1 X 2 Y type; 5 species (0.7%) have an X 1 X 2 X 3 X 4 0 system; 5 species (0.7%) have an XY SCS; 5 species (0.7%) have an SCS of the X 1 X 2 X 3 Y type; 1 species (0.1%) has an SCS of the X 1 X 2 X 3 X 4 X 5 Y type; and 1 species exhibits variations of a multiple X n Y n SCS.In 31 species (4.6%), the SCS has not been identified (Fig. 2).The number of cytogenetic records (678) in spiders is slightly higher than the number of spider species analysed chromosomally (665) because more than one type of SCS has been registered for some species.

Meiotic behaviour of sex chromosomes
The pioneering works describing the X 1 X 2 0 and X0 SCS in spiders were those of Wallace (1900Wallace ( , 1905) ) and Berry (1906), respectively.These researchers identified the sex chromosomes based on the positive heteropycnotic behaviour of these elements during meiosis (Fig. 3).According to White (1940), the term heteropycnosis was introduced to describe the different levels of condensation and staining that certain chromosomes exhibit in the course of mitosis and/or meiosis.This heteropycnotic pattern can be positive or negative, and it is related to a high or low degree of chromosome condensation, respectively.
Manifestation of heteropycnosis is commonly visualised in the sex chromosomes, especially in male meiotic cells; the high level of chromosome condensation in these cells seems to prevent recombination between nonhomologous regions of heteromorphic sex chromosomes (McKee & Handel, 1993).However, the autosomes and female sex chromosomes can also exhibit chromatin differentiation in some stages of the cell cycle.
In spider spermatogenesis, a heteropycnotic pattern of the sex chromosomes has been recorded for roughly 25% of the species that have been cytogenetically examined, which belong to different suborders (Mygalomorphae and Araneomorphae) and families.Regardless of the type of SCS, 95% of these spider species showed positively heteropycnotic sex chromosomes in premeiotic interphase and prophase I nuclei (Fig. 3a-c, e-g, i-m) and, occasionally, also in metaphase II cells (Fig. 3d).In late meiotic stages, the sex chromosomes usually appeared to be isopycnotic (Fig. 3o-p).The heteropycnotic pattern of sex chromosomes can be used as an additional criterion to determine the type of SCS in spiders, as the number of positive heteropycnotic corpuscles frequently corresponds to the number of sex chromosomes.
It is of note that in some SCS that were originated through relatively recent rearrangements between autosomes and sex chromosomes, such as the X 1 X 2 X 3 Y system of Evarcha hoyi (under Pellenes hoyi) (Maddison, 1982), the X n Y n system observed in Delena cancerides (Rowell, 1985), and the X 1 X 2 X 3 X 4 X 5 Y system found in Malthonica ferruginea (Král, 2007), only the ancient sex chromosomes exhibit positive heteropycnosis in the course of male meiosis.
Recently, positive heteropycnotic behaviour of one autosomal bivalent was verified during male meiosis of some spiders belonging to the families Dipluridae, Theraphosidae (Mygalomorphae), Diguetidae, and Sicariidae (basal Araneomorphae).According to Král et al. (2006Král et al. ( , 2011)), this bivalent could represent sex chromosomes in an early stage of differentiation because in addition to positive heteropycnosis, this bivalent showed a recurrent association with the sex chromosomes during the initial prophase I substages.Conclusive proof of the relationship between this homomorphic chromosome pair and sex determination has been obtained through ultrastructural chromosome analysis of the synaptonemal complex.
The modes of sex chromosome association and segregation are interesting features to investigate in meiotic cells.In spiders with an XY SCS, the sex chromosomes can present associations that vary from chiasmatic, such as those found in Leptoneta infuscata (Leptonetidae), to terminal pairing, such as in Diguetia albolineata and Diguetia canities (Diguetidae) (Král et al. 2006).An achiasmatic terminal association of the sex chromosomes has been recorded in some basal Araneomorphae with X 1 X 2 Y SCS.In representatives of the families Drymusidae, Filistatidae, Hypochilidae, Pholcidae, and Sicariidae, in which all sex chromosomes are biarmed, the arms of the metacentric X chromosomes showed end-to-end pairing (Fig. 3n) with arms of the tiny Y chromosome (Oliveira et al. 1997, Silva et al. 2002, Král et al. 2006).In contrast, in Pholcus phalangioides (Pholcidae), which carries a submetacentric X 2 chromosome, only one arm of this sex chromosome was terminally paired with the Y chromosome (Král et al. 2006).
In derivative araneomorphs that possess other types of multiple SCS, such as X 1 X 2 X 3 Y and X 1 X 2 X 3 X 4 X 5 Y, in which the sex chromosomes do not show a high degree of morphological and/or structural differentiation due to their recent origin, the association of the sex chromosomes during meiosis can be chiasmatic between some elements and achiasmatic between others.In the X 1 X 2 X 3 Y SCS of E. hoyi, an interstitial or terminal chiasma was present between one arm of the large submetacentric Y chromosome and the long arm of the acrocentric X 2 chromosome (Maddison, 1982).In M. ferruginea (Agelenidae), the X 1 , X 2 , and X 3 sex chromosomes appeared as univalents that were terminally associated with the X 4 X 5 Y trivalent; this trivalent was composed of one large-sized Y chromosome and two small-sized acrocentric X chromosomes.Although Král (2007) did not register the presence of chiasma between these sex chromosomes due to the precocious dissociation of these elements during prophase I, ultrastructural analysis revealed the occurrence of a recombination nodule in the X 4 X 5 Y trivalent.
From the zygotene stage to metaphase I, the X chromosomes of multiple X SCS usually present a parallel disposition (Fig. 3 e-g, j-l), without evidence of chiasmata, and proximity between each X chromosome commonly involves the centromere region (Král et al. 2011).Occasionally, in some mygalomorph species of the family Theraphosidae, the sex chromosomes of the X 1 X 2 X 3 X 4 0 system exhibit an end-to-end association (X 1 X 2 Y-like pairing), and in representatives of the family Dipluridae, the sex chromosomes can appear as univalent elements that are highly condensed and separated.In addition, an X 1 X 2 Y-like pairing of the sex chromosomes has been observed in Stegodyphus lineatus (Eresidae) and Pax islamita (Zodariidae), which are carriers of the X 1 X 2 X 3 0 and X 1 X 2 0 SCS, respectively (Král et al. 2011).
Contrary to data revealed by conventional chromosome analyses, in which the sex chromosomes of multiple X systems appeared to exhibit a simple behaviour that involved only parallel pairing, more recent cytogenetic ultrastructural studies have supplied surprising information.Benavente and Wettstein (1980) were the first to describe the presence of junctional lamina between the X 1 and X 2 sex chromosomes of Schizocosa malitiosa (under Lycosa malitiosa); this junctional lamina was structurally similar to the synaptonemal complex, formed in the early substages of prophase I, and persisted to the late substages.Wise (1983) also encountered evidence of a junctional lamina in prophase I in two carriers of the X 1 X 2 0 system, Allocosa georgicola (under Lycosa georgicola) and Rabidosa rabida (under Lycosa rabida).Furthermore, a terminal association between sex chromosomes and one homomorphic bivalent was observed in S. malitiosa and A. georgicola.However, the association between sex chromosomes and a homomorphic bivalent was only explained in recent ultrastructural analyses of Pardosa morosa (Lycosidae) and the agelenids Malthonica campestris and Malthonica silvestris performed by Král (2007) and Král et al. (2011).These studies indicated that this homomorphic bivalent is an element that belongs to the SCS; that is, the system included one pair of homomorphic sex chromosomes in addition to the morphologically differentiated X 1 and X 2 or X 1 , X 2 and X 3 sex chromosomes (Fig. 3h).Fig. 3. Behaviour of the sex chromosomes during meiosis in male spiders.a -d.Hogna sternalis (X0); e. Lycosa erythrognatha (X 1 X 2 0); f.Falconina sp.(X 1 X 2 0); g.Polybetes sp.(X 1 X 2 0); h.Phoneutria sp.(X 1 X 2 0); i -k.Trachelas sp.(X 1 X 2 X 3 0); l.Xeropigo sp.(X 1 X 2 X 3 X 4 0); m -p.Loxosceles variegata (X 1 X 2 Y).a, e, i, m.Premeiotic interphase nuclei.b, f, h, j.Pachytene cells.c, g, k, l, n.Diplotene cells.d, o, p. Metaphase II nuclei.In almost all cells, the sex chromosomes can be easily recognised by their high degree of condensation and positive heteropycnosis (a-g, i-m) and/or association behaviour (e-h, j-l, n).Note the parallel pairing of the X chromosomes of the multiple sex chromosome system (e-f, j-l) and the end-to-end paring of the X 1 X 2 Y chromosomes (n).In h, observe the probable junctional lamina (arrow) between the X 1 and X 2 chromosomes and the terminal association of these sex chromosomes with a homomorphic bivalent (arrowhead).Scale bar=10 µm.
Although employing cytogenetic techniques to identify specific chromosomal regions has provided relevant data on spider chromosomes, only approximately 50 and 30 species were characterised with respect to the constitutive heterochromatin and nucleolar organiser regions (NORs), respectively.In these species, constitutive heterochromatin exhibited a similar distribution among autosomes and sex chromosomes, occurring mainly in pericentromeric regions.However, in at least three representatives of the basal araneomorphs with an X 1 X 2 Y SCS, Pholcus phalangioides, Loxosceles intermedia, and Loxosceles laeta, the Y chromosome was completely heterochromatic (Silva et al., 2002, Král et al. 2006).This reinforces the results obtained in analyses of the association behaviour of sex chromosomes during meiosis I; that is, the Y chromosome exhibits a high degree of differentiation and does not share homology with the X 1 and X 2 chromosomes.In mygalomorphs, basal and derived araneomorphs, the NORs are located predominantly on the terminal regions of one to three autosome pairs.Nevertheless, among basal araneomorph species with an X0 SCS, NORs can occur on autosomes and the X chromosome (Dysderidae, Pholcidae and Tetrablemmidae) or only on the X chromosome (Ochyroceratidae, Leptonetidae, and Scytodidae).Recently, Král et al. (2011) described the presence of NORs on the X 2 sex chromosome of derived araneomorph species belonging to the family Tetragnathidae.According to these authors, this unusual NOR localisation may be due to the translocation of rDNA cistrons from autosomes to sex chromosomes.

Early descriptions and discussions of nomenclature, function and origin
Carnoy (1885) presented the first, although inaccurate, chromosome numbers of some spider species.However, this study did not mention the existence of chromosomes that could be related to sex determination.Wallace (1900Wallace ( , 1909) ) was the first to describe double "accessory chromosomes" in a spider, Agelenopsis naevia (under Agalena naevia), and to associate the presence of these elements with sex determination.However, Wallace (1900Wallace ( , 1909) ) was not able to observe such chromosomes in females, thus concluding that male embryos were produced by fusion between a spermatozoon with accessory chromosomes and an egg without such elements, and female embryos were formed by fusion between a spermatozoon and an egg that both lack accessory chromosomes.
According to Wallace (1909), the accessory chromosomes corresponded to those that Wagner (1896) described as a "nucleolus".Subsequently, these chromosomes in spiders were also referred to as "heterochromosomes" (Montgomery, 1905) or "odd-chromosomes" (Berry, 1906).The nomenclature of "accessory chromosomes" was adopted by Painter (1914) and others.In a brief communication describing the X element in Amaurobius sp., King (1925) was the first to use the term "sex chromosome" in spiders.The designation of "accessory chromosomes" was still used by Hard (1939), but by the late 1940's, the nomenclature of "sex chromosomes" was definitively adopted in spiders.An early explanation of the origin and evolution of sex chromosomes in spiders and other groups of organisms was elaborated by Montgomery (1905).According to this author, there were two types of so-called "heterochromosomes"; those that occurred in pairs in spermatogonia and then associated to form bivalents in spermatocytes (paired type), and those that were unpaired, or single, in spermatogonia and continued to be unpaired in the spermatocytes during the course of meiosis.According to this study, unpaired heterochromosomes could originate from those of the paired type through subsequent modifications.Furthermore, Montgomery (1905) stated that heterochromosomes of the paired type were derived from "ordinary chromosomes", but these heterochromosomes no longer carried out the same activities as the "ordinary chromosomes" and had the tendency to disappear.Thus, excessively minute heterochromosomes could represent the last stage before total deletion of these elements, instead of the first stage in the origin of this type of chromosome.Montgomery (1905) also suggested that heterochromosomes arose from "ordinary chromosomes" concomitantly with a change in chromosomal number, most likely from a higher to a lower number.However, the relationship between heterochromosomes and sex determination was only a hypothesis at that time because there was no record of such chromosomes in oocytes.
According to Painter (1914), the fact that accessory chromosomes were consistent with respect to their number, form, and behaviour in spider species belonging to 13 families suggested that these elements must have a very important and constant function in the life cycle of these spiders, in contrast to the autosomes, which were numerically variable among different species.Painter (1914) also noted that the accessory chromosomes were related to sex determination.The point of view of Painter (1914) was contrary to the conclusion presented by Wallace (1909), i.e., male embryos were produced by the fusion of one spermatozoon without accessory chromosomes and one egg with such elements, whereas female embryos were formed by the fusion of a spermatozoon and egg that both carried accessory chromosomes.The conclusion of Painter (1914) can be considered correct for all spider species without a Y chromosome.

Origin of the X 1 X 2 0 sex chromosome system
The X 1 X 2 0 SCS has been considered a plesiomorphic feature in spiders because it occurs in representatives of the phylogenetically basal family Liphistiidae (Mesothelae) (Suzuki, 1954).Various hypotheses concerning the origin of this system in spiders have been put forth.Revell (1947) was the first to suggest that the X 1 X 2 0 SCS most likely originated from an X0 system in spiders, considering the proposition of White (1940), who suggested that duplication of the X chromosome from an X0 system gave rise to the multiple X chromosome systems (Fig. 4).This hypothesis was based on the similarity of the sizes of X chromosomes and the probable homology between the X chromosomes during prophase I in a multiple sex chromosome system.However, Revell (1947) verified the presence of multiple X chromosomes of different sizes in Tegenaria, and he suggested that these X chromosomes had undergone evolutionary differentiation after originating from an X0 system (Fig. 5).Fig. 4. Interpretive scheme of the origin of the X 1 X 2 0 SCS based on descriptions of White (1940).Fig. 5. Schematic representation of the origin of the X 1 X 2 0 SCS based on descriptions of Revell (1947).
Studying the meiotic cells of Araneus quadratus (under Aranea reaumuri), Pätau (1948) suggested that there was no indication of homology between the X 1 and X 2 chromosomes; however, this author did not exclude the possibility of partial homology.Absence of homology between the X 1 and X 2 chromosomes was corroborated by Hackman (1948), Suzuki (1952) and Mittal (1964).Moreover, Pätau (1948) proposed that the X 1 X 2 0 SCS was formed by centric fission of a large X chromosome in an X0 system.
Due to the fact that all X chromosomes of X0 systems that were registered at that time exhibited subterminal or terminal centromeres, Pätau (1948) suggested that the smaller X 1 and X 2 chromosomes had originated from the X0 system not only by simple centric fission, but through additional rearrangements such as 1) centric fragmentation and fission in the long arm terminal region followed by inversion of the long chromosome segment, resulting in a dicentric chromosome; 2) fission in the middle region of the dicentric chromosome, forming two acrocentric X 1 and X 2 chromosomes of similar size (Fig. 6).Fig. 6.Interpretive scheme of the origin of the X 1 X 2 0 SCS based on descriptions of Pätau (1948).
Bole-Gowda (1950) asserted that the X 1 X 2 0 SCS originated from an X0 system in the ancestor of spiders by fission in the middle of the X chromosome producing an acentric chromosome segment that then translocated to a supernumerary centric fragment.These rearrangements produced X 1 and X 2 chromosomes of similar sizes, one of which retained the original centromere of the X chromosome, while the other kept the supernumerary centromere (Fig. 7).Suzuki (1954) was in agreement with the hypothesis described by Pätau (1948) but disagreed with the assertion of Bole-Gowda (1950) regarding the origin of the X 1 X 2 0 system in spiders because no explanation was provided for the origin of the supernumerary centric fragment involved in this hypothesis.Considering the terminal position of the centromere and the size of the X 1 and X 2 chromosomes, Postiglioni & Brum-Zorrilla (1981) suggested that non-disjunction or duplication of a single telocentric X chromosome may have been responsible for the origin of the X 1 X 2 0 SCS of Lycosa sp.3 (Fig. 8a).According to these researchers, the lack of homology between these chromosomes was most likely due to the occurrence of other rearrangements.However, the authors did not exclude the possibility that the origin of the X 1 X 2 0 system of Lycosa sp.3.could have occurred by centric fission of a metacentric X chromosome (Fig. 8b).
Although previous authors reported that female X chromosomes form normal bivalents during meiosis (Hackman, 1948;Pätau, 1948;Sharma et al., 1959), Král (2007) and Král et al. (2011) found that the female X chromosomes of entelegyne and mygalomorph spiders paired during meiosis and also exhibited heterochromatinisation similar to that observed in male meiosis.This event could prevent recombination between the X chromosomes and accelerate their differentiation, a process that is consistent with the hypothesis of Postiglioni & Brum-Zorrilla (1981).Fig. 8. Interpretive scheme of the origin of the X 1 X 2 0 SCS based on descriptions of Postiglioni & Brum-Zorrilla (1981).
In addition to these hypotheses, some researchers have considered the X 1 X 2 0 SCS to have originated secondarily from other multiple SCS.Oliveira et al. (2007) proposed that the X 1 X 2 0 system could have arisen by gradual heterochromatinisation and erosion of the Y chromosome (Fig. 9).This hypothesis was based on the fact that the pholcid Spermophora senoculata, which exhibits an X 1 X 2 Y system, was considered phylogenetically basal (Bruvo-Mararic et al., 2005) in relation to Crossopriza lyoni, which shows an X 1 X 2 0 SCS.Fig. 9. Schematic representation of the origin of the X 1 X 2 0 SCS based on the descriptions of Oliveira et al. (2007).
In some species of the genus Malthonica (Agelenidae), Král (2007) encountered three types of SCS, X 1 X 2 0, X 1 X 2 X 3 0, and X 1 X 2 X 3 X 4 X 5 Y, and therefore suggested that in this genus, the X 1 X 2 X 3 0 condition was ancestral and gave rise to the X 1 X 2 0 system through a tandem fusion between two X chromosomes (Fig. 10).This proposition was based on the peculiar meiotic behaviour of the X chromosomes belonging to the X 1 X 2 X 3 0 SCS, the great difference in sizes between the X chromosomes of the X 1 X 2 0 SCS, and the presence of the X 1 X 2 X 3 0 system in Tegenaria parietina, which is morphologically closely related to Malthonica.Fig. 10.Interpretive scheme of the origin of the X 1 X 2 0 SCS based on descriptions presented by Král (2007).
2.4 Origin of the X 1 X 2 X 3 0 sex chromosome system Revell (1947) found 2n♂=40+X 1 X 2 0 in Tegenaria atrica and 2n♂=40+X 1 X 2 X 3 0 in Tegenaria domestica.Considering that the number of autosomes was constant (40 autosomes) in these two Tegenaria species, the author suggested that the X1X2X30 SCS originated from the X 1 X 2 0 system, which was derived from an X0 system.However, this author did not state the chromosome rearrangements that were responsible for the origin of the X 1 X 2 X 3 0 system.The existence of X chromosomes of different lengths in species with an X 1 X 2 X 3 0 system was considered by Revell (1947) as evidence that the multiple X chromosomes of the X 1 X 2 X 3 0 system were modified after the origination of this SCS.
According to Pätau (1948), in the Tegenaria species studied by Revell (1947), the largest X chromosome found in T. atrica (X 1 X 2 0) was equivalent to the sum of the lengths of the two smallest X chromosomes observed in T. domestica (X 1 X 2 X 3 0).In this case, an X 1 X 2 0 SCS could give rise to an X 1 X 2 X 3 0 system through rearrangements similar to those proposed by Pätau (1948) for the origin of the X 1 X 2 0 SCS (Fig. 11, 6).This explanation was supported by Suzuki (1954) and Sharma et al. (1959).However, these authors analysed Selenops radiatus, which showed three X chromosomes of equal size, and proposed that the similarity in the length of the sex chromosomes in some species with the X 1 X 2 X 3 0 system could be the result of additional rearrangements after their origin from an X 1 X 2 0 system.Fig. 11.Schematic representation of the origin of the X 1 X 2 X 3 0 SCS based on the descriptions of Pätau (1948).Sharma et al. (1959) demonstrated that the six X chromosomes in the zygotene and pachytene stages in Selenops radiatus females (males were X 1 X 2 X 3 0) did not show any positive heteropycnosis and formed only normal bivalents but did not form multivalents; similar meiotic behaviour of chromosomes was observed in female meiotic cells of a species with four X chromosomes (males were X 1 X 2 0) by Hackman (1948), Pätau (1948) and Suzuki (1954), reinforcing the suggestion that there was no homology between the X chromosomes of the X 1 X 2 0 and X 1 X 2 X 3 0 SCS.Bole-Gowda (1952) proposed that the X 1 X 2 X 3 0 system found in Heteropoda venatoria arose from an ancestor with an X 1 X 2 0 system by translocation of a segment of one X chromosome of the X 1 X 2 0 system and one supernumerary centric fragment (Fig. 12).This is similar to this author's hypothesis for the origin of the X 1 X 2 0 system from the X0 system.Fig. 12. Interpretive scheme of the origin of the X 1 X 2 X 3 0 SCS based on descriptions of Bole-Gowda (1952).
To explain the origin of the X 1 X 2 X 3 0 SCS found in Lycosa sp.(thorelli group), Postiglioni & Brum-Zorrilla (1981) hypothesised that non-disjunction of one X chromosome of the X 1 X 2 0 system, followed by loss of homology between the X chromosomes had occurred (Fig. 13).
To corroborate the hypothesis of non-disjunction, the authors cited the observation of a particular behaviour involving early condensation and isolation of one X chromosome at the pachytene stage.Taking into account that the X 1 X 2 0 system could be ancestral in spiders, Postiglioni & Brum-Zorrilla (1981) suggested that each sex chromosome would undergo independent mutations during the course of evolution; however, if one of the X chromosomes had suffered a recent non-disjunction, these last two elements would present similar behaviour during meiotic prophase I, and the X chromosome not involved in the event would conserve its individuality and would appear condensed in early stages.Fig. 13.Schematic representation of the origin of the X 1 X 2 X 3 0 SCS based on descriptions presented by Postiglioni & Brum-Zorrilla (1981).Parida & Sharma (1986) observed that in some spider species with an X 1 X 2 X 3 0 SCS, two X chromosomes were small and one was large, suggesting that this system was derived from a small fragment of the X 1 or X 2 chromosome of an X 1 X 2 0 system by deletion of most of its chromatin and a subsequent increase in length by duplications (Fig. 14).Fig. 14.Interpretive scheme of the origin of the X 1 X 2 X 3 0 SCS based on descriptions of Parida & Sharma (1986).
2.5 Origin of the X 1 X 2 X 3 X 4 0 sex chromosome system Data & Chatterjee (1983, 1988) were the first to record an X 1 X 2 X 3 X 4 0 SCS in spiders.This SCS was found in Metellina segmentata ( Meta segmentata) (Tetragnathidae) and Bhutaniella sikkimensis ( Heteropoda sikkimensis) (Sparassidae).In 1988, these researchers presented a proposal for the origin of the X 1 X 2 X 3 X 4 0 system: duplication or non-disjunction of one X chromosome of the X 1 X 2 X 3 0 SCS, with subsequent loss of homology (Fig. 15).This proposition was similar to that formulated by Postiglioni & Brum-Zorrilla (1981) to explain the origin of the X 1 X 2 0 and X 1 X 2 X 3 0 systems.Fig. 15.Interpretive scheme of the origin of the X 1 X 2 X 3 X 4 0 SCS based on descriptions of Datta & Chatterjee (1988).
In Diplura species, which employ an X 1 X 2 X 3 X 4 0 system, Král et al. (2011) found that one heteropycnotic and one isopycnotic portion could be distinguished in the X 1 and X 2 chromosomes, most likely corresponding to the original X 1 and X 2 chromosomes and the original autosomes, respectively, indicating that the X 1 and X 2 sex chromosomes originated by sex chromosome/autosome translocation.Additionally, Král et al. (2011), suggested that this system probably originated by duplication of the X 1 X 2 0 system via non-disjunctions or polyploidisation (Fig. 16) based on the chromomere pattern and size of the sex chromosomes observed in the X 1 X 2 X 3 X 4 0 Diplura.Fig. 16.Schematic representation of the origin of the X 1 X 2 X 3 X 4 0 SCS based on the descriptions of Král et al. (2011).
2.6 Origin of the X0 sex chromosome system Hackman (1948) found the supposed first case of a metacentric X chromosome in spiders in Oxyopes ramosus and noted that the X0 SCS verified in Oxyopes (metacentric), Myrmarachne, Misumena, and Xysticus (acrocentric) had most likely been derived from the X 1 X 2 0 system in two ways: 1.The metacentric X of the X0 system could have been derived by centric fusion between X 1 and X 2 chromosomes (Fig. 17).This mechanism was also employed by several authors (Bole-Gowda, 1952;Suzuki, 1954;Postiglioni & Brum-Zorrilla, 1981;ezáč et al., 2006;Král et al., 2011;Stávale et al. 2011) to explain the origin of the X0 SCS, which involves a metacentric X, in many spider groups.2. The acrocentric X of the X0 system could have originated through gradual elimination of one X chromosome of the X 1 X 2 0 SCS, as suggested by Suzuki (1952Suzuki ( , 1954)).This author put forth this proposition based on the fact that some thomisid species with an X 1 X 2 0 system presented gradual differences between the lengths of X 1 and X 2 chromosomes (with both showing the same, slightly different or markedly different sizes).Furthermore, some species even exhibited an X0 system, suggesting that elimination of one X of the X 1 X 2 0 system had taken place in the course of evolution (Fig. 18).(1948).
Bole-Gowda (1950) proposed that the X0 system found in several spider species could have evolved by reciprocal translocation between the X 1 and X 2 chromosomes, preceded by distal fission in one sex chromosome and proximal fission in the other X, giving rise to a large acrocentric X chromosome, as found in Oxyopes hindostanicus; the centric fragment produced in this process was lost (Fig. 19).Datta & Chatterjee (1989, 1992) proposed that the X0 system found in lycosid and uloborid spiders originated from the X 1 X 2 0 SCS by centric fusion of the X 1 and X 2 chromosomes, followed by pericentric inversion (Fig. 20a) or partial deletion (Fig. 20b) in one of the X chromosome arms, giving rise to an acrocentric element.Alternatively, the acrocentric X chromosome could have originated from tandem fusion between the X 1 and X 2 chromosomes (Fig. 20c).
Tandem fusion was postulated as the mechanism involved in the derivation of the acrocentric X chromosome (X0 system) of Zodarion from the acrocentric X 1 and X 2 chromosomes (X 1 X 2 0 system) present in species of the same genus (Pekár & Král, 2001).Pekár et al. (2005) proposed that two positive heteropycnotic bodies observed in the premeiotic interphase nuclei of two Zodarion species with the X0 SCS were segments of the X chromosome that corresponded to the original X 1 and X 2 .
Fig. 20.Interpretive scheme of the origin of the X0 SCS based on descriptions of Datta & Chatterjee (1989, 1992).
According to Král et al. (2006), the XY SCS, which was originated from the X 1 X 2 Y system, gave rise to an X0 SCS through loss of the Y chromosome (Fig. 21).The SCS of the X0 type was found in many pholcids and Scytodes (Scytodidae).The evidence for this degeneration and complete elimination of the Y chromosome was the high level of constitutive heterochromatin detected in this chromosome in Pholcus phalangioides.2006) described the first XY SCS in a spider of the genus Atypus (Mygalomorphae).According to these authors, this neo-XY system was formed from an X0 system, which was recorded in other species of this genus and involved rearrangements between the X chromosome and autosomes.Král et al. (2006) studied the evolution of the chromosomes in several basal araneomorphs and formulated some hypotheses about SCS evolution.These authors discovered that SCS that include a Y chromosome are more common in spiders than previously believed, at least in basal araneomorphs.Several species with XY and X 1 X 2 Y SCS were described in many families, and the authors highlighted the fact that in some species previously described as carriers of the X 1 X 2 0 system, the tiny Y chromosome could have been neglected.Based on the diversity of SCS in basal araneomorphs, specifically in pholcids, Král et al. (2006) proposed that the X 1 X 2 Y SCS involving metacentric chromosomes in this group of spiders was similar to that found in Filistatidae and Loxosceles and was converted into an XY SCS.First, by pericentric inversion transforming one of the metacentric X chromosomes into a subtelo-or acrocentric form, the X 1 X 2 Y became similar to that described in Pholcus phalangioides (Pholcidae).Subsequently, the other X chromosome of the X 1 X 2 Y system was also pericentrically inverted, forming a hypothetical configuration in which both X chromosomes of the X 1 X 2 Y system exhibit acrocentric morphology.In the next step, centric fusion between the acrocentric X chromosomes of the hypothetical X 1 X 2 Y SCS occurred, generating an XY system, as found in Smeringopus pallidus (Pholcidae) and Diguetia (Diguetidae).In Diguetia albolineata, which exhibits an XY SCS, both arms of the metacentric Y chromosome paired with only one arm of the metacentric X during meiosis.In Diguetia canities, the metacentric X chromosome was supposedly pericentrically inverted, resulting an acrocentric element (Fig. 22).The proposed origin of the XY SCS observed in Leptoneta (Leptonetidae) was quite different.The XY system was believed to have originated from the X0 system, not from the X 1 X 2 Y system, in a mechanism involving translocation between the X and one autosome constituting the neo X chromosome; the homolog of the autosome involved in the rearrangement formed the Y chromosome (Fig. 23).This hypothesis put forth by Král et al. (2006) was based on the fact that the distal part of the X chromosome in Leptoneta, which has an XY SCS (probably corresponding to the translocated autosome), presented a different pattern of condensation during meiosis I.This last characteristic was not detected in the sex chromosomes of Diguetia species.2.8 Origin of the X 1 X 2 Y sex chromosome system Silva (1988) was the first to record an X 1 X 2 Y SCS in spiders.Although the author noted the possibility that the small acrocentric chromosome found in Loxosceles laeta was a

Conclusion
Despite the fact that spider meiosis analyses have been carried out for more than 100 years, relatively little has been learned about sex chromosome origin and evolution in this group.The vast majority of studies on this topic have been based solely on assumptions or on basic chromosomal characteristics (chromosome length, number, meiotic condensation, and meiotic segregation).Recent advances in spider cytogenetics, such as ultrastructural analysis of cells during meiosis and examination of female meiosis, have added new insights into SCS evolution.However, only 665 (~1.6%) of the 42,423 known spider species (110 families) have been chromosomally characterised.Fifty-four families have not been studied cytogenetically, resulting in several gaps in the existing hypotheses on sex chromosome evolution.
To provide a broader knowledge base leading to better inferences regarding the origin and evolution of sex chromosomes, further efforts involving spider meiosis analysis should include a broader range of species and use conventional, ultrastructural (synaptonemal complex), and molecular (rDNA and telomere FISH and chromosome painting) cytogenetic techniques.

Fig. 7 .
Fig. 7. Schematic representation of the origin of the X 1 X 2 0 SCS based on the descriptions of Bole-Gowda (1950).

Fig. 17 .
Fig. 17.Schematic representation of the origin of the X0 SCS based on descriptions presented by Hackman (1948).

Fig. 18 .
Fig. 18.Interpretive scheme of the origin of the X0 SCS based on descriptions of Hackman(1948).

Fig. 19 .
Fig. 19.Schematic representation of the origin of the X0 SCS based on descriptions presented by Bole-Gowda (1950).

Fig. 21 .
Fig. 21.Schematic representation of the origin of the X0 SCS based on descriptions of Král et al. (2006).

Fig. 22 .
Fig. 22. Interpretive scheme of the origin of the XY SCS of Pholcidae and Diguetidae based on the descriptions of Král et al. (2006).

Fig. 23 .
Fig. 23.Schematic representation of the origin of the XY SCS of Leptonetidae based on the descriptions of Král et al. (2006).